Adding upstream version 115.7.0esr.upstream/115.7.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

20 files changed, 5009 insertions, 0 deletions

diff --git a/toolkit/modules/subprocess/.eslintrc.js b/toolkit/modules/subprocess/.eslintrc.js
new file mode 100644
index 0000000000..7640781589
--- /dev/null
+++ b/toolkit/modules/subprocess/.eslintrc.js
@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+module.exports = {
+  extends: "../../components/extensions/.eslintrc.js",
+
+  rules: {
+    "no-console": "off",
+  },
+};
diff --git a/toolkit/modules/subprocess/Subprocess.sys.mjs b/toolkit/modules/subprocess/Subprocess.sys.mjs
new file mode 100644
index 0000000000..f26fe50fc2
--- /dev/null
+++ b/toolkit/modules/subprocess/Subprocess.sys.mjs
@@ -0,0 +1,201 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * These modules are loosely based on the subprocess.jsm module created
+ * by Jan Gerber and Patrick Brunschwig, though the implementation
+ * differs drastically.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+import { SubprocessConstants } from "resource://gre/modules/subprocess/subprocess_common.sys.mjs";
+
+const lazy = {};
+
+if (AppConstants.platform == "win") {
+  ChromeUtils.defineESModuleGetters(lazy, {
+    SubprocessImpl: "resource://gre/modules/subprocess/subprocess_win.sys.mjs",
+  });
+} else {
+  // Ignore the "duplicate" definitions here as this are also defined
+  // in the "win" block above.
+  // eslint-disable-next-line mozilla/valid-lazy
+  ChromeUtils.defineESModuleGetters(lazy, {
+    SubprocessImpl: "resource://gre/modules/subprocess/subprocess_unix.sys.mjs",
+  });
+}
+
+function encodeEnvVar(name, value) {
+  if (typeof name === "string" && typeof value === "string") {
+    return `${name}=${value}`;
+  }
+
+  let encoder = new TextEncoder();
+  function encode(val) {
+    return typeof val === "string" ? encoder.encode(val) : val;
+  }
+
+  return Uint8Array.of(...encode(name), ...encode("="), ...encode(value), 0);
+}
+
+function platformSupportsDisclaimedSpawn() {
+  return AppConstants.isPlatformAndVersionAtLeast("macosx", 18);
+}
+
+/**
+ * Allows for creation of and communication with OS-level sub-processes.
+ *
+ * @namespace
+ */
+export var Subprocess = {
+  /**
+   * Launches a process, and returns a handle to it.
+   *
+   * @param {object} options
+   * An object describing the process to launch.
+   *
+   * @param {string} options.command
+   * The full path of the executable to launch. Relative paths are not
+   * accepted, and `$PATH` is not searched.
+   *
+   * If a path search is necessary, the {@link Subprocess.pathSearch} method may
+   * be used to map a bare executable name to a full path.
+   *
+   * @param {string[]} [options.arguments]
+   * A list of strings to pass as arguments to the process.
+   *
+   * @param {object} [options.environment] An object containing a key
+   * and value for each environment variable to pass to the
+   * process. Values that are `=== null` are ignored. Only the
+   * object's own, enumerable properties are added to the environment.
+   *
+   * @param {boolean} [options.environmentAppend] If true, append the
+   * environment variables passed in `environment` to the existing set
+   * of environment variables. Values that are `=== null` are removed
+   * from the environment. Otherwise, the values in 'environment'
+   * constitute the entire set of environment variables passed to the
+   * new process.
+   *
+   * @param {string} [options.stderr]
+   * Defines how the process's stderr output is handled. One of:
+   *
+   * - `"ignore"`: (default) The process's standard error is not redirected.
+   * - `"stdout"`: The process's stderr is merged with its stdout.
+   * - `"pipe"`: The process's stderr is redirected to a pipe, which can be read
+   *   from via its `stderr` property.
+   *
+   * @param {string} [options.workdir]
+   *        The working directory in which to launch the new process.
+   *
+   * @param {boolean} [options.disclaim]
+   * macOS-specific option for 10.14+ OS versions. If true, enables a
+   * macOS-specific process launch option allowing the parent process to
+   * disclaim responsibility for the child process with respect to privacy/
+   * security permission prompts and decisions. This option is ignored on
+   * platforms that do not support it.
+   *
+   * @returns {Promise<Process>}
+   *
+   * @throws {Error}
+   * May be rejected with an Error object if the process can not be
+   * launched. The object will include an `errorCode` property with
+   * one of the following values if it was rejected for the
+   * corresponding reason:
+   *
+   * - Subprocess.ERROR_BAD_EXECUTABLE: The given command could not
+   *   be found, or the file that it references is not executable.
+   *
+   * Note that if the process is successfully launched, but exits with
+   * a non-zero exit code, the promise will still resolve successfully.
+   */
+  call(options) {
+    options = Object.assign({}, options);
+
+    options.stderr = options.stderr || "ignore";
+    options.workdir = options.workdir || null;
+    options.disclaim = options.disclaim || false;
+
+    let environment = {};
+    if (!options.environment || options.environmentAppend) {
+      environment = this.getEnvironment();
+    }
+
+    if (options.environment) {
+      Object.assign(environment, options.environment);
+    }
+
+    options.environment = Object.entries(environment)
+      .map(([key, val]) => (val !== null ? encodeEnvVar(key, val) : null))
+      .filter(s => s);
+
+    options.arguments = Array.from(options.arguments || []);
+
+    if (options.disclaim && !platformSupportsDisclaimedSpawn()) {
+      options.disclaim = false;
+    }
+
+    return Promise.resolve(
+      lazy.SubprocessImpl.isExecutableFile(options.command)
+    ).then(isExecutable => {
+      if (!isExecutable) {
+        let error = new Error(
+          `File at path "${options.command}" does not exist, or is not executable`
+        );
+        error.errorCode = SubprocessConstants.ERROR_BAD_EXECUTABLE;
+        throw error;
+      }
+
+      options.arguments.unshift(options.command);
+
+      return lazy.SubprocessImpl.call(options);
+    });
+  },
+
+  /**
+   * Returns an object with a key-value pair for every variable in the process's
+   * current environment.
+   *
+   * @returns {object}
+   */
+  getEnvironment() {
+    let environment = Object.create(null);
+    for (let [k, v] of lazy.SubprocessImpl.getEnvironment()) {
+      environment[k] = v;
+    }
+    return environment;
+  },
+
+  /**
+   * Searches for the given executable file in the system executable
+   * file paths as specified by the PATH environment variable.
+   *
+   * On Windows, if the unadorned filename cannot be found, the
+   * extensions in the semicolon-separated list in the PATHSEP
+   * environment variable are successively appended to the original
+   * name and searched for in turn.
+   *
+   * @param {string} command
+   *        The name of the executable to find.
+   * @param {object} [environment]
+   *        An object containing a key for each environment variable to be used
+   *        in the search. If not provided, full the current process environment
+   *        is used.
+   * @returns {Promise<string>}
+   */
+  pathSearch(command, environment = this.getEnvironment()) {
+    // Promise.resolve lets us get around returning one of the Promise.jsm
+    // pseudo-promises returned by Task.jsm.
+    let path = lazy.SubprocessImpl.pathSearch(command, environment);
+    return Promise.resolve(path);
+  },
+};
+
+Object.assign(Subprocess, SubprocessConstants);
+Object.freeze(Subprocess);
+
+export function getSubprocessImplForTest() {
+  return lazy.SubprocessImpl;
+}
diff --git a/toolkit/modules/subprocess/docs/index.rst b/toolkit/modules/subprocess/docs/index.rst
new file mode 100644
index 0000000000..983fe30d4d
--- /dev/null
+++ b/toolkit/modules/subprocess/docs/index.rst
@@ -0,0 +1,226 @@
+.. _Subprocess:
+
+=================
+Subprocess Module
+=================
+
+The Subprocess module allows a caller to spawn a native host executable, and
+communicate with it asynchronously over its standard input and output pipes.
+
+Processes are launched asynchronously ``Subprocess.call`` method, based
+on the properties of a single options object. The method returns a promise
+which resolves, once the process has successfully launched, to a ``Process``
+object, which can be used to communicate with and control the process.
+
+A simple Hello World invocation, which writes a message to a process, reads it
+back, logs it, and waits for the process to exit looks something like:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/cat",
+    });
+
+    proc.stdin.write("Hello World!");
+
+    let result = await proc.stdout.readString();
+    console.log(result);
+
+    proc.stdin.close();
+    let {exitCode} = await proc.wait();
+
+Input and Output Redirection
+============================
+
+Communication with the child process happens entirely via one-way pipes tied
+to its standard input, standard output, and standard error file descriptors.
+While standard input and output are always redirected to pipes, standard error
+is inherited from the parent process by default. Standard error can, however,
+optionally be either redirected to its own pipe or merged into the standard
+output pipe.
+
+The module is designed primarily for use with processes following a strict
+IO protocol, with predictable message sizes. Its read operations, therefore,
+either complete after reading the exact amount of data specified, or do not
+complete at all. For cases where this is not desirable, ``read()`` and
+``readString`` may be called without any length argument, and will return a
+chunk of data of an arbitrary size.
+
+
+Process and Pipe Lifecycles
+===========================
+
+Once the process exits, any buffered data from its output pipes may still be
+read until the pipe is explicitly closed. Unless the pipe is explicitly
+closed, however, any pending buffered data *must* be read from the pipe, or
+the resources associated with the pipe will not be freed.
+
+Beyond this, no explicit cleanup is required for either processes or their
+pipes. So long as the caller ensures that the process exits, and there is no
+pending input to be read on its ``stdout`` or ``stderr`` pipes, all resources
+will be freed automatically.
+
+The preferred way to ensure that a process exits is to close its input pipe
+and wait for it to exit gracefully. Processes which haven't exited gracefully
+by shutdown time, however, must be forcibly terminated:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/usr/bin/subprocess.py",
+    });
+
+    // Kill the process if it hasn't gracefully exited by shutdown time.
+    let blocker = () => proc.kill();
+
+    AsyncShutdown.profileBeforeChange.addBlocker(
+      "Subprocess: Killing hung process",
+      blocker);
+
+    proc.wait().then(() => {
+      // Remove the shutdown blocker once we've exited.
+      AsyncShutdown.profileBeforeChange.removeBlocker(blocker);
+
+      // Close standard output, in case there's any buffered data we haven't read.
+      proc.stdout.close();
+    });
+
+    // Send a message to the process, and close stdin, so the process knows to
+    // exit.
+    proc.stdin.write(message);
+    proc.stdin.close();
+
+In the simpler case of a short-running process which takes no input, and exits
+immediately after producing output, it's generally enough to simply read its
+output stream until EOF:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: await Subprocess.pathSearch("ifconfig"),
+    });
+
+    // Read all of the process output.
+    let result = "";
+    let string;
+    while ((string = await proc.stdout.readString())) {
+      result += string;
+    }
+    console.log(result);
+
+    // The output pipe is closed and no buffered data remains to be read.
+    // This means the process has exited, and no further cleanup is necessary.
+
+
+Bidirectional IO
+================
+
+When performing bidirectional IO, special care needs to be taken to avoid
+deadlocks. While all IO operations in the Subprocess API are asynchronous,
+careless ordering of operations can still lead to a state where both processes
+are blocked on a read or write operation at the same time. For example,
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/cat",
+    });
+
+    let size = 1024 * 1024;
+    await proc.stdin.write(new ArrayBuffer(size));
+
+    let result = await proc.stdout.read(size);
+
+The code attempts to write 1MB of data to an input pipe, and then read it back
+from the output pipe. Because the data is big enough to fill both the input
+and output pipe buffers, though, and because the code waits for the write
+operation to complete before attempting any reads, the ``cat`` process will
+block trying to write to its output indefinitely, and never finish reading the
+data from its standard input.
+
+In order to avoid the deadlock, we need to avoid blocking on the write
+operation:
+
+.. code-block:: javascript
+
+    let size = 1024 * 1024;
+    proc.stdin.write(new ArrayBuffer(size));
+
+    let result = await proc.stdout.read(size);
+
+There is no silver bullet to avoiding deadlocks in this type of situation,
+though. Any input operations that depend on output operations, or vice versa,
+have the possibility of triggering deadlocks, and need to be thought out
+carefully.
+
+Arguments
+=========
+
+Arguments may be passed to the process in the form an array of strings.
+Arguments are never split, or subjected to any sort of shell expansion, so the
+target process will receive the exact arguments array as passed to
+``Subprocess.call``. Argument 0 will always be the full path to the
+executable, as passed via the ``command`` argument:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/sh",
+      arguments: ["-c", "echo -n $0"],
+    });
+
+    let output = await proc.stdout.readString();
+    assert(output === "/bin/sh");
+
+
+Process Environment
+===================
+
+By default, the process is launched with the same environment variables and
+working directory as the parent process, but either can be changed if
+necessary. The working directory may be changed simply by passing a
+``workdir`` option:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/pwd",
+      workdir: "/tmp",
+    });
+
+    let output = await proc.stdout.readString();
+    assert(output === "/tmp\n");
+
+The process's environment variables can be changed using the ``environment``
+and ``environmentAppend`` options. By default, passing an ``environment``
+object replaces the process's entire environment with the properties in that
+object:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/pwd",
+      environment: {FOO: "BAR"},
+    });
+
+    let output = await proc.stdout.readString();
+    assert(output === "FOO=BAR\n");
+
+In order to add variables to, or change variables from, the current set of
+environment variables, the ``environmentAppend`` object must be passed in
+addition:
+
+.. code-block:: javascript
+
+    let proc = await Subprocess.call({
+      command: "/bin/pwd",
+      environment: {FOO: "BAR"},
+      environmentAppend: true,
+    });
+
+    let output = "";
+    while ((string = await proc.stdout.readString())) {
+      output += string;
+    }
+
+    assert(output.includes("FOO=BAR\n"));
diff --git a/toolkit/modules/subprocess/moz.build b/toolkit/modules/subprocess/moz.build
new file mode 100644
index 0000000000..3b3ec21246
--- /dev/null
+++ b/toolkit/modules/subprocess/moz.build
@@ -0,0 +1,35 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+EXTRA_JS_MODULES += [
+    "Subprocess.sys.mjs",
+]
+
+EXTRA_JS_MODULES.subprocess += [
+    "subprocess_common.sys.mjs",
+    "subprocess_shared.js",
+    "subprocess_worker_common.js",
+]
+
+if CONFIG["OS_TARGET"] == "WINNT":
+    EXTRA_JS_MODULES.subprocess += [
+        "subprocess_shared_win.js",
+        "subprocess_win.sys.mjs",
+        "subprocess_worker_win.js",
+    ]
+else:
+    EXTRA_JS_MODULES.subprocess += [
+        "subprocess_shared_unix.js",
+        "subprocess_unix.sys.mjs",
+        "subprocess_worker_unix.js",
+    ]
+
+XPCSHELL_TESTS_MANIFESTS += ["test/xpcshell/xpcshell.ini"]
+
+SPHINX_TREES["toolkit_modules/subprocess"] = ["docs"]
+
+with Files("docs/**"):
+    SCHEDULES.exclusive = ["docs"]
diff --git a/toolkit/modules/subprocess/subprocess_common.sys.mjs b/toolkit/modules/subprocess/subprocess_common.sys.mjs
new file mode 100644
index 0000000000..aa3a28fb7b
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_common.sys.mjs
@@ -0,0 +1,711 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* eslint-disable mozilla/balanced-listeners */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+var obj = {};
+Services.scriptloader.loadSubScript(
+  "resource://gre/modules/subprocess/subprocess_shared.js",
+  obj
+);
+
+const { ArrayBuffer_transfer } = obj;
+export const SubprocessConstants = obj.SubprocessConstants;
+
+const BUFFER_SIZE = 32768;
+
+let nextResponseId = 0;
+
+/**
+ * Wraps a ChromeWorker so that messages sent to it return a promise which
+ * resolves when the message has been received and the operation it triggers is
+ * complete.
+ */
+export class PromiseWorker extends ChromeWorker {
+  constructor(url) {
+    super(url);
+
+    this.listeners = new Map();
+    this.pendingResponses = new Map();
+
+    this.addListener("close", this.onClose.bind(this));
+    this.addListener("failure", this.onFailure.bind(this));
+    this.addListener("success", this.onSuccess.bind(this));
+    this.addListener("debug", this.onDebug.bind(this));
+
+    this.addEventListener("message", this.onmessage);
+
+    this.shutdown = this.shutdown.bind(this);
+    lazy.AsyncShutdown.webWorkersShutdown.addBlocker(
+      "Subprocess.sys.mjs: Shut down IO worker",
+      this.shutdown
+    );
+  }
+
+  onClose() {
+    lazy.AsyncShutdown.webWorkersShutdown.removeBlocker(this.shutdown);
+  }
+
+  shutdown() {
+    return this.call("shutdown", []);
+  }
+
+  /**
+   * Adds a listener for the given message from the worker. Any message received
+   * from the worker with a `data.msg` property matching the given `msg`
+   * parameter are passed to the given listener.
+   *
+   * @param {string} msg
+   *        The message to listen for.
+   * @param {function(Event)} listener
+   *        The listener to call when matching messages are received.
+   */
+  addListener(msg, listener) {
+    if (!this.listeners.has(msg)) {
+      this.listeners.set(msg, new Set());
+    }
+    this.listeners.get(msg).add(listener);
+  }
+
+  /**
+   * Removes the given message listener.
+   *
+   * @param {string} msg
+   *        The message to stop listening for.
+   * @param {function(Event)} listener
+   *        The listener to remove.
+   */
+  removeListener(msg, listener) {
+    let listeners = this.listeners.get(msg);
+    if (listeners) {
+      listeners.delete(listener);
+
+      if (!listeners.size) {
+        this.listeners.delete(msg);
+      }
+    }
+  }
+
+  onmessage(event) {
+    let { msg } = event.data;
+    let listeners = this.listeners.get(msg) || new Set();
+
+    for (let listener of listeners) {
+      try {
+        listener(event.data);
+      } catch (e) {
+        console.error(e);
+      }
+    }
+  }
+
+  /**
+   * Called when a message sent to the worker has failed, and rejects its
+   * corresponding promise.
+   *
+   * @private
+   */
+  onFailure({ msgId, error }) {
+    this.pendingResponses.get(msgId).reject(error);
+    this.pendingResponses.delete(msgId);
+  }
+
+  /**
+   * Called when a message sent to the worker has succeeded, and resolves its
+   * corresponding promise.
+   *
+   * @private
+   */
+  onSuccess({ msgId, data }) {
+    this.pendingResponses.get(msgId).resolve(data);
+    this.pendingResponses.delete(msgId);
+  }
+
+  onDebug({ message }) {
+    dump(`Worker debug: ${message}\n`);
+  }
+
+  /**
+   * Calls the given method in the worker, and returns a promise which resolves
+   * or rejects when the method has completed.
+   *
+   * @param {string} method
+   *        The name of the method to call.
+   * @param {Array} args
+   *        The arguments to pass to the method.
+   * @param {Array} [transferList]
+   *        A list of objects to transfer to the worker, rather than cloning.
+   * @returns {Promise}
+   */
+  call(method, args, transferList = []) {
+    let msgId = nextResponseId++;
+
+    return new Promise((resolve, reject) => {
+      this.pendingResponses.set(msgId, { resolve, reject });
+
+      let message = {
+        msg: method,
+        msgId,
+        args,
+      };
+
+      this.postMessage(message, transferList);
+    });
+  }
+}
+
+/**
+ * Represents an input or output pipe connected to a subprocess.
+ *
+ * @property {integer} fd
+ *           The file descriptor number of the pipe on the child process's side.
+ *           @readonly
+ */
+class Pipe {
+  /**
+   * @param {Process} process
+   *        The child process that this pipe is connected to.
+   * @param {integer} fd
+   *        The file descriptor number of the pipe on the child process's side.
+   * @param {integer} id
+   *        The internal ID of the pipe, which ties it to the corresponding Pipe
+   *        object on the Worker side.
+   */
+  constructor(process, fd, id) {
+    this.id = id;
+    this.fd = fd;
+    this.processId = process.id;
+    this.worker = process.worker;
+
+    /**
+     * @property {boolean} closed
+     *           True if the file descriptor has been closed, and can no longer
+     *           be read from or written to. Pending IO operations may still
+     *           complete, but new operations may not be initiated.
+     *           @readonly
+     */
+    this.closed = false;
+  }
+
+  /**
+   * Closes the end of the pipe which belongs to this process.
+   *
+   * @param {boolean} force
+   *        If true, the pipe is closed immediately, regardless of any pending
+   *        IO operations. If false, the pipe is closed after any existing
+   *        pending IO operations have completed.
+   * @returns {Promise<object>}
+   *          Resolves to an object with no properties once the pipe has been
+   *          closed.
+   */
+  close(force = false) {
+    this.closed = true;
+    return this.worker.call("close", [this.id, force]);
+  }
+}
+
+/**
+ * Represents an output-only pipe, to which data may be written.
+ */
+class OutputPipe extends Pipe {
+  constructor(...args) {
+    super(...args);
+
+    this.encoder = new TextEncoder();
+  }
+
+  /**
+   * Writes the given data to the stream.
+   *
+   * When given an array buffer or typed array, ownership of the buffer is
+   * transferred to the IO worker, and it may no longer be used from this
+   * thread.
+   *
+   * @param {ArrayBuffer|TypedArray|string} buffer
+   *        Data to write to the stream.
+   * @returns {Promise<object>}
+   *          Resolves to an object with a `bytesWritten` property, containing
+   *          the number of bytes successfully written, once the operation has
+   *          completed.
+   *
+   * @throws {object}
+   *          May be rejected with an Error object, or an object with similar
+   *          properties. The object will include an `errorCode` property with
+   *          one of the following values if it was rejected for the
+   *          corresponding reason:
+   *
+   *            - Subprocess.ERROR_END_OF_FILE: The pipe was closed before
+   *              all of the data in `buffer` could be written to it.
+   */
+  write(buffer) {
+    if (typeof buffer === "string") {
+      buffer = this.encoder.encode(buffer);
+    }
+
+    if (Cu.getClassName(buffer, true) !== "ArrayBuffer") {
+      if (buffer.byteLength === buffer.buffer.byteLength) {
+        buffer = buffer.buffer;
+      } else {
+        buffer = buffer.buffer.slice(
+          buffer.byteOffset,
+          buffer.byteOffset + buffer.byteLength
+        );
+      }
+    }
+
+    let args = [this.id, buffer];
+
+    return this.worker.call("write", args, [buffer]);
+  }
+}
+
+/**
+ * Represents an input-only pipe, from which data may be read.
+ */
+class InputPipe extends Pipe {
+  constructor(...args) {
+    super(...args);
+
+    this.buffers = [];
+
+    /**
+     * @property {integer} dataAvailable
+     *           The number of readable bytes currently stored in the input
+     *           buffer.
+     *           @readonly
+     */
+    this.dataAvailable = 0;
+
+    this.decoder = new TextDecoder();
+
+    this.pendingReads = [];
+
+    this._pendingBufferRead = null;
+
+    this.fillBuffer();
+  }
+
+  /**
+   * @property {integer} bufferSize
+   *           The current size of the input buffer. This varies depending on
+   *           the size of pending read operations.
+   *           @readonly
+   */
+  get bufferSize() {
+    if (this.pendingReads.length) {
+      return Math.max(this.pendingReads[0].length, BUFFER_SIZE);
+    }
+    return BUFFER_SIZE;
+  }
+
+  /**
+   * Attempts to fill the input buffer.
+   *
+   * @private
+   */
+  fillBuffer() {
+    let dataWanted = this.bufferSize - this.dataAvailable;
+
+    if (!this._pendingBufferRead && dataWanted > 0) {
+      this._pendingBufferRead = this._read(dataWanted);
+
+      this._pendingBufferRead.then(result => {
+        this._pendingBufferRead = null;
+
+        if (result) {
+          this.onInput(result.buffer);
+
+          this.fillBuffer();
+        }
+      });
+    }
+  }
+
+  _read(size) {
+    let args = [this.id, size];
+
+    return this.worker.call("read", args).catch(e => {
+      this.closed = true;
+
+      for (let { length, resolve, reject } of this.pendingReads.splice(0)) {
+        if (
+          length === null &&
+          e.errorCode === SubprocessConstants.ERROR_END_OF_FILE
+        ) {
+          resolve(new ArrayBuffer(0));
+        } else {
+          reject(e);
+        }
+      }
+    });
+  }
+
+  /**
+   * Adds the given data to the end of the input buffer.
+   *
+   * @param {ArrayBuffer} buffer
+   *        An input buffer to append to the current buffered input.
+   * @private
+   */
+  onInput(buffer) {
+    this.buffers.push(buffer);
+    this.dataAvailable += buffer.byteLength;
+    this.checkPendingReads();
+  }
+
+  /**
+   * Checks the topmost pending read operations and fulfills as many as can be
+   * filled from the current input buffer.
+   *
+   * @private
+   */
+  checkPendingReads() {
+    this.fillBuffer();
+
+    let reads = this.pendingReads;
+    while (
+      reads.length &&
+      this.dataAvailable &&
+      reads[0].length <= this.dataAvailable
+    ) {
+      let pending = this.pendingReads.shift();
+
+      let length = pending.length || this.dataAvailable;
+
+      let result;
+      let byteLength = this.buffers[0].byteLength;
+      if (byteLength == length) {
+        result = this.buffers.shift();
+      } else if (byteLength > length) {
+        let buffer = this.buffers[0];
+
+        this.buffers[0] = buffer.slice(length);
+        result = ArrayBuffer_transfer(buffer, length);
+      } else {
+        result = ArrayBuffer_transfer(this.buffers.shift(), length);
+        let u8result = new Uint8Array(result);
+
+        while (byteLength < length) {
+          let buffer = this.buffers[0];
+          let u8buffer = new Uint8Array(buffer);
+
+          let remaining = length - byteLength;
+
+          if (buffer.byteLength <= remaining) {
+            this.buffers.shift();
+
+            u8result.set(u8buffer, byteLength);
+          } else {
+            this.buffers[0] = buffer.slice(remaining);
+
+            u8result.set(u8buffer.subarray(0, remaining), byteLength);
+          }
+
+          byteLength += Math.min(buffer.byteLength, remaining);
+        }
+      }
+
+      this.dataAvailable -= result.byteLength;
+      pending.resolve(result);
+    }
+  }
+
+  /**
+   * Reads exactly `length` bytes of binary data from the input stream, or, if
+   * length is not provided, reads the first chunk of data to become available.
+   * In the latter case, returns an empty array buffer on end of file.
+   *
+   * The read operation will not complete until enough data is available to
+   * fulfill the request. If the pipe closes without enough available data to
+   * fulfill the read, the operation fails, and any remaining buffered data is
+   * lost.
+   *
+   * @param {integer} [length]
+   *        The number of bytes to read.
+   * @returns {Promise<ArrayBuffer>}
+   *
+   * @throws {object}
+   *          May be rejected with an Error object, or an object with similar
+   *          properties. The object will include an `errorCode` property with
+   *          one of the following values if it was rejected for the
+   *          corresponding reason:
+   *
+   *            - Subprocess.ERROR_END_OF_FILE: The pipe was closed before
+   *              enough input could be read to satisfy the request.
+   */
+  read(length = null) {
+    if (length !== null && !(Number.isInteger(length) && length >= 0)) {
+      throw new RangeError("Length must be a non-negative integer");
+    }
+
+    if (length == 0) {
+      return Promise.resolve(new ArrayBuffer(0));
+    }
+
+    return new Promise((resolve, reject) => {
+      this.pendingReads.push({ length, resolve, reject });
+      this.checkPendingReads();
+    });
+  }
+
+  /**
+   * Reads exactly `length` bytes from the input stream, and parses them as
+   * UTF-8 JSON data.
+   *
+   * @param {integer} length
+   *        The number of bytes to read.
+   * @returns {Promise<object>}
+   *
+   * @throws {object}
+   *          May be rejected with an Error object, or an object with similar
+   *          properties. The object will include an `errorCode` property with
+   *          one of the following values if it was rejected for the
+   *          corresponding reason:
+   *
+   *            - Subprocess.ERROR_END_OF_FILE: The pipe was closed before
+   *              enough input could be read to satisfy the request.
+   *            - Subprocess.ERROR_INVALID_JSON: The data read from the pipe
+   *              could not be parsed as a valid JSON string.
+   */
+  readJSON(length) {
+    if (!Number.isInteger(length) || length <= 0) {
+      throw new RangeError("Length must be a positive integer");
+    }
+
+    return this.readString(length).then(string => {
+      try {
+        return JSON.parse(string);
+      } catch (e) {
+        e.errorCode = SubprocessConstants.ERROR_INVALID_JSON;
+        throw e;
+      }
+    });
+  }
+
+  /**
+   * Reads a chunk of UTF-8 data from the input stream, and converts it to a
+   * JavaScript string.
+   *
+   * If `length` is provided, reads exactly `length` bytes. Otherwise, reads the
+   * first chunk of data to become available, and returns an empty string on end
+   * of file. In the latter case, the chunk is decoded in streaming mode, and
+   * any incomplete UTF-8 sequences at the end of a chunk are returned at the
+   * start of a subsequent read operation.
+   *
+   * @param {integer} [length]
+   *        The number of bytes to read.
+   * @param {object} [options]
+   *        An options object as expected by TextDecoder.decode.
+   * @returns {Promise<string>}
+   *
+   * @throws {object}
+   *          May be rejected with an Error object, or an object with similar
+   *          properties. The object will include an `errorCode` property with
+   *          one of the following values if it was rejected for the
+   *          corresponding reason:
+   *
+   *            - Subprocess.ERROR_END_OF_FILE: The pipe was closed before
+   *              enough input could be read to satisfy the request.
+   */
+  readString(length = null, options = { stream: length === null }) {
+    if (length !== null && !(Number.isInteger(length) && length >= 0)) {
+      throw new RangeError("Length must be a non-negative integer");
+    }
+
+    return this.read(length).then(buffer => {
+      return this.decoder.decode(buffer, options);
+    });
+  }
+
+  /**
+   * Reads 4 bytes from the input stream, and parses them as an unsigned
+   * integer, in native byte order.
+   *
+   * @returns {Promise<integer>}
+   *
+   * @throws {object}
+   *          May be rejected with an Error object, or an object with similar
+   *          properties. The object will include an `errorCode` property with
+   *          one of the following values if it was rejected for the
+   *          corresponding reason:
+   *
+   *            - Subprocess.ERROR_END_OF_FILE: The pipe was closed before
+   *              enough input could be read to satisfy the request.
+   */
+  readUint32() {
+    return this.read(4).then(buffer => {
+      return new Uint32Array(buffer)[0];
+    });
+  }
+}
+
+/**
+ * @class Process
+ * @augments BaseProcess
+ */
+
+/**
+ * Represents a currently-running process, and allows interaction with it.
+ */
+export class BaseProcess {
+  /**
+   * @param {PromiseWorker} worker
+   *        The worker instance which owns the process.
+   * @param {integer} processId
+   *        The internal ID of the Process object, which ties it to the
+   *        corresponding process on the Worker side.
+   * @param {integer[]} fds
+   *        An array of internal Pipe IDs, one for each standard file descriptor
+   *        in the child process.
+   * @param {integer} pid
+   *        The operating system process ID of the process.
+   */
+  constructor(worker, processId, fds, pid) {
+    this.id = processId;
+    this.worker = worker;
+
+    /**
+     * @property {integer} pid
+     *           The process ID of the process, assigned by the operating system.
+     *           @readonly
+     */
+    this.pid = pid;
+
+    this.exitCode = null;
+
+    this.exitPromise = new Promise(resolve => {
+      this.worker.call("wait", [this.id]).then(({ exitCode }) => {
+        resolve(Object.freeze({ exitCode }));
+        this.exitCode = exitCode;
+      });
+    });
+
+    if (fds[0] !== undefined) {
+      /**
+       * @property {OutputPipe} stdin
+       *           A Pipe object which allows writing to the process's standard
+       *           input.
+       *           @readonly
+       */
+      this.stdin = new OutputPipe(this, 0, fds[0]);
+    }
+    if (fds[1] !== undefined) {
+      /**
+       * @property {InputPipe} stdout
+       *           A Pipe object which allows reading from the process's standard
+       *           output.
+       *           @readonly
+       */
+      this.stdout = new InputPipe(this, 1, fds[1]);
+    }
+    if (fds[2] !== undefined) {
+      /**
+       * @property {InputPipe} [stderr]
+       *           An optional Pipe object which allows reading from the
+       *           process's standard error output.
+       *           @readonly
+       */
+      this.stderr = new InputPipe(this, 2, fds[2]);
+    }
+  }
+
+  /**
+   * Spawns a process, and resolves to a BaseProcess instance on success.
+   *
+   * @param {object} options
+   *        An options object as passed to `Subprocess.call`.
+   *
+   * @returns {Promise<BaseProcess>}
+   */
+  static create(options) {
+    let worker = this.getWorker();
+
+    return worker.call("spawn", [options]).then(({ processId, fds, pid }) => {
+      return new this(worker, processId, fds, pid);
+    });
+  }
+
+  static get WORKER_URL() {
+    throw new Error("Not implemented");
+  }
+
+  static get WorkerClass() {
+    return PromiseWorker;
+  }
+
+  /**
+   * Gets the current subprocess worker, or spawns a new one if it does not
+   * currently exist.
+   *
+   * @returns {PromiseWorker}
+   */
+  static getWorker() {
+    if (!this._worker) {
+      this._worker = new this.WorkerClass(this.WORKER_URL);
+    }
+    return this._worker;
+  }
+
+  /**
+   * Kills the process.
+   *
+   * @param {integer} [timeout=300]
+   *        A timeout, in milliseconds, after which the process will be forcibly
+   *        killed. On platforms which support it, the process will be sent
+   *        a `SIGTERM` signal immediately, so that it has a chance to terminate
+   *        gracefully, and a `SIGKILL` signal if it hasn't exited within
+   *        `timeout` milliseconds. On other platforms (namely Windows), the
+   *        process will be forcibly terminated immediately.
+   *
+   * @returns {Promise<object>}
+   *          Resolves to an object with an `exitCode` property when the process
+   *          has exited.
+   */
+  kill(timeout = 300) {
+    // If the process has already exited, don't bother sending a signal.
+    if (this.exitCode != null) {
+      return this.wait();
+    }
+
+    let force = timeout <= 0;
+    this.worker.call("kill", [this.id, force]);
+
+    if (!force) {
+      lazy.setTimeout(() => {
+        if (this.exitCode == null) {
+          this.worker.call("kill", [this.id, true]);
+        }
+      }, timeout);
+    }
+
+    return this.wait();
+  }
+
+  /**
+   * Returns a promise which resolves to the process's exit code, once it has
+   * exited.
+   *
+   * @returns {Promise<object>}
+   * Resolves to an object with an `exitCode` property, containing the
+   * process's exit code, once the process has exited.
+   *
+   * On Unix-like systems, a negative exit code indicates that the
+   * process was killed by a signal whose signal number is the absolute
+   * value of the error code. On Windows, an exit code of -9 indicates
+   * that the process was killed via the {@linkcode BaseProcess#kill kill()}
+   * method.
+   */
+  wait() {
+    return this.exitPromise;
+  }
+}
diff --git a/toolkit/modules/subprocess/subprocess_shared.js b/toolkit/modules/subprocess/subprocess_shared.js
new file mode 100644
index 0000000000..d59a14a351
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_shared.js
@@ -0,0 +1,108 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+/* exported ArrayBuffer_transfer, Library, SubprocessConstants */
+
+// ctypes is either already available in the chrome worker scope, or defined
+// in scope via loadSubScript.
+/* global ctypes */
+
+/**
+ * Returns a new ArrayBuffer whose contents have been taken from the `buffer`'s
+ * data and then is either truncated or zero-extended by `size`. If `size` is
+ * undefined, the `byteLength` of the `buffer` is used. This operation leaves
+ * `buffer` in a detached state.
+ *
+ * @param {ArrayBuffer} buffer
+ * @param {integer} [size = buffer.byteLength]
+ * @returns {ArrayBuffer}
+ */
+var ArrayBuffer_transfer = function (buffer, size = buffer.byteLength) {
+  let u8out = new Uint8Array(size);
+  let u8buffer = new Uint8Array(buffer, 0, Math.min(size, buffer.byteLength));
+  u8out.set(u8buffer);
+  return u8out.buffer;
+};
+
+var libraries = {};
+
+var Library = class Library {
+  constructor(name, names, definitions) {
+    if (name in libraries) {
+      return libraries[name];
+    }
+
+    for (let name of names) {
+      try {
+        if (!this.library) {
+          this.library = ctypes.open(name);
+        }
+      } catch (e) {
+        // Ignore errors until we've tried all the options.
+      }
+    }
+    if (!this.library) {
+      throw new Error("Could not load libc");
+    }
+
+    libraries[name] = this;
+
+    for (let symbol of Object.keys(definitions)) {
+      this.declare(symbol, ...definitions[symbol]);
+    }
+  }
+
+  declare(name, ...args) {
+    Object.defineProperty(this, name, {
+      configurable: true,
+      get() {
+        Object.defineProperty(this, name, {
+          configurable: true,
+          value: this.library.declare(name, ...args),
+        });
+
+        return this[name];
+      },
+    });
+  }
+};
+
+/**
+ * Holds constants which apply to various Subprocess operations.
+ *
+ * @namespace
+ * @lends Subprocess
+ */
+var SubprocessConstants = {
+  /**
+   * @property {integer} ERROR_END_OF_FILE
+   *           The operation failed because the end of the file was reached.
+   *           @constant
+   */
+  ERROR_END_OF_FILE: 0xff7a0001,
+  /**
+   * @property {integer} ERROR_INVALID_JSON
+   *           The operation failed because an invalid JSON was encountered.
+   *           @constant
+   */
+  ERROR_INVALID_JSON: 0xff7a0002,
+  /**
+   * @property {integer} ERROR_BAD_EXECUTABLE
+   *           The operation failed because the given file did not exist, or
+   *           could not be executed.
+   *           @constant
+   */
+  ERROR_BAD_EXECUTABLE: 0xff7a0003,
+  /**
+   * @property {integer} ERROR_INVALID_OPTION
+   *           The operation failed because an invalid option was provided.
+   *           @constant
+   */
+  ERROR_INVALID_OPTION: 0xff7a0004,
+};
+
+Object.freeze(SubprocessConstants);
diff --git a/toolkit/modules/subprocess/subprocess_shared_unix.js b/toolkit/modules/subprocess/subprocess_shared_unix.js
new file mode 100644
index 0000000000..ad07dd7854
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_shared_unix.js
@@ -0,0 +1,123 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+if (typeof Components !== "undefined") {
+  /* global OS */
+  Cc["@mozilla.org/net/osfileconstantsservice;1"]
+    .getService(Ci.nsIOSFileConstantsService)
+    .init();
+}
+
+/* exported LIBC, libc */
+
+// ctypes is either already available in the chrome worker scope, or defined
+// in scope via loadSubScript.
+/* global ctypes */
+
+// This file is loaded into the same scope as subprocess_shared.js.
+/* import-globals-from subprocess_shared.js */
+
+var LIBC = OS.Constants.libc;
+
+const LIBC_CHOICES = ["libc.so", "libSystem.B.dylib", "a.out"];
+
+const unix = {
+  pid_t: ctypes.int32_t,
+
+  pollfd: new ctypes.StructType("pollfd", [
+    { fd: ctypes.int },
+    { events: ctypes.short },
+    { revents: ctypes.short },
+  ]),
+
+  WEXITSTATUS(status) {
+    return (status >> 8) & 0xff;
+  },
+
+  WTERMSIG(status) {
+    return status & 0x7f;
+  },
+};
+
+var libc = new Library("libc", LIBC_CHOICES, {
+  environ: [ctypes.char.ptr.ptr],
+
+  // Darwin-only.
+  _NSGetEnviron: [ctypes.default_abi, ctypes.char.ptr.ptr.ptr],
+
+  setenv: [
+    ctypes.default_abi,
+    ctypes.int,
+    ctypes.char.ptr,
+    ctypes.char.ptr,
+    ctypes.int,
+  ],
+
+  chdir: [ctypes.default_abi, ctypes.int, ctypes.char.ptr /* path */],
+
+  close: [ctypes.default_abi, ctypes.int, ctypes.int /* fildes */],
+
+  fcntl: [
+    ctypes.default_abi,
+    ctypes.int,
+    ctypes.int /* fildes */,
+    ctypes.int /* cmd */,
+    ctypes.int /* ... */,
+  ],
+
+  getcwd: [
+    ctypes.default_abi,
+    ctypes.char.ptr,
+    ctypes.char.ptr /* buf */,
+    ctypes.size_t /* size */,
+  ],
+
+  kill: [
+    ctypes.default_abi,
+    ctypes.int,
+    unix.pid_t /* pid */,
+    ctypes.int /* signal */,
+  ],
+
+  pipe: [ctypes.default_abi, ctypes.int, ctypes.int.array(2) /* pipefd */],
+
+  poll: [
+    ctypes.default_abi,
+    ctypes.int,
+    unix.pollfd.array() /* fds */,
+    ctypes.unsigned_int /* nfds */,
+    ctypes.int /* timeout */,
+  ],
+
+  read: [
+    ctypes.default_abi,
+    ctypes.ssize_t,
+    ctypes.int /* fildes */,
+    ctypes.char.ptr /* buf */,
+    ctypes.size_t /* nbyte */,
+  ],
+
+  waitpid: [
+    ctypes.default_abi,
+    unix.pid_t,
+    unix.pid_t /* pid */,
+    ctypes.int.ptr /* status */,
+    ctypes.int /* options */,
+  ],
+
+  write: [
+    ctypes.default_abi,
+    ctypes.ssize_t,
+    ctypes.int /* fildes */,
+    ctypes.char.ptr /* buf */,
+    ctypes.size_t /* nbyte */,
+  ],
+});
+
+unix.Fd = function (fd) {
+  return ctypes.CDataFinalizer(ctypes.int(fd), libc.close);
+};
diff --git a/toolkit/modules/subprocess/subprocess_shared_win.js b/toolkit/modules/subprocess/subprocess_shared_win.js
new file mode 100644
index 0000000000..2b9eb69c6d
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_shared_win.js
@@ -0,0 +1,538 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+if (typeof Components !== "undefined") {
+  /* global OS */
+  Cc["@mozilla.org/net/osfileconstantsservice;1"]
+    .getService(Ci.nsIOSFileConstantsService)
+    .init();
+}
+
+/* exported LIBC, Win, createPipe, libc, win32 */
+
+// ctypes is either already available in the chrome worker scope, or defined
+// in scope via loadSubScript.
+/* global ctypes */
+
+// This file is loaded into the same scope as subprocess_shared.js.
+/* import-globals-from subprocess_shared.js */
+
+const LIBC = OS.Constants.libc;
+
+const Win = OS.Constants.Win;
+
+const LIBC_CHOICES = ["kernel32.dll"];
+
+var win32 = {
+  // On Windows 64, winapi_abi is an alias for default_abi.
+  WINAPI: ctypes.winapi_abi,
+
+  VOID: ctypes.void_t,
+
+  BYTE: ctypes.uint8_t,
+  WORD: ctypes.uint16_t,
+  DWORD: ctypes.uint32_t,
+  LONG: ctypes.long,
+  LARGE_INTEGER: ctypes.int64_t,
+  ULONGLONG: ctypes.uint64_t,
+
+  UINT: ctypes.unsigned_int,
+  UCHAR: ctypes.unsigned_char,
+
+  BOOL: ctypes.bool,
+
+  HANDLE: ctypes.voidptr_t,
+  PVOID: ctypes.voidptr_t,
+  LPVOID: ctypes.voidptr_t,
+
+  CHAR: ctypes.char,
+  WCHAR: ctypes.jschar,
+
+  ULONG_PTR: ctypes.uintptr_t,
+
+  SIZE_T: ctypes.size_t,
+  PSIZE_T: ctypes.size_t.ptr,
+};
+
+Object.assign(win32, {
+  DWORD_PTR: win32.ULONG_PTR,
+
+  LPSTR: win32.CHAR.ptr,
+  LPWSTR: win32.WCHAR.ptr,
+
+  LPBYTE: win32.BYTE.ptr,
+  LPDWORD: win32.DWORD.ptr,
+  LPHANDLE: win32.HANDLE.ptr,
+
+  // This is an opaque type.
+  PROC_THREAD_ATTRIBUTE_LIST: ctypes.char.array(),
+  LPPROC_THREAD_ATTRIBUTE_LIST: ctypes.char.ptr,
+});
+
+Object.assign(win32, {
+  LPCSTR: win32.LPSTR,
+  LPCWSTR: win32.LPWSTR,
+  LPCVOID: win32.LPVOID,
+});
+
+Object.assign(win32, {
+  INVALID_HANDLE_VALUE: ctypes.cast(ctypes.int64_t(-1), win32.HANDLE),
+  NULL_HANDLE_VALUE: ctypes.cast(ctypes.uintptr_t(0), win32.HANDLE),
+
+  CREATE_SUSPENDED: 0x00000004,
+  CREATE_NEW_CONSOLE: 0x00000010,
+  CREATE_UNICODE_ENVIRONMENT: 0x00000400,
+  CREATE_NO_WINDOW: 0x08000000,
+  CREATE_BREAKAWAY_FROM_JOB: 0x01000000,
+  EXTENDED_STARTUPINFO_PRESENT: 0x00080000,
+
+  STARTF_USESTDHANDLES: 0x0100,
+
+  DUPLICATE_CLOSE_SOURCE: 0x01,
+  DUPLICATE_SAME_ACCESS: 0x02,
+
+  ERROR_HANDLE_EOF: 38,
+  ERROR_BROKEN_PIPE: 109,
+  ERROR_INSUFFICIENT_BUFFER: 122,
+
+  FILE_FLAG_OVERLAPPED: 0x40000000,
+
+  PIPE_TYPE_BYTE: 0x00,
+
+  PIPE_ACCESS_INBOUND: 0x01,
+  PIPE_ACCESS_OUTBOUND: 0x02,
+  PIPE_ACCESS_DUPLEX: 0x03,
+
+  PIPE_WAIT: 0x00,
+  PIPE_NOWAIT: 0x01,
+
+  STILL_ACTIVE: 259,
+
+  PROC_THREAD_ATTRIBUTE_HANDLE_LIST: 0x00020002,
+
+  JobObjectBasicLimitInformation: 2,
+  JobObjectExtendedLimitInformation: 9,
+
+  JOB_OBJECT_LIMIT_BREAKAWAY_OK: 0x00000800,
+
+  // These constants are 32-bit unsigned integers, but Windows defines
+  // them as negative integers cast to an unsigned type.
+  STD_INPUT_HANDLE: -10 + 0x100000000,
+  STD_OUTPUT_HANDLE: -11 + 0x100000000,
+  STD_ERROR_HANDLE: -12 + 0x100000000,
+
+  WAIT_TIMEOUT: 0x00000102,
+  WAIT_FAILED: 0xffffffff,
+});
+
+Object.assign(win32, {
+  JOBOBJECT_BASIC_LIMIT_INFORMATION: new ctypes.StructType(
+    "JOBOBJECT_BASIC_LIMIT_INFORMATION",
+    [
+      { PerProcessUserTimeLimit: win32.LARGE_INTEGER },
+      { PerJobUserTimeLimit: win32.LARGE_INTEGER },
+      { LimitFlags: win32.DWORD },
+      { MinimumWorkingSetSize: win32.SIZE_T },
+      { MaximumWorkingSetSize: win32.SIZE_T },
+      { ActiveProcessLimit: win32.DWORD },
+      { Affinity: win32.ULONG_PTR },
+      { PriorityClass: win32.DWORD },
+      { SchedulingClass: win32.DWORD },
+    ]
+  ),
+
+  IO_COUNTERS: new ctypes.StructType("IO_COUNTERS", [
+    { ReadOperationCount: win32.ULONGLONG },
+    { WriteOperationCount: win32.ULONGLONG },
+    { OtherOperationCount: win32.ULONGLONG },
+    { ReadTransferCount: win32.ULONGLONG },
+    { WriteTransferCount: win32.ULONGLONG },
+    { OtherTransferCount: win32.ULONGLONG },
+  ]),
+});
+
+Object.assign(win32, {
+  JOBOBJECT_EXTENDED_LIMIT_INFORMATION: new ctypes.StructType(
+    "JOBOBJECT_EXTENDED_LIMIT_INFORMATION",
+    [
+      { BasicLimitInformation: win32.JOBOBJECT_BASIC_LIMIT_INFORMATION },
+      { IoInfo: win32.IO_COUNTERS },
+      { ProcessMemoryLimit: win32.SIZE_T },
+      { JobMemoryLimit: win32.SIZE_T },
+      { PeakProcessMemoryUsed: win32.SIZE_T },
+      { PeakJobMemoryUsed: win32.SIZE_T },
+    ]
+  ),
+
+  OVERLAPPED: new ctypes.StructType("OVERLAPPED", [
+    { Internal: win32.ULONG_PTR },
+    { InternalHigh: win32.ULONG_PTR },
+    { Offset: win32.DWORD },
+    { OffsetHigh: win32.DWORD },
+    { hEvent: win32.HANDLE },
+  ]),
+
+  PROCESS_INFORMATION: new ctypes.StructType("PROCESS_INFORMATION", [
+    { hProcess: win32.HANDLE },
+    { hThread: win32.HANDLE },
+    { dwProcessId: win32.DWORD },
+    { dwThreadId: win32.DWORD },
+  ]),
+
+  SECURITY_ATTRIBUTES: new ctypes.StructType("SECURITY_ATTRIBUTES", [
+    { nLength: win32.DWORD },
+    { lpSecurityDescriptor: win32.LPVOID },
+    { bInheritHandle: win32.BOOL },
+  ]),
+
+  STARTUPINFOW: new ctypes.StructType("STARTUPINFOW", [
+    { cb: win32.DWORD },
+    { lpReserved: win32.LPWSTR },
+    { lpDesktop: win32.LPWSTR },
+    { lpTitle: win32.LPWSTR },
+    { dwX: win32.DWORD },
+    { dwY: win32.DWORD },
+    { dwXSize: win32.DWORD },
+    { dwYSize: win32.DWORD },
+    { dwXCountChars: win32.DWORD },
+    { dwYCountChars: win32.DWORD },
+    { dwFillAttribute: win32.DWORD },
+    { dwFlags: win32.DWORD },
+    { wShowWindow: win32.WORD },
+    { cbReserved2: win32.WORD },
+    { lpReserved2: win32.LPBYTE },
+    { hStdInput: win32.HANDLE },
+    { hStdOutput: win32.HANDLE },
+    { hStdError: win32.HANDLE },
+  ]),
+});
+
+Object.assign(win32, {
+  STARTUPINFOEXW: new ctypes.StructType("STARTUPINFOEXW", [
+    { StartupInfo: win32.STARTUPINFOW },
+    { lpAttributeList: win32.LPPROC_THREAD_ATTRIBUTE_LIST },
+  ]),
+});
+
+var libc = new Library("libc", LIBC_CHOICES, {
+  AssignProcessToJobObject: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hJob */,
+    win32.HANDLE /* hProcess */,
+  ],
+
+  CloseHandle: [win32.WINAPI, win32.BOOL, win32.HANDLE /* hObject */],
+
+  CreateEventW: [
+    win32.WINAPI,
+    win32.HANDLE,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpEventAttributes */,
+    win32.BOOL /* bManualReset */,
+    win32.BOOL /* bInitialState */,
+    win32.LPWSTR /* lpName */,
+  ],
+
+  CreateFileW: [
+    win32.WINAPI,
+    win32.HANDLE,
+    win32.LPWSTR /* lpFileName */,
+    win32.DWORD /* dwDesiredAccess */,
+    win32.DWORD /* dwShareMode */,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpSecurityAttributes */,
+    win32.DWORD /* dwCreationDisposition */,
+    win32.DWORD /* dwFlagsAndAttributes */,
+    win32.HANDLE /* opt hTemplateFile */,
+  ],
+
+  CreateJobObjectW: [
+    win32.WINAPI,
+    win32.HANDLE,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpJobAttributes */,
+    win32.LPWSTR /* lpName */,
+  ],
+
+  CreateNamedPipeW: [
+    win32.WINAPI,
+    win32.HANDLE,
+    win32.LPWSTR /* lpName */,
+    win32.DWORD /* dwOpenMode */,
+    win32.DWORD /* dwPipeMode */,
+    win32.DWORD /* nMaxInstances */,
+    win32.DWORD /* nOutBufferSize */,
+    win32.DWORD /* nInBufferSize */,
+    win32.DWORD /* nDefaultTimeOut */,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpSecurityAttributes */,
+  ],
+
+  CreatePipe: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.LPHANDLE /* out hReadPipe */,
+    win32.LPHANDLE /* out hWritePipe */,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpPipeAttributes */,
+    win32.DWORD /* nSize */,
+  ],
+
+  CreateProcessW: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.LPCWSTR /* lpApplicationName */,
+    win32.LPWSTR /* lpCommandLine */,
+    win32.SECURITY_ATTRIBUTES.ptr /* lpProcessAttributes */,
+    win32.SECURITY_ATTRIBUTES.ptr /* lpThreadAttributes */,
+    win32.BOOL /* bInheritHandle */,
+    win32.DWORD /* dwCreationFlags */,
+    win32.LPVOID /* opt lpEnvironment */,
+    win32.LPCWSTR /* opt lpCurrentDirectory */,
+    win32.STARTUPINFOW.ptr /* lpStartupInfo */,
+    win32.PROCESS_INFORMATION.ptr /* out lpProcessInformation */,
+  ],
+
+  CreateSemaphoreW: [
+    win32.WINAPI,
+    win32.HANDLE,
+    win32.SECURITY_ATTRIBUTES.ptr /* opt lpSemaphoreAttributes */,
+    win32.LONG /* lInitialCount */,
+    win32.LONG /* lMaximumCount */,
+    win32.LPCWSTR /* opt lpName */,
+  ],
+
+  DeleteProcThreadAttributeList: [
+    win32.WINAPI,
+    win32.VOID,
+    win32.LPPROC_THREAD_ATTRIBUTE_LIST /* in/out lpAttributeList */,
+  ],
+
+  DuplicateHandle: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hSourceProcessHandle */,
+    win32.HANDLE /* hSourceHandle */,
+    win32.HANDLE /* hTargetProcessHandle */,
+    win32.LPHANDLE /* out lpTargetHandle */,
+    win32.DWORD /* dwDesiredAccess */,
+    win32.BOOL /* bInheritHandle */,
+    win32.DWORD /* dwOptions */,
+  ],
+
+  FreeEnvironmentStringsW: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.LPCWSTR /* lpszEnvironmentBlock */,
+  ],
+
+  GetCurrentProcess: [win32.WINAPI, win32.HANDLE],
+
+  GetCurrentProcessId: [win32.WINAPI, win32.DWORD],
+
+  GetEnvironmentStringsW: [win32.WINAPI, win32.LPCWSTR],
+
+  GetExitCodeProcess: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hProcess */,
+    win32.LPDWORD /* lpExitCode */,
+  ],
+
+  GetOverlappedResult: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hFile */,
+    win32.OVERLAPPED.ptr /* lpOverlapped */,
+    win32.LPDWORD /* lpNumberOfBytesTransferred */,
+    win32.BOOL /* bWait */,
+  ],
+
+  GetStdHandle: [win32.WINAPI, win32.HANDLE, win32.DWORD /* nStdHandle */],
+
+  InitializeProcThreadAttributeList: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.LPPROC_THREAD_ATTRIBUTE_LIST /* out opt lpAttributeList */,
+    win32.DWORD /* dwAttributeCount */,
+    win32.DWORD /* dwFlags */,
+    win32.PSIZE_T /* in/out lpSize */,
+  ],
+
+  ReadFile: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hFile */,
+    win32.LPVOID /* out lpBuffer */,
+    win32.DWORD /* nNumberOfBytesToRead */,
+    win32.LPDWORD /* opt out lpNumberOfBytesRead */,
+    win32.OVERLAPPED.ptr /* opt in/out lpOverlapped */,
+  ],
+
+  ReleaseSemaphore: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hSemaphore */,
+    win32.LONG /* lReleaseCount */,
+    win32.LONG.ptr /* opt out lpPreviousCount */,
+  ],
+
+  ResumeThread: [win32.WINAPI, win32.DWORD, win32.HANDLE /* hThread */],
+
+  SetInformationJobObject: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hJob */,
+    ctypes.int /* JobObjectInfoClass */,
+    win32.LPVOID /* lpJobObjectInfo */,
+    win32.DWORD /* cbJobObjectInfoLengt */,
+  ],
+
+  TerminateJobObject: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hJob */,
+    win32.UINT /* uExitCode */,
+  ],
+
+  TerminateProcess: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hProcess */,
+    win32.UINT /* uExitCode */,
+  ],
+
+  UpdateProcThreadAttribute: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.LPPROC_THREAD_ATTRIBUTE_LIST /* in/out lpAttributeList */,
+    win32.DWORD /* dwFlags */,
+    win32.DWORD_PTR /* Attribute */,
+    win32.PVOID /* lpValue */,
+    win32.SIZE_T /* cbSize */,
+    win32.PVOID /* out opt lpPreviousValue */,
+    win32.PSIZE_T /* opt lpReturnSize */,
+  ],
+
+  WaitForMultipleObjects: [
+    win32.WINAPI,
+    win32.DWORD,
+    win32.DWORD /* nCount */,
+    win32.HANDLE.ptr /* hHandles */,
+    win32.BOOL /* bWaitAll */,
+    win32.DWORD /* dwMilliseconds */,
+  ],
+
+  WaitForSingleObject: [
+    win32.WINAPI,
+    win32.DWORD,
+    win32.HANDLE /* hHandle */,
+    win32.BOOL /* bWaitAll */,
+    win32.DWORD /* dwMilliseconds */,
+  ],
+
+  WriteFile: [
+    win32.WINAPI,
+    win32.BOOL,
+    win32.HANDLE /* hFile */,
+    win32.LPCVOID /* lpBuffer */,
+    win32.DWORD /* nNumberOfBytesToRead */,
+    win32.LPDWORD /* opt out lpNumberOfBytesWritten */,
+    win32.OVERLAPPED.ptr /* opt in/out lpOverlapped */,
+  ],
+});
+
+let nextNamedPipeId = 0;
+
+win32.Handle = function (handle) {
+  return ctypes.CDataFinalizer(win32.HANDLE(handle), libc.CloseHandle);
+};
+
+win32.createPipe = function (secAttr, readFlags = 0, writeFlags = 0, size = 0) {
+  readFlags |= win32.PIPE_ACCESS_INBOUND;
+  writeFlags |= Win.FILE_ATTRIBUTE_NORMAL;
+
+  if (size == 0) {
+    size = 4096;
+  }
+
+  let pid = libc.GetCurrentProcessId();
+  let pipeName = String.raw`\\.\Pipe\SubProcessPipe.${pid}.${nextNamedPipeId++}`;
+
+  let readHandle = libc.CreateNamedPipeW(
+    pipeName,
+    readFlags,
+    win32.PIPE_TYPE_BYTE | win32.PIPE_WAIT,
+    1 /* number of connections */,
+    size /* output buffer size */,
+    size /* input buffer size */,
+    0 /* timeout */,
+    secAttr.address()
+  );
+
+  let isInvalid = handle =>
+    String(handle) == String(win32.HANDLE(Win.INVALID_HANDLE_VALUE));
+
+  if (isInvalid(readHandle)) {
+    return [];
+  }
+
+  let writeHandle = libc.CreateFileW(
+    pipeName,
+    Win.GENERIC_WRITE,
+    0,
+    secAttr.address(),
+    Win.OPEN_EXISTING,
+    writeFlags,
+    null
+  );
+
+  if (isInvalid(writeHandle)) {
+    libc.CloseHandle(readHandle);
+    return [];
+  }
+
+  return [win32.Handle(readHandle), win32.Handle(writeHandle)];
+};
+
+win32.createThreadAttributeList = function (handles) {
+  try {
+    void libc.InitializeProcThreadAttributeList;
+    void libc.DeleteProcThreadAttributeList;
+    void libc.UpdateProcThreadAttribute;
+  } catch (e) {
+    // This is only supported in Windows Vista and later.
+    return null;
+  }
+
+  let size = win32.SIZE_T();
+  if (
+    !libc.InitializeProcThreadAttributeList(null, 1, 0, size.address()) &&
+    ctypes.winLastError != win32.ERROR_INSUFFICIENT_BUFFER
+  ) {
+    return null;
+  }
+
+  let attrList = win32.PROC_THREAD_ATTRIBUTE_LIST(size.value);
+
+  if (!libc.InitializeProcThreadAttributeList(attrList, 1, 0, size.address())) {
+    return null;
+  }
+
+  let ok = libc.UpdateProcThreadAttribute(
+    attrList,
+    0,
+    win32.PROC_THREAD_ATTRIBUTE_HANDLE_LIST,
+    handles,
+    handles.constructor.size,
+    null,
+    null
+  );
+
+  if (!ok) {
+    libc.DeleteProcThreadAttributeList(attrList);
+    return null;
+  }
+
+  return attrList;
+};
diff --git a/toolkit/modules/subprocess/subprocess_unix.sys.mjs b/toolkit/modules/subprocess/subprocess_unix.sys.mjs
new file mode 100644
index 0000000000..8f5349b94b
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_unix.sys.mjs
@@ -0,0 +1,203 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import {
+  BaseProcess,
+  PromiseWorker,
+} from "resource://gre/modules/subprocess/subprocess_common.sys.mjs";
+
+import { ctypes } from "resource://gre/modules/ctypes.sys.mjs";
+
+var obj = { ctypes };
+Services.scriptloader.loadSubScript(
+  "resource://gre/modules/subprocess/subprocess_shared.js",
+  obj
+);
+Services.scriptloader.loadSubScript(
+  "resource://gre/modules/subprocess/subprocess_shared_unix.js",
+  obj
+);
+
+const { SubprocessConstants, LIBC } = obj;
+
+// libc is exported for tests.
+export var libc = obj.libc;
+
+class UnixPromiseWorker extends PromiseWorker {
+  constructor(...args) {
+    super(...args);
+
+    let fds = ctypes.int.array(2)();
+    let res = libc.pipe(fds);
+    if (res == -1) {
+      throw new Error("Unable to create pipe");
+    }
+
+    this.signalFd = fds[1];
+
+    libc.fcntl(fds[0], LIBC.F_SETFL, LIBC.O_NONBLOCK);
+    libc.fcntl(fds[0], LIBC.F_SETFD, LIBC.FD_CLOEXEC);
+    libc.fcntl(fds[1], LIBC.F_SETFD, LIBC.FD_CLOEXEC);
+
+    this.call("init", [{ signalFd: fds[0] }]);
+  }
+
+  closePipe() {
+    if (this.signalFd) {
+      libc.close(this.signalFd);
+      this.signalFd = null;
+    }
+  }
+
+  onClose() {
+    this.closePipe();
+    super.onClose();
+  }
+
+  signalWorker() {
+    libc.write(this.signalFd, new ArrayBuffer(1), 1);
+  }
+
+  postMessage(...args) {
+    this.signalWorker();
+    return super.postMessage(...args);
+  }
+}
+
+class Process extends BaseProcess {
+  static get WORKER_URL() {
+    return "resource://gre/modules/subprocess/subprocess_worker_unix.js";
+  }
+
+  static get WorkerClass() {
+    return UnixPromiseWorker;
+  }
+}
+
+// Convert a null-terminated char pointer into a sized char array, and then
+// convert that into a JS typed array.
+// The resulting array will not be null-terminated.
+function ptrToUint8Array(input) {
+  let { cast, uint8_t } = ctypes;
+
+  let len = 0;
+  for (
+    let ptr = cast(input, uint8_t.ptr);
+    ptr.contents;
+    ptr = ptr.increment()
+  ) {
+    len++;
+  }
+
+  let aryPtr = cast(input, uint8_t.array(len).ptr);
+  return new Uint8Array(aryPtr.contents);
+}
+
+var SubprocessUnix = {
+  Process,
+
+  call(options) {
+    return Process.create(options);
+  },
+
+  *getEnvironment() {
+    let environ;
+    if (Services.appinfo.OS === "Darwin") {
+      environ = libc._NSGetEnviron().contents;
+    } else {
+      environ = libc.environ;
+    }
+
+    const EQUAL = "=".charCodeAt(0);
+    let decoder = new TextDecoder("utf-8", { fatal: true });
+
+    function decode(array) {
+      try {
+        return decoder.decode(array);
+      } catch (e) {
+        return array;
+      }
+    }
+
+    for (
+      let envp = environ;
+      !envp.isNull() && !envp.contents.isNull();
+      envp = envp.increment()
+    ) {
+      let buf = ptrToUint8Array(envp.contents);
+
+      for (let i = 0; i < buf.length; i++) {
+        if (buf[i] == EQUAL) {
+          yield [decode(buf.subarray(0, i)), decode(buf.subarray(i + 1))];
+          break;
+        }
+      }
+    }
+  },
+
+  isExecutableFile: async function isExecutable(path) {
+    if (!PathUtils.isAbsolute(path)) {
+      return false;
+    }
+
+    try {
+      let info = await IOUtils.stat(path);
+
+      // FIXME: We really want access(path, X_OK) here, but IOUtils does not
+      // support it.
+      return info.type !== "directory" && info.permissions & 0o111;
+    } catch (e) {
+      return false;
+    }
+  },
+
+  /**
+   * Searches for the given executable file in the system executable
+   * file paths as specified by the PATH environment variable.
+   *
+   * On Windows, if the unadorned filename cannot be found, the
+   * extensions in the semicolon-separated list in the PATHEXT
+   * environment variable are successively appended to the original
+   * name and searched for in turn.
+   *
+   * @param {string} bin
+   *        The name of the executable to find.
+   * @param {object} environment
+   *        An object containing a key for each environment variable to be used
+   *        in the search.
+   * @returns {Promise<string>}
+   */
+  async pathSearch(bin, environment) {
+    if (PathUtils.isAbsolute(bin)) {
+      if (await this.isExecutableFile(bin)) {
+        return bin;
+      }
+      let error = new Error(
+        `File at path "${bin}" does not exist, or is not executable`
+      );
+      error.errorCode = SubprocessConstants.ERROR_BAD_EXECUTABLE;
+      throw error;
+    }
+
+    let dirs = [];
+    if (typeof environment.PATH === "string") {
+      dirs = environment.PATH.split(":");
+    }
+
+    for (let dir of dirs) {
+      let path = PathUtils.join(dir, bin);
+
+      if (await this.isExecutableFile(path)) {
+        return path;
+      }
+    }
+    let error = new Error(`Executable not found: ${bin}`);
+    error.errorCode = SubprocessConstants.ERROR_BAD_EXECUTABLE;
+    throw error;
+  },
+};
+
+export var SubprocessImpl = SubprocessUnix;
diff --git a/toolkit/modules/subprocess/subprocess_win.sys.mjs b/toolkit/modules/subprocess/subprocess_win.sys.mjs
new file mode 100644
index 0000000000..5150f4a4b1
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_win.sys.mjs
@@ -0,0 +1,178 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+import {
+  BaseProcess,
+  PromiseWorker,
+} from "resource://gre/modules/subprocess/subprocess_common.sys.mjs";
+
+import { ctypes } from "resource://gre/modules/ctypes.sys.mjs";
+
+var obj = { ctypes };
+Services.scriptloader.loadSubScript(
+  "resource://gre/modules/subprocess/subprocess_shared.js",
+  obj
+);
+Services.scriptloader.loadSubScript(
+  "resource://gre/modules/subprocess/subprocess_shared_win.js",
+  obj
+);
+
+const { SubprocessConstants } = obj;
+
+// libc and win32 are exported for tests.
+export const libc = obj.libc;
+export const win32 = obj.win32;
+
+class WinPromiseWorker extends PromiseWorker {
+  constructor(...args) {
+    super(...args);
+
+    this.signalEvent = libc.CreateSemaphoreW(null, 0, 32, null);
+
+    this.call("init", [
+      {
+        breakAwayFromJob: !AppConstants.isPlatformAndVersionAtLeast(
+          "win",
+          "6.2"
+        ),
+        comspec: Services.env.get("COMSPEC"),
+        signalEvent: String(
+          ctypes.cast(this.signalEvent, ctypes.uintptr_t).value
+        ),
+      },
+    ]);
+  }
+
+  signalWorker() {
+    libc.ReleaseSemaphore(this.signalEvent, 1, null);
+  }
+
+  postMessage(...args) {
+    this.signalWorker();
+    return super.postMessage(...args);
+  }
+}
+
+class Process extends BaseProcess {
+  static get WORKER_URL() {
+    return "resource://gre/modules/subprocess/subprocess_worker_win.js";
+  }
+
+  static get WorkerClass() {
+    return WinPromiseWorker;
+  }
+}
+
+var SubprocessWin = {
+  Process,
+
+  call(options) {
+    return Process.create(options);
+  },
+
+  *getEnvironment() {
+    let env = libc.GetEnvironmentStringsW();
+    try {
+      for (let p = env, q = env; ; p = p.increment()) {
+        if (p.contents == "\0") {
+          if (String(p) == String(q)) {
+            break;
+          }
+
+          let str = q.readString();
+          q = p.increment();
+
+          let idx = str.indexOf("=");
+          if (idx == 0) {
+            idx = str.indexOf("=", 1);
+          }
+
+          if (idx >= 0) {
+            yield [str.slice(0, idx), str.slice(idx + 1)];
+          }
+        }
+      }
+    } finally {
+      libc.FreeEnvironmentStringsW(env);
+    }
+  },
+
+  async isExecutableFile(path) {
+    if (!PathUtils.isAbsolute(path)) {
+      return false;
+    }
+
+    try {
+      let info = await IOUtils.stat(path);
+      // On Windows, a FileType of "other" indicates it is a reparse point
+      // (i.e., a link).
+      return info.type !== "directory" && info.type !== "other";
+    } catch (e) {
+      return false;
+    }
+  },
+
+  /**
+   * Searches for the given executable file in the system executable
+   * file paths as specified by the PATH environment variable.
+   *
+   * On Windows, if the unadorned filename cannot be found, the
+   * extensions in the semicolon-separated list in the PATHEXT
+   * environment variable are successively appended to the original
+   * name and searched for in turn.
+   *
+   * @param {string} bin
+   *        The name of the executable to find.
+   * @param {object} environment
+   *        An object containing a key for each environment variable to be used
+   *        in the search.
+   * @returns {Promise<string>}
+   */
+  async pathSearch(bin, environment) {
+    if (PathUtils.isAbsolute(bin)) {
+      if (await this.isExecutableFile(bin)) {
+        return bin;
+      }
+      let error = new Error(
+        `File at path "${bin}" does not exist, or is not a normal file`
+      );
+      error.errorCode = SubprocessConstants.ERROR_BAD_EXECUTABLE;
+      throw error;
+    }
+
+    let dirs = [];
+    let exts = [];
+    if (environment.PATH) {
+      dirs = environment.PATH.split(";");
+    }
+    if (environment.PATHEXT) {
+      exts = environment.PATHEXT.split(";");
+    }
+
+    for (let dir of dirs) {
+      let path = PathUtils.join(dir, bin);
+
+      if (await this.isExecutableFile(path)) {
+        return path;
+      }
+
+      for (let ext of exts) {
+        let file = path + ext;
+
+        if (await this.isExecutableFile(file)) {
+          return file;
+        }
+      }
+    }
+    let error = new Error(`Executable not found: ${bin}`);
+    error.errorCode = SubprocessConstants.ERROR_BAD_EXECUTABLE;
+    throw error;
+  },
+};
+
+export var SubprocessImpl = SubprocessWin;
diff --git a/toolkit/modules/subprocess/subprocess_worker_common.js b/toolkit/modules/subprocess/subprocess_worker_common.js
new file mode 100644
index 0000000000..a0370326d7
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_worker_common.js
@@ -0,0 +1,211 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+// This file is loaded into the same context as subprocess_worker_unix.js
+// and subprocess_worker_win.js
+/* import-globals-from subprocess_worker_unix.js */
+
+/* exported BasePipe, BaseProcess, debug */
+
+function debug(message) {
+  self.postMessage({ msg: "debug", message });
+}
+
+class BasePipe {
+  constructor() {
+    this.closing = false;
+    this.closed = false;
+
+    this.closedPromise = new Promise(resolve => {
+      this.resolveClosed = resolve;
+    });
+
+    this.pending = [];
+  }
+
+  shiftPending() {
+    let result = this.pending.shift();
+
+    if (this.closing && !this.pending.length) {
+      this.close();
+    }
+
+    return result;
+  }
+}
+
+let nextProcessId = 0;
+
+class BaseProcess {
+  constructor(options) {
+    this.id = nextProcessId++;
+
+    this.exitCode = null;
+
+    this.exitPromise = new Promise(resolve => {
+      this.resolveExit = resolve;
+    });
+    this.exitPromise.then(() => {
+      // The input file descriptors will be closed after poll
+      // reports that their input buffers are empty. If we close
+      // them now, we may lose output.
+      this.pipes[0].close(true);
+    });
+
+    this.pid = null;
+    this.pipes = [];
+
+    this.spawn(options);
+  }
+
+  /**
+   * Waits for the process to exit and all of its pending IO operations to
+   * complete.
+   *
+   * @returns {Promise<void>}
+   */
+  awaitFinished() {
+    return Promise.all([
+      this.exitPromise,
+      ...this.pipes.map(pipe => pipe.closedPromise),
+    ]);
+  }
+}
+
+let requests = {
+  init(details) {
+    io.init(details);
+
+    return { data: {} };
+  },
+
+  shutdown() {
+    io.shutdown();
+
+    return { data: {} };
+  },
+
+  close(pipeId, force = false) {
+    let pipe = io.getPipe(pipeId);
+
+    return pipe.close(force).then(() => ({ data: {} }));
+  },
+
+  spawn(options) {
+    let process = new Process(options);
+    let processId = process.id;
+
+    io.addProcess(process);
+
+    let fds = process.pipes.map(pipe => pipe.id);
+
+    return { data: { processId, fds, pid: process.pid } };
+  },
+
+  kill(processId, force = false) {
+    let process = io.getProcess(processId);
+
+    process.kill(force ? 9 : 15);
+
+    return { data: {} };
+  },
+
+  wait(processId) {
+    let process = io.getProcess(processId);
+
+    process.wait();
+
+    process.awaitFinished().then(() => {
+      io.cleanupProcess(process);
+    });
+
+    return process.exitPromise.then(exitCode => {
+      return { data: { exitCode } };
+    });
+  },
+
+  read(pipeId, count) {
+    let pipe = io.getPipe(pipeId);
+
+    return pipe.read(count).then(buffer => {
+      return { data: { buffer } };
+    });
+  },
+
+  write(pipeId, buffer) {
+    let pipe = io.getPipe(pipeId);
+
+    return pipe.write(buffer).then(bytesWritten => {
+      return { data: { bytesWritten } };
+    });
+  },
+
+  getOpenFiles() {
+    return { data: new Set(io.pipes.keys()) };
+  },
+
+  getProcesses() {
+    let data = new Map(
+      Array.from(io.processes.values())
+        .filter(proc => proc.exitCode == null)
+        .map(proc => [proc.id, proc.pid])
+    );
+    return { data };
+  },
+
+  waitForNoProcesses() {
+    return Promise.all(
+      Array.from(io.processes.values(), proc => proc.awaitFinished())
+    );
+  },
+};
+
+onmessage = event => {
+  io.messageCount--;
+
+  let { msg, msgId, args } = event.data;
+
+  new Promise(resolve => {
+    resolve(requests[msg](...args));
+  })
+    .then(result => {
+      let response = {
+        msg: "success",
+        msgId,
+        data: result.data,
+      };
+
+      self.postMessage(response, result.transfer || []);
+    })
+    .catch(error => {
+      if (error instanceof Error) {
+        error = {
+          message: error.message,
+          fileName: error.fileName,
+          lineNumber: error.lineNumber,
+          column: error.column,
+          stack: error.stack,
+          errorCode: error.errorCode,
+        };
+      }
+
+      self.postMessage({
+        msg: "failure",
+        msgId,
+        error,
+      });
+    })
+    .catch(error => {
+      console.error(error);
+
+      self.postMessage({
+        msg: "failure",
+        msgId,
+        error: {},
+      });
+    });
+};
diff --git a/toolkit/modules/subprocess/subprocess_worker_unix.js b/toolkit/modules/subprocess/subprocess_worker_unix.js
new file mode 100644
index 0000000000..74df22f349
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_worker_unix.js
@@ -0,0 +1,612 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+/* eslint-env mozilla/chrome-worker */
+/* exported Process */
+
+/* import-globals-from subprocess_shared.js */
+/* import-globals-from subprocess_shared_unix.js */
+/* import-globals-from subprocess_worker_common.js */
+importScripts(
+  "resource://gre/modules/subprocess/subprocess_shared.js",
+  "resource://gre/modules/subprocess/subprocess_shared_unix.js",
+  "resource://gre/modules/subprocess/subprocess_worker_common.js"
+);
+
+const POLL_TIMEOUT = 5000;
+
+let io;
+
+let nextPipeId = 0;
+
+class Pipe extends BasePipe {
+  constructor(process, fd) {
+    super();
+
+    this.process = process;
+    this.fd = fd;
+    this.id = nextPipeId++;
+  }
+
+  get pollEvents() {
+    throw new Error("Not implemented");
+  }
+
+  /**
+   * Closes the file descriptor.
+   *
+   * @param {boolean} [force=false]
+   *        If true, the file descriptor is closed immediately. If false, the
+   *        file descriptor is closed after all current pending IO operations
+   *        have completed.
+   *
+   * @returns {Promise<void>}
+   *          Resolves when the file descriptor has been closed.
+   */
+  close(force = false) {
+    if (!force && this.pending.length) {
+      this.closing = true;
+      return this.closedPromise;
+    }
+
+    for (let { reject } of this.pending) {
+      let error = new Error("File closed");
+      error.errorCode = SubprocessConstants.ERROR_END_OF_FILE;
+      reject(error);
+    }
+    this.pending.length = 0;
+
+    if (!this.closed) {
+      this.fd.dispose();
+
+      this.closed = true;
+      this.resolveClosed();
+
+      io.pipes.delete(this.id);
+      io.updatePollFds();
+    }
+    return this.closedPromise;
+  }
+
+  /**
+   * Called when an error occurred while polling our file descriptor.
+   */
+  onError() {
+    this.close(true);
+    this.process.wait();
+  }
+}
+
+class InputPipe extends Pipe {
+  /**
+   * A bit mask of poll() events which we currently wish to be notified of on
+   * this file descriptor.
+   */
+  get pollEvents() {
+    if (this.pending.length) {
+      return LIBC.POLLIN;
+    }
+    return 0;
+  }
+
+  /**
+   * Asynchronously reads at most `length` bytes of binary data from the file
+   * descriptor into an ArrayBuffer of the same size. Returns a promise which
+   * resolves when the operation is complete.
+   *
+   * @param {integer} length
+   *        The number of bytes to read.
+   *
+   * @returns {Promise<ArrayBuffer>}
+   */
+  read(length) {
+    if (this.closing || this.closed) {
+      throw new Error("Attempt to read from closed pipe");
+    }
+
+    return new Promise((resolve, reject) => {
+      this.pending.push({ resolve, reject, length });
+      io.updatePollFds();
+    });
+  }
+
+  /**
+   * Synchronously reads at most `count` bytes of binary data into an
+   * ArrayBuffer, and returns that buffer. If no data can be read without
+   * blocking, returns null instead.
+   *
+   * @param {integer} count
+   *        The number of bytes to read.
+   *
+   * @returns {ArrayBuffer|null}
+   */
+  readBuffer(count) {
+    let buffer = new ArrayBuffer(count);
+
+    let read = +libc.read(this.fd, buffer, buffer.byteLength);
+    if (read < 0 && ctypes.errno != LIBC.EAGAIN) {
+      this.onError();
+    }
+
+    if (read <= 0) {
+      return null;
+    }
+
+    if (read < buffer.byteLength) {
+      return ArrayBuffer_transfer(buffer, read);
+    }
+
+    return buffer;
+  }
+
+  /**
+   * Called when one of the IO operations matching the `pollEvents` mask may be
+   * performed without blocking.
+   *
+   * @returns {boolean}
+   *        True if any data was successfully read.
+   */
+  onReady() {
+    let result = false;
+    let reads = this.pending;
+    while (reads.length) {
+      let { resolve, length } = reads[0];
+
+      let buffer = this.readBuffer(length);
+      if (buffer) {
+        result = true;
+        this.shiftPending();
+        resolve(buffer);
+      } else {
+        break;
+      }
+    }
+
+    if (!reads.length) {
+      io.updatePollFds();
+    }
+    return result;
+  }
+}
+
+class OutputPipe extends Pipe {
+  /**
+   * A bit mask of poll() events which we currently wish to be notified of on
+   * this file discriptor.
+   */
+  get pollEvents() {
+    if (this.pending.length) {
+      return LIBC.POLLOUT;
+    }
+    return 0;
+  }
+
+  /**
+   * Asynchronously writes the given buffer to our file descriptor, and returns
+   * a promise which resolves when the operation is complete.
+   *
+   * @param {ArrayBuffer} buffer
+   *        The buffer to write.
+   *
+   * @returns {Promise<integer>}
+   *          Resolves to the number of bytes written when the operation is
+   *          complete.
+   */
+  write(buffer) {
+    if (this.closing || this.closed) {
+      throw new Error("Attempt to write to closed pipe");
+    }
+
+    return new Promise((resolve, reject) => {
+      this.pending.push({ resolve, reject, buffer, length: buffer.byteLength });
+      io.updatePollFds();
+    });
+  }
+
+  /**
+   * Attempts to synchronously write the given buffer to our file descriptor.
+   * Writes only as many bytes as can be written without blocking, and returns
+   * the number of byes successfully written.
+   *
+   * Closes the file descriptor if an IO error occurs.
+   *
+   * @param {ArrayBuffer} buffer
+   *        The buffer to write.
+   *
+   * @returns {integer}
+   *          The number of bytes successfully written.
+   */
+  writeBuffer(buffer) {
+    let bytesWritten = libc.write(this.fd, buffer, buffer.byteLength);
+
+    if (bytesWritten < 0 && ctypes.errno != LIBC.EAGAIN) {
+      this.onError();
+    }
+
+    return bytesWritten;
+  }
+
+  /**
+   * Called when one of the IO operations matching the `pollEvents` mask may be
+   * performed without blocking.
+   */
+  onReady() {
+    let writes = this.pending;
+    while (writes.length) {
+      let { buffer, resolve, length } = writes[0];
+
+      let written = this.writeBuffer(buffer);
+
+      if (written == buffer.byteLength) {
+        resolve(length);
+        this.shiftPending();
+      } else if (written > 0) {
+        writes[0].buffer = buffer.slice(written);
+      } else {
+        break;
+      }
+    }
+
+    if (!writes.length) {
+      io.updatePollFds();
+    }
+  }
+}
+
+class Signal {
+  constructor(fd) {
+    this.fd = fd;
+  }
+
+  cleanup() {
+    libc.close(this.fd);
+    this.fd = null;
+  }
+
+  get pollEvents() {
+    return LIBC.POLLIN;
+  }
+
+  /**
+   * Called when an error occurred while polling our file descriptor.
+   */
+  onError() {
+    io.shutdown();
+  }
+
+  /**
+   * Called when one of the IO operations matching the `pollEvents` mask may be
+   * performed without blocking.
+   */
+  onReady() {
+    let buffer = new ArrayBuffer(16);
+    let count = +libc.read(this.fd, buffer, buffer.byteLength);
+    if (count > 0) {
+      io.messageCount += count;
+    }
+  }
+}
+
+class Process extends BaseProcess {
+  /**
+   * Each Process object opens an additional pipe from the target object, which
+   * will be automatically closed when the process exits, but otherwise
+   * carries no data.
+   *
+   * This property contains a bit mask of poll() events which we wish to be
+   * notified of on this descriptor. We're not expecting any input from this
+   * pipe, but we need to poll for input until the process exits in order to be
+   * notified when the pipe closes.
+   */
+  get pollEvents() {
+    if (this.exitCode === null) {
+      return LIBC.POLLIN;
+    }
+    return 0;
+  }
+
+  /**
+   * Kills the process with the given signal.
+   *
+   * @param {integer} signal
+   */
+  kill(signal) {
+    libc.kill(this.pid, signal);
+    this.wait();
+  }
+
+  /**
+   * Initializes the IO pipes for use as standard input, output, and error
+   * descriptors in the spawned process.
+   *
+   * @param {object} options
+   *        The Subprocess options object for this process.
+   * @returns {unix.Fd[]}
+   *          The array of file descriptors belonging to the spawned process.
+   */
+  initPipes(options) {
+    let stderr = options.stderr;
+
+    let our_pipes = [];
+    let their_pipes = new Map();
+
+    let pipe = input => {
+      let fds = ctypes.int.array(2)();
+
+      let res = libc.pipe(fds);
+      if (res == -1) {
+        throw new Error("Unable to create pipe");
+      }
+
+      fds = Array.from(fds, unix.Fd);
+
+      if (input) {
+        fds.reverse();
+      }
+
+      if (input) {
+        our_pipes.push(new InputPipe(this, fds[1]));
+      } else {
+        our_pipes.push(new OutputPipe(this, fds[1]));
+      }
+
+      libc.fcntl(fds[0], LIBC.F_SETFD, LIBC.FD_CLOEXEC);
+      libc.fcntl(fds[1], LIBC.F_SETFD, LIBC.FD_CLOEXEC);
+      libc.fcntl(fds[1], LIBC.F_SETFL, LIBC.O_NONBLOCK);
+
+      return fds[0];
+    };
+
+    their_pipes.set(0, pipe(false));
+    their_pipes.set(1, pipe(true));
+
+    if (stderr == "pipe") {
+      their_pipes.set(2, pipe(true));
+    } else if (stderr == "stdout") {
+      their_pipes.set(2, their_pipes.get(1));
+    }
+
+    // Create an additional pipe that we can use to monitor for process exit.
+    their_pipes.set(3, pipe(true));
+    this.fd = our_pipes.pop().fd;
+
+    this.pipes = our_pipes;
+
+    return their_pipes;
+  }
+
+  spawn(options) {
+    let fds = this.initPipes(options);
+
+    let launchOptions = {
+      environment: options.environment,
+      disclaim: options.disclaim,
+      fdMap: [],
+    };
+
+    // Check for truthiness to avoid chdir("null")
+    if (options.workdir) {
+      launchOptions.workdir = options.workdir;
+    }
+
+    for (let [dst, src] of fds.entries()) {
+      launchOptions.fdMap.push({ src, dst });
+    }
+
+    try {
+      this.pid = IOUtils.launchProcess(options.arguments, launchOptions);
+    } finally {
+      for (let fd of new Set(fds.values())) {
+        fd.dispose();
+      }
+    }
+  }
+
+  /**
+   * Called when input is available on our sentinel file descriptor.
+   *
+   * @see pollEvents
+   */
+  onReady() {
+    // We're not actually expecting any input on this pipe. If we get any, we
+    // can't poll the pipe any further without reading it.
+    if (this.wait() == undefined) {
+      this.kill(9);
+    }
+  }
+
+  /**
+   * Called when an error occurred while polling our sentinel file descriptor.
+   *
+   * @see pollEvents
+   */
+  onError() {
+    this.wait();
+  }
+
+  /**
+   * Attempts to wait for the process's exit status, without blocking. If
+   * successful, resolves the `exitPromise` to the process's exit value.
+   *
+   * @returns {integer|null}
+   *          The process's exit status, if it has already exited.
+   */
+  wait() {
+    if (this.exitCode !== null) {
+      return this.exitCode;
+    }
+
+    let status = ctypes.int();
+
+    let res = libc.waitpid(this.pid, status.address(), LIBC.WNOHANG);
+    // If there's a failure here and we get any errno other than EINTR, it
+    // means that the process has been reaped by another thread (most likely
+    // the nspr process wait thread), and its actual exit status is not
+    // available to us. In that case, we have to assume success.
+    if (res == 0 || (res == -1 && ctypes.errno == LIBC.EINTR)) {
+      return null;
+    }
+
+    let sig = unix.WTERMSIG(status.value);
+    if (sig) {
+      this.exitCode = -sig;
+    } else {
+      this.exitCode = unix.WEXITSTATUS(status.value);
+    }
+
+    this.fd.dispose();
+    io.updatePollFds();
+    this.resolveExit(this.exitCode);
+    return this.exitCode;
+  }
+}
+
+io = {
+  pollFds: null,
+  pollHandlers: null,
+
+  pipes: new Map(),
+
+  processes: new Map(),
+
+  messageCount: 0,
+
+  running: true,
+
+  polling: false,
+
+  init(details) {
+    this.signal = new Signal(details.signalFd);
+    this.updatePollFds();
+
+    setTimeout(this.loop.bind(this), 0);
+  },
+
+  shutdown() {
+    if (this.running) {
+      this.running = false;
+
+      this.signal.cleanup();
+      this.signal = null;
+
+      self.postMessage({ msg: "close" });
+      self.close();
+    }
+  },
+
+  getPipe(pipeId) {
+    let pipe = this.pipes.get(pipeId);
+
+    if (!pipe) {
+      let error = new Error("File closed");
+      error.errorCode = SubprocessConstants.ERROR_END_OF_FILE;
+      throw error;
+    }
+    return pipe;
+  },
+
+  getProcess(processId) {
+    let process = this.processes.get(processId);
+
+    if (!process) {
+      throw new Error(`Invalid process ID: ${processId}`);
+    }
+    return process;
+  },
+
+  updatePollFds() {
+    let handlers = [
+      this.signal,
+      ...this.pipes.values(),
+      ...this.processes.values(),
+    ];
+
+    handlers = handlers.filter(handler => handler.pollEvents);
+
+    // Our poll loop is only useful if we've got at least 1 thing to poll other than our own
+    // signal.
+    if (handlers.length == 1) {
+      this.polling = false;
+    } else if (!this.polling && this.running) {
+      // Restart the poll loop if necessary:
+      setTimeout(this.loop.bind(this), 0);
+      this.polling = true;
+    }
+
+    let pollfds = unix.pollfd.array(handlers.length)();
+
+    for (let [i, handler] of handlers.entries()) {
+      let pollfd = pollfds[i];
+
+      pollfd.fd = handler.fd;
+      pollfd.events = handler.pollEvents;
+      pollfd.revents = 0;
+    }
+
+    this.pollFds = pollfds;
+    this.pollHandlers = handlers;
+  },
+
+  loop() {
+    this.poll();
+    if (this.running && this.polling) {
+      setTimeout(this.loop.bind(this), 0);
+    }
+  },
+
+  poll() {
+    let handlers = this.pollHandlers;
+    let pollfds = this.pollFds;
+
+    let timeout = this.messageCount > 0 ? 0 : POLL_TIMEOUT;
+    let count = libc.poll(pollfds, pollfds.length, timeout);
+
+    for (let i = 0; count && i < pollfds.length; i++) {
+      let pollfd = pollfds[i];
+      if (pollfd.revents) {
+        count--;
+
+        let handler = handlers[i];
+        try {
+          let success = false;
+          if (pollfd.revents & handler.pollEvents) {
+            success = handler.onReady();
+          }
+          // Only call the error handler in this iteration if we didn't also
+          // have a success. This is necessary because Linux systems set POLLHUP
+          // on a pipe when it's closed but there's still buffered data to be
+          // read, and Darwin sets POLLIN and POLLHUP on a closed pipe, even
+          // when there's no data to be read.
+          if (
+            !success &&
+            pollfd.revents & (LIBC.POLLERR | LIBC.POLLHUP | LIBC.POLLNVAL)
+          ) {
+            handler.onError();
+          }
+        } catch (e) {
+          console.error(e);
+          debug(`Worker error: ${e} :: ${e.stack}`);
+          handler.onError();
+        }
+
+        pollfd.revents = 0;
+      }
+    }
+  },
+
+  addProcess(process) {
+    this.processes.set(process.id, process);
+
+    for (let pipe of process.pipes) {
+      this.pipes.set(pipe.id, pipe);
+    }
+  },
+
+  cleanupProcess(process) {
+    this.processes.delete(process.id);
+  },
+};
diff --git a/toolkit/modules/subprocess/subprocess_worker_win.js b/toolkit/modules/subprocess/subprocess_worker_win.js
new file mode 100644
index 0000000000..7e07361e58
--- /dev/null
+++ b/toolkit/modules/subprocess/subprocess_worker_win.js
@@ -0,0 +1,792 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+/* eslint-env mozilla/chrome-worker */
+/* exported Process */
+
+/* import-globals-from subprocess_shared.js */
+/* import-globals-from subprocess_shared_win.js */
+/* import-globals-from subprocess_worker_common.js */
+importScripts(
+  "resource://gre/modules/subprocess/subprocess_shared.js",
+  "resource://gre/modules/subprocess/subprocess_shared_win.js",
+  "resource://gre/modules/subprocess/subprocess_worker_common.js"
+);
+
+const POLL_TIMEOUT = 5000;
+
+// The exit code that we send when we forcibly terminate a process.
+const TERMINATE_EXIT_CODE = 0x7f;
+
+let io;
+
+let nextPipeId = 0;
+
+class Pipe extends BasePipe {
+  constructor(process, origHandle) {
+    super();
+
+    let handle = win32.HANDLE();
+
+    let curProc = libc.GetCurrentProcess();
+    libc.DuplicateHandle(
+      curProc,
+      origHandle,
+      curProc,
+      handle.address(),
+      0,
+      false /* inheritable */,
+      win32.DUPLICATE_SAME_ACCESS
+    );
+
+    origHandle.dispose();
+
+    this.id = nextPipeId++;
+    this.process = process;
+
+    this.handle = win32.Handle(handle);
+
+    let event = libc.CreateEventW(null, false, false, null);
+
+    this.overlapped = win32.OVERLAPPED();
+    this.overlapped.hEvent = event;
+
+    this._event = win32.Handle(event);
+
+    this.buffer = null;
+  }
+
+  get event() {
+    if (this.pending.length) {
+      return this._event;
+    }
+    return null;
+  }
+
+  maybeClose() {}
+
+  /**
+   * Closes the file handle.
+   *
+   * @param {boolean} [force=false]
+   *        If true, the file handle is closed immediately. If false, the
+   *        file handle is closed after all current pending IO operations
+   *        have completed.
+   *
+   * @returns {Promise<void>}
+   *          Resolves when the file handle has been closed.
+   */
+  close(force = false) {
+    if (!force && this.pending.length) {
+      this.closing = true;
+      return this.closedPromise;
+    }
+
+    for (let { reject } of this.pending) {
+      let error = new Error("File closed");
+      error.errorCode = SubprocessConstants.ERROR_END_OF_FILE;
+      reject(error);
+    }
+    this.pending.length = 0;
+
+    this.buffer = null;
+
+    if (!this.closed) {
+      this.handle.dispose();
+      this._event.dispose();
+
+      io.pipes.delete(this.id);
+
+      this.handle = null;
+      this.closed = true;
+      this.resolveClosed();
+
+      io.updatePollEvents();
+    }
+    return this.closedPromise;
+  }
+
+  /**
+   * Called when an error occurred while attempting an IO operation on our file
+   * handle.
+   */
+  onError() {
+    this.close(true);
+  }
+}
+
+class InputPipe extends Pipe {
+  /**
+   * Queues the next chunk of data to be read from the pipe if, and only if,
+   * there is no IO operation currently pending.
+   */
+  readNext() {
+    if (this.buffer === null) {
+      this.readBuffer(this.pending[0].length);
+    }
+  }
+
+  /**
+   * Closes the pipe if there is a pending read operation with no more
+   * buffered data to be read.
+   */
+  maybeClose() {
+    if (this.buffer) {
+      let read = win32.DWORD();
+
+      let ok = libc.GetOverlappedResult(
+        this.handle,
+        this.overlapped.address(),
+        read.address(),
+        false
+      );
+
+      if (!ok) {
+        this.onError();
+      }
+    }
+  }
+
+  /**
+   * Asynchronously reads at most `length` bytes of binary data from the file
+   * descriptor into an ArrayBuffer of the same size. Returns a promise which
+   * resolves when the operation is complete.
+   *
+   * @param {integer} length
+   *        The number of bytes to read.
+   *
+   * @returns {Promise<ArrayBuffer>}
+   */
+  read(length) {
+    if (this.closing || this.closed) {
+      throw new Error("Attempt to read from closed pipe");
+    }
+
+    return new Promise((resolve, reject) => {
+      this.pending.push({ resolve, reject, length });
+      this.readNext();
+    });
+  }
+
+  /**
+   * Initializes an overlapped IO read operation to read exactly `count` bytes
+   * into a new ArrayBuffer, which is stored in the `buffer` property until the
+   * operation completes.
+   *
+   * @param {integer} count
+   *        The number of bytes to read.
+   */
+  readBuffer(count) {
+    this.buffer = new ArrayBuffer(count);
+
+    let ok = libc.ReadFile(
+      this.handle,
+      this.buffer,
+      count,
+      null,
+      this.overlapped.address()
+    );
+
+    if (!ok && (!this.process.handle || libc.winLastError)) {
+      this.onError();
+    } else {
+      io.updatePollEvents();
+    }
+  }
+
+  /**
+   * Called when our pending overlapped IO operation has completed, whether
+   * successfully or in failure.
+   */
+  onReady() {
+    let read = win32.DWORD();
+
+    let ok = libc.GetOverlappedResult(
+      this.handle,
+      this.overlapped.address(),
+      read.address(),
+      false
+    );
+
+    read = read.value;
+
+    if (!ok) {
+      this.onError();
+    } else if (read > 0) {
+      let buffer = this.buffer;
+      this.buffer = null;
+
+      let { resolve } = this.shiftPending();
+
+      if (read == buffer.byteLength) {
+        resolve(buffer);
+      } else {
+        resolve(ArrayBuffer_transfer(buffer, read));
+      }
+
+      if (this.pending.length) {
+        this.readNext();
+      } else {
+        io.updatePollEvents();
+      }
+    }
+  }
+}
+
+class OutputPipe extends Pipe {
+  /**
+   * Queues the next chunk of data to be written to the pipe if, and only if,
+   * there is no IO operation currently pending.
+   */
+  writeNext() {
+    if (this.buffer === null) {
+      this.writeBuffer(this.pending[0].buffer);
+    }
+  }
+
+  /**
+   * Asynchronously writes the given buffer to our file descriptor, and returns
+   * a promise which resolves when the operation is complete.
+   *
+   * @param {ArrayBuffer} buffer
+   *        The buffer to write.
+   *
+   * @returns {Promise<integer>}
+   *          Resolves to the number of bytes written when the operation is
+   *          complete.
+   */
+  write(buffer) {
+    if (this.closing || this.closed) {
+      throw new Error("Attempt to write to closed pipe");
+    }
+
+    return new Promise((resolve, reject) => {
+      this.pending.push({ resolve, reject, buffer });
+      this.writeNext();
+    });
+  }
+
+  /**
+   * Initializes an overapped IO read operation to write the data in `buffer` to
+   * our file descriptor.
+   *
+   * @param {ArrayBuffer} buffer
+   *        The buffer to write.
+   */
+  writeBuffer(buffer) {
+    this.buffer = buffer;
+
+    let ok = libc.WriteFile(
+      this.handle,
+      buffer,
+      buffer.byteLength,
+      null,
+      this.overlapped.address()
+    );
+
+    if (!ok && libc.winLastError) {
+      this.onError();
+    } else {
+      io.updatePollEvents();
+    }
+  }
+
+  /**
+   * Called when our pending overlapped IO operation has completed, whether
+   * successfully or in failure.
+   */
+  onReady() {
+    let written = win32.DWORD();
+
+    let ok = libc.GetOverlappedResult(
+      this.handle,
+      this.overlapped.address(),
+      written.address(),
+      false
+    );
+
+    written = written.value;
+
+    if (!ok || written != this.buffer.byteLength) {
+      this.onError();
+    } else if (written > 0) {
+      let { resolve } = this.shiftPending();
+
+      this.buffer = null;
+      resolve(written);
+
+      if (this.pending.length) {
+        this.writeNext();
+      } else {
+        io.updatePollEvents();
+      }
+    }
+  }
+}
+
+class Signal {
+  constructor(event) {
+    this.event = event;
+  }
+
+  cleanup() {
+    libc.CloseHandle(this.event);
+    this.event = null;
+  }
+
+  onError() {
+    io.shutdown();
+  }
+
+  onReady() {
+    io.messageCount += 1;
+  }
+}
+
+class Process extends BaseProcess {
+  constructor(...args) {
+    super(...args);
+
+    this.killed = false;
+  }
+
+  /**
+   * Returns our process handle for use as an event in a WaitForMultipleObjects
+   * call.
+   */
+  get event() {
+    return this.handle;
+  }
+
+  /**
+   * Forcibly terminates the process.
+   */
+  kill() {
+    this.killed = true;
+    libc.TerminateJobObject(this.jobHandle, TERMINATE_EXIT_CODE);
+  }
+
+  /**
+   * Initializes the IO pipes for use as standard input, output, and error
+   * descriptors in the spawned process.
+   *
+   * @returns {win32.Handle[]}
+   *          The array of file handles belonging to the spawned process.
+   */
+  initPipes({ stderr }) {
+    let our_pipes = [];
+    let their_pipes = [];
+
+    let secAttr = new win32.SECURITY_ATTRIBUTES();
+    secAttr.nLength = win32.SECURITY_ATTRIBUTES.size;
+    secAttr.bInheritHandle = true;
+
+    let pipe = input => {
+      if (input) {
+        let handles = win32.createPipe(secAttr, win32.FILE_FLAG_OVERLAPPED);
+        our_pipes.push(new InputPipe(this, handles[0]));
+        return handles[1];
+      }
+      let handles = win32.createPipe(secAttr, 0, win32.FILE_FLAG_OVERLAPPED);
+      our_pipes.push(new OutputPipe(this, handles[1]));
+      return handles[0];
+    };
+
+    their_pipes[0] = pipe(false);
+    their_pipes[1] = pipe(true);
+
+    if (stderr == "pipe") {
+      their_pipes[2] = pipe(true);
+    } else {
+      let srcHandle;
+      if (stderr == "stdout") {
+        srcHandle = their_pipes[1];
+      } else {
+        srcHandle = libc.GetStdHandle(win32.STD_ERROR_HANDLE);
+      }
+
+      // If we don't have a valid stderr handle, just pass it along without duplicating.
+      if (
+        String(srcHandle) == win32.INVALID_HANDLE_VALUE ||
+        String(srcHandle) == win32.NULL_HANDLE_VALUE
+      ) {
+        their_pipes[2] = srcHandle;
+      } else {
+        let handle = win32.HANDLE();
+
+        let curProc = libc.GetCurrentProcess();
+        let ok = libc.DuplicateHandle(
+          curProc,
+          srcHandle,
+          curProc,
+          handle.address(),
+          0,
+          true /* inheritable */,
+          win32.DUPLICATE_SAME_ACCESS
+        );
+
+        their_pipes[2] = ok && win32.Handle(handle);
+      }
+    }
+
+    if (!their_pipes.every(handle => handle)) {
+      throw new Error("Failed to create pipe");
+    }
+
+    this.pipes = our_pipes;
+
+    return their_pipes;
+  }
+
+  /**
+   * Creates a null-separated, null-terminated string list.
+   *
+   * @param {Array<string>} strings
+   * @returns {win32.WCHAR.array}
+   */
+  stringList(strings) {
+    // Remove empty strings, which would terminate the list early.
+    strings = strings.filter(string => string);
+
+    let string = strings.join("\0") + "\0\0";
+
+    return win32.WCHAR.array()(string);
+  }
+
+  /**
+   * Quotes a string for use as a single command argument, using Windows quoting
+   * conventions.
+   *
+   * @see https://msdn.microsoft.com/en-us/library/17w5ykft(v=vs.85).aspx
+   *
+   * @param {string} str
+   *        The argument string to quote.
+   * @returns {string}
+   */
+  quoteString(str) {
+    if (!/[\s"]/.test(str)) {
+      return str;
+    }
+
+    let escaped = str.replace(/(\\*)("|$)/g, (m0, m1, m2) => {
+      if (m2) {
+        m2 = `\\${m2}`;
+      }
+      return `${m1}${m1}${m2}`;
+    });
+
+    return `"${escaped}"`;
+  }
+
+  spawn(options) {
+    let { command, arguments: args } = options;
+
+    if (
+      /\\cmd\.exe$/i.test(command) &&
+      args.length == 3 &&
+      /^(\/S)?\/C$/i.test(args[1])
+    ) {
+      // cmd.exe is insane and requires special treatment.
+      args = [this.quoteString(args[0]), "/S/C", `"${args[2]}"`];
+    } else {
+      args = args.map(arg => this.quoteString(arg));
+    }
+
+    if (/\.(bat|cmd)$/i.test(command)) {
+      command = io.comspec;
+      args = ["cmd.exe", "/s/c", `"${args.join(" ")}"`];
+    }
+
+    let envp = this.stringList(options.environment);
+
+    let handles = this.initPipes(options);
+
+    let processFlags =
+      win32.CREATE_NO_WINDOW |
+      win32.CREATE_SUSPENDED |
+      win32.CREATE_UNICODE_ENVIRONMENT;
+
+    if (io.breakAwayFromJob) {
+      processFlags |= win32.CREATE_BREAKAWAY_FROM_JOB;
+    }
+
+    let startupInfoEx = new win32.STARTUPINFOEXW();
+    let startupInfo = startupInfoEx.StartupInfo;
+
+    startupInfo.cb = win32.STARTUPINFOW.size;
+    startupInfo.dwFlags = win32.STARTF_USESTDHANDLES;
+
+    startupInfo.hStdInput = handles[0];
+    startupInfo.hStdOutput = handles[1];
+    startupInfo.hStdError = handles[2];
+
+    // Note: This needs to be kept alive until we destroy the attribute list.
+    let handleArray = win32.HANDLE.array()(handles);
+
+    let threadAttrs = win32.createThreadAttributeList(handleArray);
+    if (threadAttrs) {
+      // If have thread attributes to pass, pass the size of the full extended
+      // startup info struct.
+      processFlags |= win32.EXTENDED_STARTUPINFO_PRESENT;
+      startupInfo.cb = win32.STARTUPINFOEXW.size;
+
+      startupInfoEx.lpAttributeList = threadAttrs;
+    }
+
+    let procInfo = new win32.PROCESS_INFORMATION();
+
+    let errorMessage = "Failed to create process";
+    let ok = libc.CreateProcessW(
+      command,
+      args.join(" "),
+      null /* Security attributes */,
+      null /* Thread security attributes */,
+      true /* Inherits handles */,
+      processFlags,
+      envp,
+      options.workdir,
+      startupInfo.address(),
+      procInfo.address()
+    );
+
+    for (let handle of new Set(handles)) {
+      // If any of our handles are invalid, they don't have finalizers.
+      if (handle && handle.dispose) {
+        handle.dispose();
+      }
+    }
+
+    if (threadAttrs) {
+      libc.DeleteProcThreadAttributeList(threadAttrs);
+    }
+
+    if (ok) {
+      this.jobHandle = win32.Handle(libc.CreateJobObjectW(null, null));
+
+      let info = win32.JOBOBJECT_EXTENDED_LIMIT_INFORMATION();
+      info.BasicLimitInformation.LimitFlags =
+        win32.JOB_OBJECT_LIMIT_BREAKAWAY_OK;
+
+      ok = libc.SetInformationJobObject(
+        this.jobHandle,
+        win32.JobObjectExtendedLimitInformation,
+        ctypes.cast(info.address(), ctypes.voidptr_t),
+        info.constructor.size
+      );
+      errorMessage = `Failed to set job limits: 0x${(
+        ctypes.winLastError || 0
+      ).toString(16)}`;
+    }
+
+    if (ok) {
+      ok = libc.AssignProcessToJobObject(this.jobHandle, procInfo.hProcess);
+      if (!ok) {
+        errorMessage = `Failed to attach process to job object: 0x${(
+          ctypes.winLastError || 0
+        ).toString(16)}`;
+        libc.TerminateProcess(procInfo.hProcess, TERMINATE_EXIT_CODE);
+      }
+    }
+
+    if (!ok) {
+      for (let pipe of this.pipes) {
+        pipe.close();
+      }
+      throw new Error(errorMessage);
+    }
+
+    this.handle = win32.Handle(procInfo.hProcess);
+    this.pid = procInfo.dwProcessId;
+
+    libc.ResumeThread(procInfo.hThread);
+    libc.CloseHandle(procInfo.hThread);
+  }
+
+  /**
+   * Called when our process handle is signaled as active, meaning the process
+   * has exited.
+   */
+  onReady() {
+    this.wait();
+  }
+
+  /**
+   * Attempts to wait for the process's exit status, without blocking. If
+   * successful, resolves the `exitPromise` to the process's exit value.
+   *
+   * @returns {integer|null}
+   *          The process's exit status, if it has already exited.
+   */
+  wait() {
+    if (this.exitCode !== null) {
+      return this.exitCode;
+    }
+
+    let status = win32.DWORD();
+
+    let ok = libc.GetExitCodeProcess(this.handle, status.address());
+    if (ok && status.value != win32.STILL_ACTIVE) {
+      let exitCode = status.value;
+      if (this.killed && exitCode == TERMINATE_EXIT_CODE) {
+        // If we forcibly terminated the process, return the force kill exit
+        // code that we return on other platforms.
+        exitCode = -9;
+      }
+
+      this.resolveExit(exitCode);
+      this.exitCode = exitCode;
+
+      this.handle.dispose();
+      this.handle = null;
+
+      libc.TerminateJobObject(this.jobHandle, TERMINATE_EXIT_CODE);
+      this.jobHandle.dispose();
+      this.jobHandle = null;
+
+      for (let pipe of this.pipes) {
+        pipe.maybeClose();
+      }
+
+      io.updatePollEvents();
+
+      return exitCode;
+    }
+  }
+}
+
+io = {
+  events: null,
+  eventHandlers: null,
+
+  pipes: new Map(),
+
+  processes: new Map(),
+
+  messageCount: 0,
+
+  running: true,
+
+  polling: false,
+
+  init(details) {
+    this.comspec = details.comspec;
+
+    let signalEvent = ctypes.cast(
+      ctypes.uintptr_t(details.signalEvent),
+      win32.HANDLE
+    );
+    this.signal = new Signal(signalEvent);
+    this.updatePollEvents();
+
+    this.breakAwayFromJob = details.breakAwayFromJob;
+
+    setTimeout(this.loop.bind(this), 0);
+  },
+
+  shutdown() {
+    if (this.running) {
+      this.running = false;
+
+      this.signal.cleanup();
+      this.signal = null;
+
+      self.postMessage({ msg: "close" });
+      self.close();
+    }
+  },
+
+  getPipe(pipeId) {
+    let pipe = this.pipes.get(pipeId);
+
+    if (!pipe) {
+      let error = new Error("File closed");
+      error.errorCode = SubprocessConstants.ERROR_END_OF_FILE;
+      throw error;
+    }
+    return pipe;
+  },
+
+  getProcess(processId) {
+    let process = this.processes.get(processId);
+
+    if (!process) {
+      throw new Error(`Invalid process ID: ${processId}`);
+    }
+    return process;
+  },
+
+  updatePollEvents() {
+    let handlers = [
+      this.signal,
+      ...this.pipes.values(),
+      ...this.processes.values(),
+    ];
+
+    handlers = handlers.filter(handler => handler.event);
+
+    // Our poll loop is only useful if we've got at least 1 thing to poll other than our own
+    // signal.
+    if (handlers.length == 1) {
+      this.polling = false;
+    } else if (!this.polling && this.running) {
+      // Restart the poll loop if necessary:
+      setTimeout(this.loop.bind(this), 0);
+      this.polling = true;
+    }
+
+    this.eventHandlers = handlers;
+
+    let handles = handlers.map(handler => handler.event);
+    this.events = win32.HANDLE.array()(handles);
+  },
+
+  loop() {
+    this.poll();
+    if (this.running && this.polling) {
+      setTimeout(this.loop.bind(this), 0);
+    }
+  },
+
+  poll() {
+    let timeout = this.messageCount > 0 ? 0 : POLL_TIMEOUT;
+    for (; ; timeout = 0) {
+      let events = this.events;
+      let handlers = this.eventHandlers;
+
+      let result = libc.WaitForMultipleObjects(
+        events.length,
+        events,
+        false,
+        timeout
+      );
+
+      if (result < handlers.length) {
+        try {
+          handlers[result].onReady();
+        } catch (e) {
+          console.error(e);
+          debug(`Worker error: ${e} :: ${e.stack}`);
+          handlers[result].onError();
+        }
+      } else {
+        break;
+      }
+    }
+  },
+
+  addProcess(process) {
+    this.processes.set(process.id, process);
+
+    for (let pipe of process.pipes) {
+      this.pipes.set(pipe.id, pipe);
+    }
+  },
+
+  cleanupProcess(process) {
+    this.processes.delete(process.id);
+  },
+};
diff --git a/toolkit/modules/subprocess/test/xpcshell/data_test_script.py b/toolkit/modules/subprocess/test/xpcshell/data_test_script.py
new file mode 100644
index 0000000000..e1f5f5de93
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/data_test_script.py
@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+
+import os
+import signal
+import struct
+import sys
+
+
+def output(line, stream=sys.stdout, print_only=False):
+    if isinstance(line, str):
+        line = line.encode("utf-8", "surrogateescape")
+    if not print_only:
+        stream.buffer.write(struct.pack("@I", len(line)))
+    stream.buffer.write(line)
+    stream.flush()
+
+
+def echo_loop():
+    while True:
+        line = sys.stdin.buffer.readline()
+        if not line:
+            break
+
+        output(line)
+
+
+if sys.platform == "win32":
+    import msvcrt
+
+    msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
+
+
+cmd = sys.argv[1]
+if cmd == "echo":
+    echo_loop()
+elif cmd == "exit":
+    sys.exit(int(sys.argv[2]))
+elif cmd == "env":
+    for var in sys.argv[2:]:
+        output(os.environ.get(var, "!"))
+elif cmd == "pwd":
+    output(os.path.abspath(os.curdir))
+elif cmd == "print_args":
+    for arg in sys.argv[2:]:
+        output(arg)
+elif cmd == "ignore_sigterm":
+    signal.signal(signal.SIGTERM, signal.SIG_IGN)
+
+    output("Ready")
+    while True:
+        try:
+            signal.pause()
+        except AttributeError:
+            import time
+
+            time.sleep(3600)
+elif cmd == "print":
+    output(sys.argv[2], stream=sys.stdout, print_only=True)
+    output(sys.argv[3], stream=sys.stderr, print_only=True)
diff --git a/toolkit/modules/subprocess/test/xpcshell/data_text_file.txt b/toolkit/modules/subprocess/test/xpcshell/data_text_file.txt
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/data_text_file.txt
diff --git a/toolkit/modules/subprocess/test/xpcshell/head.js b/toolkit/modules/subprocess/test/xpcshell/head.js
new file mode 100644
index 0000000000..a2b85047d3
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/head.js
@@ -0,0 +1,13 @@
+"use strict";
+
+var { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+// eslint-disable-next-line no-unused-vars
+ChromeUtils.defineESModuleGetters(this, {
+  Subprocess: "resource://gre/modules/Subprocess.sys.mjs",
+});
diff --git a/toolkit/modules/subprocess/test/xpcshell/test_subprocess.js b/toolkit/modules/subprocess/test/xpcshell/test_subprocess.js
new file mode 100644
index 0000000000..77d09f41ae
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/test_subprocess.js
@@ -0,0 +1,868 @@
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+"use strict";
+
+const { setTimeout } = ChromeUtils.importESModule(
+  "resource://gre/modules/Timer.sys.mjs"
+);
+
+const MAX_ROUND_TRIP_TIME_MS = AppConstants.DEBUG || AppConstants.ASAN ? 18 : 9;
+const MAX_RETRIES = 5;
+
+let PYTHON;
+let PYTHON_BIN;
+let PYTHON_DIR;
+
+const TEST_SCRIPT = do_get_file("data_test_script.py").path;
+
+let read = pipe => {
+  return pipe.readUint32().then(count => {
+    return pipe.readString(count);
+  });
+};
+
+let readAll = async function (pipe) {
+  let result = [];
+  let string;
+  while ((string = await pipe.readString())) {
+    result.push(string);
+  }
+
+  return result.join("");
+};
+
+add_task(async function setup() {
+  PYTHON = await Subprocess.pathSearch(Services.env.get("PYTHON"));
+
+  PYTHON_BIN = PathUtils.filename(PYTHON);
+  PYTHON_DIR = PathUtils.parent(PYTHON);
+});
+
+add_task(async function test_subprocess_io() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  Assert.throws(() => {
+    proc.stdout.read(-1);
+  }, /non-negative integer/);
+  Assert.throws(() => {
+    proc.stdout.read(1.1);
+  }, /non-negative integer/);
+
+  Assert.throws(() => {
+    proc.stdout.read(Infinity);
+  }, /non-negative integer/);
+  Assert.throws(() => {
+    proc.stdout.read(NaN);
+  }, /non-negative integer/);
+
+  Assert.throws(() => {
+    proc.stdout.readString(-1);
+  }, /non-negative integer/);
+  Assert.throws(() => {
+    proc.stdout.readString(1.1);
+  }, /non-negative integer/);
+
+  Assert.throws(() => {
+    proc.stdout.readJSON(-1);
+  }, /positive integer/);
+  Assert.throws(() => {
+    proc.stdout.readJSON(0);
+  }, /positive integer/);
+  Assert.throws(() => {
+    proc.stdout.readJSON(1.1);
+  }, /positive integer/);
+
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let outputPromise = read(proc.stdout);
+
+  await new Promise(resolve => setTimeout(resolve, 100));
+
+  let [output] = await Promise.all([outputPromise, proc.stdin.write(LINE1)]);
+
+  equal(output, LINE1, "Got expected output");
+
+  // Make sure it succeeds whether the write comes before or after the
+  // read.
+  let inputPromise = proc.stdin.write(LINE2);
+
+  await new Promise(resolve => setTimeout(resolve, 100));
+
+  [output] = await Promise.all([read(proc.stdout), inputPromise]);
+
+  equal(output, LINE2, "Got expected output");
+
+  let JSON_BLOB = { foo: { bar: "baz" } };
+
+  inputPromise = proc.stdin.write(JSON.stringify(JSON_BLOB) + "\n");
+
+  output = await proc.stdout.readUint32().then(count => {
+    return proc.stdout.readJSON(count);
+  });
+
+  Assert.deepEqual(output, JSON_BLOB, "Got expected JSON output");
+
+  await proc.stdin.close();
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_large_io() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  const LINE = "I'm a leaf on the wind.\n";
+  const BUFFER_SIZE = 4096;
+
+  // Create a message that's ~3/4 the input buffer size.
+  let msg =
+    Array(((BUFFER_SIZE * 0.75) / 16) | 0)
+      .fill("0123456789abcdef")
+      .join("") + "\n";
+
+  // This sequence of writes and reads crosses several buffer size
+  // boundaries, and causes some branches of the read buffer code to be
+  // exercised which are not exercised by other tests.
+  proc.stdin.write(msg);
+  proc.stdin.write(msg);
+  proc.stdin.write(LINE);
+
+  let output = await read(proc.stdout);
+  equal(output, msg, "Got the expected output");
+
+  output = await read(proc.stdout);
+  equal(output, msg, "Got the expected output");
+
+  output = await read(proc.stdout);
+  equal(output, LINE, "Got the expected output");
+
+  proc.stdin.close();
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_huge() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  // This should be large enough to fill most pipe input/output buffers.
+  const MESSAGE_SIZE = 1024 * 16;
+
+  let msg = Array(MESSAGE_SIZE).fill("0123456789abcdef").join("") + "\n";
+
+  proc.stdin.write(msg);
+
+  let output = await read(proc.stdout);
+  equal(output, msg, "Got the expected output");
+
+  proc.stdin.close();
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(
+  { skip_if: () => mozinfo.ccov },
+  async function test_subprocess_round_trip_perf() {
+    let roundTripTime = Infinity;
+    for (
+      let i = 0;
+      i < MAX_RETRIES && roundTripTime > MAX_ROUND_TRIP_TIME_MS;
+      i++
+    ) {
+      let proc = await Subprocess.call({
+        command: PYTHON,
+        arguments: ["-u", TEST_SCRIPT, "echo"],
+      });
+
+      const LINE = "I'm a leaf on the wind.\n";
+
+      let now = Date.now();
+      const COUNT = 1000;
+      for (let j = 0; j < COUNT; j++) {
+        let [output] = await Promise.all([
+          read(proc.stdout),
+          proc.stdin.write(LINE),
+        ]);
+
+        // We don't want to log this for every iteration, but we still need
+        // to fail if it goes wrong.
+        if (output !== LINE) {
+          equal(output, LINE, "Got expected output");
+        }
+      }
+
+      roundTripTime = (Date.now() - now) / COUNT;
+
+      await proc.stdin.close();
+
+      let { exitCode } = await proc.wait();
+
+      equal(exitCode, 0, "Got expected exit code");
+    }
+
+    ok(
+      roundTripTime <= MAX_ROUND_TRIP_TIME_MS,
+      `Expected round trip time (${roundTripTime}ms) to be less than ${MAX_ROUND_TRIP_TIME_MS}ms`
+    );
+  }
+);
+
+add_task(async function test_subprocess_stderr_default() {
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "print", LINE1, LINE2],
+  });
+
+  equal(proc.stderr, undefined, "There should be no stderr pipe by default");
+
+  let stdout = await readAll(proc.stdout);
+
+  equal(stdout, LINE1, "Got the expected stdout output");
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_stderr_pipe() {
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "print", LINE1, LINE2],
+    stderr: "pipe",
+  });
+
+  let [stdout, stderr] = await Promise.all([
+    readAll(proc.stdout),
+    readAll(proc.stderr),
+  ]);
+
+  equal(stdout, LINE1, "Got the expected stdout output");
+  equal(stderr, LINE2, "Got the expected stderr output");
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_stderr_merged() {
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "print", LINE1, LINE2],
+    stderr: "stdout",
+  });
+
+  equal(proc.stderr, undefined, "There should be no stderr pipe by default");
+
+  let stdout = await readAll(proc.stdout);
+
+  equal(stdout, LINE1 + LINE2, "Got the expected merged stdout output");
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_read_after_exit() {
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "print", LINE1, LINE2],
+    stderr: "pipe",
+  });
+
+  let { exitCode } = await proc.wait();
+  equal(exitCode, 0, "Process exited with expected code");
+
+  let [stdout, stderr] = await Promise.all([
+    readAll(proc.stdout),
+    readAll(proc.stderr),
+  ]);
+
+  equal(stdout, LINE1, "Got the expected stdout output");
+  equal(stderr, LINE2, "Got the expected stderr output");
+});
+
+add_task(async function test_subprocess_lazy_close_output() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  const LINE1 = "I'm a leaf on the wind.\n";
+  const LINE2 = "Watch how I soar.\n";
+
+  let writePromises = [proc.stdin.write(LINE1), proc.stdin.write(LINE2)];
+  let closedPromise = proc.stdin.close();
+
+  let output1 = await read(proc.stdout);
+  let output2 = await read(proc.stdout);
+
+  await Promise.all([...writePromises, closedPromise]);
+
+  equal(output1, LINE1, "Got expected output");
+  equal(output2, LINE2, "Got expected output");
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_lazy_close_input() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  let readPromise = proc.stdout.readUint32();
+  let closedPromise = proc.stdout.close();
+
+  const LINE = "I'm a leaf on the wind.\n";
+
+  proc.stdin.write(LINE);
+  proc.stdin.close();
+
+  let len = await readPromise;
+  equal(len, LINE.length);
+
+  await closedPromise;
+
+  // Don't test for a successful exit here. The process may exit with a
+  // write error if we close the pipe after it's written the message
+  // size but before it's written the message.
+  await proc.wait();
+});
+
+add_task(async function test_subprocess_force_close() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  let readPromise = proc.stdout.readUint32();
+  let closedPromise = proc.stdout.close(true);
+
+  await Assert.rejects(
+    readPromise,
+    function (e) {
+      equal(
+        e.errorCode,
+        Subprocess.ERROR_END_OF_FILE,
+        "Got the expected error code"
+      );
+      return /File closed/.test(e.message);
+    },
+    "Promise should be rejected when file is closed"
+  );
+
+  await closedPromise;
+  await proc.stdin.close();
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_eof() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  let readPromise = proc.stdout.readUint32();
+
+  await proc.stdin.close();
+
+  await Assert.rejects(
+    readPromise,
+    function (e) {
+      equal(
+        e.errorCode,
+        Subprocess.ERROR_END_OF_FILE,
+        "Got the expected error code"
+      );
+      return /File closed/.test(e.message);
+    },
+    "Promise should be rejected on EOF"
+  );
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_invalid_json() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  const LINE = "I'm a leaf on the wind.\n";
+
+  proc.stdin.write(LINE);
+  proc.stdin.close();
+
+  let count = await proc.stdout.readUint32();
+  let readPromise = proc.stdout.readJSON(count);
+
+  await Assert.rejects(
+    readPromise,
+    function (e) {
+      equal(
+        e.errorCode,
+        Subprocess.ERROR_INVALID_JSON,
+        "Got the expected error code"
+      );
+      return /SyntaxError/.test(e);
+    },
+    "Promise should be rejected on EOF"
+  );
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+if (AppConstants.isPlatformAndVersionAtLeast("win", "6")) {
+  add_task(async function test_subprocess_inherited_descriptors() {
+    let { libc, win32 } = ChromeUtils.importESModule(
+      "resource://gre/modules/subprocess/subprocess_win.sys.mjs"
+    );
+    const { ctypes } = ChromeUtils.importESModule(
+      "resource://gre/modules/ctypes.sys.mjs"
+    );
+
+    let secAttr = new win32.SECURITY_ATTRIBUTES();
+    secAttr.nLength = win32.SECURITY_ATTRIBUTES.size;
+    secAttr.bInheritHandle = true;
+
+    let handles = win32.createPipe(secAttr, 0);
+
+    let proc = await Subprocess.call({
+      command: PYTHON,
+      arguments: ["-u", TEST_SCRIPT, "echo"],
+    });
+
+    // Close the output end of the pipe.
+    // Ours should be the only copy, so reads should fail after this.
+    handles[1].dispose();
+
+    let buffer = new ArrayBuffer(1);
+    let succeeded = libc.ReadFile(
+      handles[0],
+      buffer,
+      buffer.byteLength,
+      null,
+      null
+    );
+
+    ok(!succeeded, "ReadFile should fail on broken pipe");
+    equal(
+      ctypes.winLastError,
+      win32.ERROR_BROKEN_PIPE,
+      "Read should fail with ERROR_BROKEN_PIPE"
+    );
+
+    proc.stdin.close();
+
+    let { exitCode } = await proc.wait();
+
+    equal(exitCode, 0, "Got expected exit code");
+  });
+}
+
+add_task(async function test_subprocess_wait() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "exit", "42"],
+  });
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 42, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_pathSearch() {
+  let promise = Subprocess.call({
+    command: PYTHON_BIN,
+    arguments: ["-u", TEST_SCRIPT, "exit", "13"],
+    environment: {
+      PATH: PYTHON_DIR,
+    },
+  });
+
+  await Assert.rejects(
+    promise,
+    function (error) {
+      return error.errorCode == Subprocess.ERROR_BAD_EXECUTABLE;
+    },
+    "Subprocess.call should fail for a bad executable"
+  );
+});
+
+add_task(async function test_subprocess_workdir() {
+  let procDir = Services.dirsvc.get("CurWorkD", Ci.nsIFile).path;
+  let tmpDir = PathUtils.normalize(PathUtils.tempDir);
+
+  notEqual(
+    procDir,
+    tmpDir,
+    "Current process directory must not be the current temp directory"
+  );
+
+  async function pwd(options) {
+    let proc = await Subprocess.call(
+      Object.assign(
+        {
+          command: PYTHON,
+          arguments: ["-u", TEST_SCRIPT, "pwd"],
+        },
+        options
+      )
+    );
+
+    let pwdOutput = read(proc.stdout);
+
+    let { exitCode } = await proc.wait();
+    equal(exitCode, 0, "Got expected exit code");
+
+    return pwdOutput;
+  }
+
+  let dir = await pwd({});
+  equal(
+    dir,
+    procDir,
+    "Process should normally launch in current process directory"
+  );
+
+  dir = await pwd({ workdir: tmpDir });
+  equal(
+    dir,
+    tmpDir,
+    "Process should launch in the directory specified in `workdir`"
+  );
+});
+
+add_task(async function test_subprocess_term() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  // Windows does not support killing processes gracefully, so they will
+  // always exit with -9 there.
+  let retVal = AppConstants.platform == "win" ? -9 : -15;
+
+  // Kill gracefully with the default timeout of 300ms.
+  let { exitCode } = await proc.kill();
+
+  equal(exitCode, retVal, "Got expected exit code");
+
+  ({ exitCode } = await proc.wait());
+
+  equal(exitCode, retVal, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_kill() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "echo"],
+  });
+
+  // Force kill with no gracefull termination timeout.
+  let { exitCode } = await proc.kill(0);
+
+  equal(exitCode, -9, "Got expected exit code");
+
+  ({ exitCode } = await proc.wait());
+
+  equal(exitCode, -9, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_kill_timeout() {
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "ignore_sigterm"],
+  });
+
+  // Wait for the process to set up its signal handler and tell us it's
+  // ready.
+  let msg = await read(proc.stdout);
+  equal(msg, "Ready", "Process is ready");
+
+  // Kill gracefully with the default timeout of 300ms.
+  // Expect a force kill after 300ms, since the process traps SIGTERM.
+  const TIMEOUT = 300;
+  let startTime = Date.now();
+
+  let { exitCode } = await proc.kill(TIMEOUT);
+
+  // Graceful termination is not supported on Windows, so don't bother
+  // testing the timeout there.
+  if (AppConstants.platform != "win") {
+    let diff = Date.now() - startTime;
+    ok(
+      diff >= TIMEOUT,
+      `Process was killed after ${diff}ms (expected ~${TIMEOUT}ms)`
+    );
+  }
+
+  equal(exitCode, -9, "Got expected exit code");
+
+  ({ exitCode } = await proc.wait());
+
+  equal(exitCode, -9, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_arguments() {
+  let args = [
+    String.raw`C:\Program Files\Company\Program.exe`,
+    String.raw`\\NETWORK SHARE\Foo Directory${"\\"}`,
+    String.raw`foo bar baz`,
+    String.raw`"foo bar baz"`,
+    String.raw`foo " bar`,
+    String.raw`Thing \" with "" "\" \\\" \\\\" quotes\\" \\`,
+  ];
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "print_args", ...args],
+  });
+
+  for (let [i, arg] of args.entries()) {
+    let val = await read(proc.stdout);
+    equal(val, arg, `Got correct value for args[${i}]`);
+  }
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_environment() {
+  let environment = {
+    FOO: "BAR",
+    EMPTY: "",
+    IGNORED: null,
+  };
+
+  // Our Windows environment can't handle launching python without
+  // PATH variables.
+  if (AppConstants.platform == "win") {
+    Object.assign(environment, {
+      PATH: Services.env.get("PATH"),
+      PATHEXT: Services.env.get("PATHEXT"),
+      SYSTEMROOT: Services.env.get("SYSTEMROOT"),
+    });
+  }
+
+  Services.env.set("BAR", "BAZ");
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: ["-u", TEST_SCRIPT, "env", "FOO", "BAR", "EMPTY", "IGNORED"],
+    environment,
+  });
+
+  let foo = await read(proc.stdout);
+  let bar = await read(proc.stdout);
+  let empty = await read(proc.stdout);
+  let ignored = await read(proc.stdout);
+
+  equal(foo, "BAR", "Got expected $FOO value");
+  equal(bar, "!", "Got expected $BAR value");
+  equal(empty, "", "Got expected $EMPTY value");
+  equal(ignored, "!", "Got expected $IGNORED value");
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+add_task(async function test_subprocess_environmentAppend() {
+  Services.env.set("VALUE_FROM_BASE_ENV", "untouched");
+  Services.env.set("VALUE_FROM_BASE_ENV_EMPTY", "untouched");
+  Services.env.set("VALUE_FROM_BASE_ENV_REMOVED", "untouched");
+
+  let proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: [
+      "-u",
+      TEST_SCRIPT,
+      "env",
+      "VALUE_FROM_BASE_ENV",
+      "VALUE_FROM_BASE_ENV_EMPTY",
+      "VALUE_FROM_BASE_ENV_REMOVED",
+      "VALUE_APPENDED_ONCE",
+    ],
+    environmentAppend: true,
+    environment: {
+      VALUE_FROM_BASE_ENV_EMPTY: "",
+      VALUE_FROM_BASE_ENV_REMOVED: null,
+      VALUE_APPENDED_ONCE: "soon empty",
+    },
+  });
+
+  let valueFromBaseEnv = await read(proc.stdout);
+  let valueFromBaseEnvEmpty = await read(proc.stdout);
+  let valueFromBaseEnvRemoved = await read(proc.stdout);
+  let valueAppendedOnce = await read(proc.stdout);
+
+  equal(
+    valueFromBaseEnv,
+    "untouched",
+    "Got expected $VALUE_FROM_BASE_ENV value"
+  );
+  equal(
+    valueFromBaseEnvEmpty,
+    "",
+    "Got expected $VALUE_FROM_BASE_ENV_EMPTY value"
+  );
+  equal(
+    valueFromBaseEnvRemoved,
+    "!",
+    "Got expected $VALUE_FROM_BASE_ENV_REMOVED value"
+  );
+  equal(
+    valueAppendedOnce,
+    "soon empty",
+    "Got expected $VALUE_APPENDED_ONCE value"
+  );
+
+  let { exitCode } = await proc.wait();
+
+  equal(exitCode, 0, "Got expected exit code");
+
+  proc = await Subprocess.call({
+    command: PYTHON,
+    arguments: [
+      "-u",
+      TEST_SCRIPT,
+      "env",
+      "VALUE_FROM_BASE_ENV",
+      "VALUE_APPENDED_ONCE",
+    ],
+    environmentAppend: true,
+  });
+
+  valueFromBaseEnv = await read(proc.stdout);
+  valueAppendedOnce = await read(proc.stdout);
+
+  equal(
+    valueFromBaseEnv,
+    "untouched",
+    "Got expected $VALUE_FROM_BASE_ENV value"
+  );
+  equal(valueAppendedOnce, "!", "Got expected $VALUE_APPENDED_ONCE value");
+
+  ({ exitCode } = await proc.wait());
+
+  equal(exitCode, 0, "Got expected exit code");
+});
+
+if (AppConstants.platform !== "win") {
+  add_task(async function test_subprocess_nonASCII() {
+    const { libc } = ChromeUtils.importESModule(
+      "resource://gre/modules/subprocess/subprocess_unix.sys.mjs"
+    );
+
+    // Use TextDecoder rather than a string with a \xff escape, since
+    // the latter will automatically be normalized to valid UTF-8.
+    let val = new TextDecoder().decode(Uint8Array.of(1, 255));
+
+    libc.setenv(
+      "FOO",
+      Uint8Array.from(val + "\0", c => c.charCodeAt(0)),
+      1
+    );
+
+    let proc = await Subprocess.call({
+      command: PYTHON,
+      arguments: ["-u", TEST_SCRIPT, "env", "FOO"],
+    });
+
+    let foo = await read(proc.stdout);
+
+    equal(foo, val, "Got expected $FOO value");
+
+    Services.env.set("FOO", "");
+
+    let { exitCode } = await proc.wait();
+
+    equal(exitCode, 0, "Got expected exit code");
+  });
+}
+
+add_task(async function test_bad_executable() {
+  // Test with a non-executable file.
+
+  let textFile = do_get_file("data_text_file.txt").path;
+
+  let promise = Subprocess.call({
+    command: textFile,
+    arguments: [],
+  });
+
+  await Assert.rejects(
+    promise,
+    function (error) {
+      if (AppConstants.platform == "win") {
+        return /Failed to create process/.test(error.message);
+      }
+      return error.errorCode == Subprocess.ERROR_BAD_EXECUTABLE;
+    },
+    "Subprocess.call should fail for a bad executable"
+  );
+
+  // Test with a nonexistent file.
+  promise = Subprocess.call({
+    command: textFile + ".doesNotExist",
+    arguments: [],
+  });
+
+  await Assert.rejects(
+    promise,
+    function (error) {
+      return error.errorCode == Subprocess.ERROR_BAD_EXECUTABLE;
+    },
+    "Subprocess.call should fail for a bad executable"
+  );
+});
+
+add_task(async function test_cleanup() {
+  let { getSubprocessImplForTest } = ChromeUtils.importESModule(
+    "resource://gre/modules/Subprocess.sys.mjs"
+  );
+
+  let worker = getSubprocessImplForTest().Process.getWorker();
+
+  let openFiles = await worker.call("getOpenFiles", []);
+  let processes = await worker.call("getProcesses", []);
+
+  equal(openFiles.size, 0, "No remaining open files");
+  equal(processes.size, 0, "No remaining processes");
+});
diff --git a/toolkit/modules/subprocess/test/xpcshell/test_subprocess_getEnvironment.js b/toolkit/modules/subprocess/test/xpcshell/test_subprocess_getEnvironment.js
new file mode 100644
index 0000000000..cb4ea7247d
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/test_subprocess_getEnvironment.js
@@ -0,0 +1,15 @@
+"use strict";
+
+add_task(async function test_getEnvironment() {
+  Services.env.set("FOO", "BAR");
+
+  let environment = Subprocess.getEnvironment();
+
+  equal(environment.FOO, "BAR");
+  equal(environment.PATH, Services.env.get("PATH"));
+
+  Services.env.set("FOO", null);
+
+  environment = Subprocess.getEnvironment();
+  equal(environment.FOO || "", "");
+});
diff --git a/toolkit/modules/subprocess/test/xpcshell/test_subprocess_pathSearch.js b/toolkit/modules/subprocess/test/xpcshell/test_subprocess_pathSearch.js
new file mode 100644
index 0000000000..9d9374f536
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/test_subprocess_pathSearch.js
@@ -0,0 +1,87 @@
+"use strict";
+
+const PYTHON = Services.env.get("PYTHON");
+
+const PYTHON_BIN = PathUtils.filename(PYTHON);
+const PYTHON_DIR = PathUtils.parent(PYTHON);
+
+const DOES_NOT_EXIST = PathUtils.join(
+  PathUtils.tempDir,
+  "ThisPathDoesNotExist"
+);
+
+const PATH_SEP = AppConstants.platform == "win" ? ";" : ":";
+
+add_task(async function test_pathSearchAbsolute() {
+  let env = {};
+
+  let path = await Subprocess.pathSearch(PYTHON, env);
+  equal(path, PYTHON, "Full path resolves even with no PATH.");
+
+  env.PATH = "";
+  path = await Subprocess.pathSearch(PYTHON, env);
+  equal(path, PYTHON, "Full path resolves even with empty PATH.");
+
+  await Assert.rejects(
+    Subprocess.pathSearch(DOES_NOT_EXIST, env),
+    function (e) {
+      equal(
+        e.errorCode,
+        Subprocess.ERROR_BAD_EXECUTABLE,
+        "Got the expected error code"
+      );
+      return /File at path .* does not exist, or is not (executable|a normal file)/.test(
+        e.message
+      );
+    },
+    "Absolute path should throw for a nonexistent execuable"
+  );
+});
+
+add_task(async function test_pathSearchRelative() {
+  let env = {};
+
+  await Assert.rejects(
+    Subprocess.pathSearch(PYTHON_BIN, env),
+    function (e) {
+      equal(
+        e.errorCode,
+        Subprocess.ERROR_BAD_EXECUTABLE,
+        "Got the expected error code"
+      );
+      return /Executable not found:/.test(e.message);
+    },
+    "Relative path should not be found when PATH is missing"
+  );
+
+  env.PATH = [DOES_NOT_EXIST, PYTHON_DIR].join(PATH_SEP);
+
+  let path = await Subprocess.pathSearch(PYTHON_BIN, env);
+  equal(path, PYTHON, "Correct executable should be found in the path");
+});
+
+add_task(
+  {
+    skip_if: () => AppConstants.platform != "win",
+  },
+  async function test_pathSearch_PATHEXT() {
+    ok(PYTHON_BIN.endsWith(".exe"), "Python executable must end with .exe");
+
+    const python_bin = PYTHON_BIN.slice(0, -4);
+
+    let env = {
+      PATH: PYTHON_DIR,
+      PATHEXT: [".com", ".exe", ".foobar"].join(";"),
+    };
+
+    let path = await Subprocess.pathSearch(python_bin, env);
+    equal(
+      path,
+      PYTHON,
+      "Correct executable should be found in the path, with guessed extension"
+    );
+  }
+);
+// IMPORTANT: Do not add any tests beyond this point without removing
+// the `skip_if` condition from the previous task, or it will prevent
+// all succeeding tasks from running when it does not match.
diff --git a/toolkit/modules/subprocess/test/xpcshell/xpcshell.ini b/toolkit/modules/subprocess/test/xpcshell/xpcshell.ini
new file mode 100644
index 0000000000..ffed6b6de0
--- /dev/null
+++ b/toolkit/modules/subprocess/test/xpcshell/xpcshell.ini
@@ -0,0 +1,16 @@
+[DEFAULT]
+head = head.js
+firefox-appdir = browser
+skip-if = os == 'android'
+subprocess = true
+support-files =
+  data_text_file.txt
+  data_test_script.py
+
+[test_subprocess.js]
+skip-if =
+  verify
+  apple_silicon # bug 1729546
+run-sequentially = very high failure rate in parallel
+[test_subprocess_getEnvironment.js]
+[test_subprocess_pathSearch.js]