Adding upstream version 115.7.0esr.upstream/115.7.0esr

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

14 files changed, 2197 insertions, 0 deletions

diff --git a/third_party/rust/jobserver/.cargo-checksum.json b/third_party/rust/jobserver/.cargo-checksum.json
new file mode 100644
index 0000000000..7ee2166816
--- /dev/null
+++ b/third_party/rust/jobserver/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"Cargo.toml":"8bdb529d87cf3cd9eed1909a2f32216ea34caa2f30ac552f57cb29a1bdad5568","LICENSE-APACHE":"a60eea817514531668d7e00765731449fe14d059d3249e0bc93b36de45f759f2","LICENSE-MIT":"378f5840b258e2779c39418f3f2d7b2ba96f1c7917dd6be0713f88305dbda397","README.md":"07d8d79f8f6b6a94321fe8db78d26ed409de47cee49290947bd6bbfa29d05e9c","src/lib.rs":"4e841d58f95ea5f5cd4f975d0ddefaeef4dc364717cda16d6f0b908850b31ab8","src/unix.rs":"68b089c8c96ee0f8ac867db66520f0bd05cb6ed818f9f3c06aa1a079c9379aec","src/wasm.rs":"65d3d8ed45972b4459581505906481d32a50d2f7514cd7ff2a595fceeaa672f0","src/windows.rs":"8e0fa3ab29757d809d4fa03c8101870435ce8c4ceaebe491df3144d62fe0aaaf","tests/client-of-myself.rs":"ca09bf398f69df4bac1730999e954dbbc3faf3c6512678c136e0938e7e9cd0ab","tests/client.rs":"d4745cdd650c86d19bc81f6c9b35df498996deffb86ae6412ad040af96a19183","tests/helper.rs":"c0e6c00eaf849295d8ec23e374690b6645c0f7d993e91abf7ad53ac960f71762","tests/make-as-a-client.rs":"8be1f3fef1e9e65c7904dbaa04364bf0f44e9deab84a2a247a5a94b5cf0df9bc","tests/server.rs":"da15bf12e1df1883f660892b996c9e0d92485aace3f7b50ee70c4a8e6deae8da"},"package":"068b1ee6743e4d11fb9c6a1e6064b3693a1b600e7f5f5988047d98b3dc9fb90b"}
\ No newline at end of file
diff --git a/third_party/rust/jobserver/Cargo.toml b/third_party/rust/jobserver/Cargo.toml
new file mode 100644
index 0000000000..c1d0481e97
--- /dev/null
+++ b/third_party/rust/jobserver/Cargo.toml
@@ -0,0 +1,65 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+name = "jobserver"
+version = "0.1.25"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+description = """
+An implementation of the GNU make jobserver for Rust
+"""
+homepage = "https://github.com/alexcrichton/jobserver-rs"
+documentation = "https://docs.rs/jobserver"
+readme = "README.md"
+license = "MIT/Apache-2.0"
+repository = "https://github.com/alexcrichton/jobserver-rs"
+
+[[test]]
+name = "client"
+path = "tests/client.rs"
+harness = false
+
+[[test]]
+name = "server"
+path = "tests/server.rs"
+
+[[test]]
+name = "client-of-myself"
+path = "tests/client-of-myself.rs"
+harness = false
+
+[[test]]
+name = "make-as-a-client"
+path = "tests/make-as-a-client.rs"
+harness = false
+
+[[test]]
+name = "helper"
+path = "tests/helper.rs"
+
+[dev-dependencies.futures]
+version = "0.1"
+
+[dev-dependencies.num_cpus]
+version = "1.0"
+
+[dev-dependencies.tempfile]
+version = "3"
+
+[dev-dependencies.tokio-core]
+version = "0.1"
+
+[dev-dependencies.tokio-process]
+version = "0.2"
+
+[target."cfg(unix)".dependencies.libc]
+version = "0.2.50"
diff --git a/third_party/rust/jobserver/LICENSE-APACHE b/third_party/rust/jobserver/LICENSE-APACHE
new file mode 100644
index 0000000000..16fe87b06e
--- /dev/null
+++ b/third_party/rust/jobserver/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
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+Licensed under the Apache License, Version 2.0 (the "License");
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diff --git a/third_party/rust/jobserver/LICENSE-MIT b/third_party/rust/jobserver/LICENSE-MIT
new file mode 100644
index 0000000000..39e0ed6602
--- /dev/null
+++ b/third_party/rust/jobserver/LICENSE-MIT
@@ -0,0 +1,25 @@
+Copyright (c) 2014 Alex Crichton
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
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+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/third_party/rust/jobserver/README.md b/third_party/rust/jobserver/README.md
new file mode 100644
index 0000000000..7b06c3eb0a
--- /dev/null
+++ b/third_party/rust/jobserver/README.md
@@ -0,0 +1,39 @@
+# jobserver-rs
+
+An implementation of the GNU make jobserver for Rust
+
+[![Crates.io](https://img.shields.io/crates/v/jobserver.svg?maxAge=2592000)](https://crates.io/crates/jobserver)
+
+[Documentation](https://docs.rs/jobserver)
+
+## Usage
+
+First, add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+jobserver = "0.1"
+```
+
+Next, add this to your crate:
+
+```rust
+extern crate jobserver;
+```
+
+# License
+
+This project is licensed under either of
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+   http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+   http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in jobserver-rs by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
diff --git a/third_party/rust/jobserver/src/lib.rs b/third_party/rust/jobserver/src/lib.rs
new file mode 100644
index 0000000000..6d07884b18
--- /dev/null
+++ b/third_party/rust/jobserver/src/lib.rs
@@ -0,0 +1,541 @@
+//! An implementation of the GNU make jobserver.
+//!
+//! This crate is an implementation, in Rust, of the GNU `make` jobserver for
+//! CLI tools that are interoperating with make or otherwise require some form
+//! of parallelism limiting across process boundaries. This was originally
+//! written for usage in Cargo to both (a) work when `cargo` is invoked from
+//! `make` (using `make`'s jobserver) and (b) work when `cargo` invokes build
+//! scripts, exporting a jobserver implementation for `make` processes to
+//! transitively use.
+//!
+//! The jobserver implementation can be found in [detail online][docs] but
+//! basically boils down to a cross-process semaphore. On Unix this is
+//! implemented with the `pipe` syscall and read/write ends of a pipe and on
+//! Windows this is implemented literally with IPC semaphores.
+//!
+//! The jobserver protocol in `make` also dictates when tokens are acquired to
+//! run child work, and clients using this crate should take care to implement
+//! such details to ensure correct interoperation with `make` itself.
+//!
+//! ## Examples
+//!
+//! Connect to a jobserver that was set up by `make` or a different process:
+//!
+//! ```no_run
+//! use jobserver::Client;
+//!
+//! // See API documentation for why this is `unsafe`
+//! let client = match unsafe { Client::from_env() } {
+//!     Some(client) => client,
+//!     None => panic!("client not configured"),
+//! };
+//! ```
+//!
+//! Acquire and release token from a jobserver:
+//!
+//! ```no_run
+//! use jobserver::Client;
+//!
+//! let client = unsafe { Client::from_env().unwrap() };
+//! let token = client.acquire().unwrap(); // blocks until it is available
+//! drop(token); // releases the token when the work is done
+//! ```
+//!
+//! Create a new jobserver and configure a child process to have access:
+//!
+//! ```
+//! use std::process::Command;
+//! use jobserver::Client;
+//!
+//! let client = Client::new(4).expect("failed to create jobserver");
+//! let mut cmd = Command::new("make");
+//! client.configure(&mut cmd);
+//! ```
+//!
+//! ## Caveats
+//!
+//! This crate makes no attempt to release tokens back to a jobserver on
+//! abnormal exit of a process. If a process which acquires a token is killed
+//! with ctrl-c or some similar signal then tokens will not be released and the
+//! jobserver may be in a corrupt state.
+//!
+//! Note that this is typically ok as ctrl-c means that an entire build process
+//! is being torn down, but it's worth being aware of at least!
+//!
+//! ## Windows caveats
+//!
+//! There appear to be two implementations of `make` on Windows. On MSYS2 one
+//! typically comes as `mingw32-make` and the other as `make` itself. I'm not
+//! personally too familiar with what's going on here, but for jobserver-related
+//! information the `mingw32-make` implementation uses Windows semaphores
+//! whereas the `make` program does not. The `make` program appears to use file
+//! descriptors and I'm not really sure how it works, so this crate is not
+//! compatible with `make` on Windows. It is, however, compatible with
+//! `mingw32-make`.
+//!
+//! [docs]: http://make.mad-scientist.net/papers/jobserver-implementation/
+
+#![deny(missing_docs, missing_debug_implementations)]
+#![doc(html_root_url = "https://docs.rs/jobserver/0.1")]
+
+use std::env;
+use std::io;
+use std::process::Command;
+use std::sync::{Arc, Condvar, Mutex, MutexGuard};
+
+#[cfg(unix)]
+#[path = "unix.rs"]
+mod imp;
+#[cfg(windows)]
+#[path = "windows.rs"]
+mod imp;
+#[cfg(not(any(unix, windows)))]
+#[path = "wasm.rs"]
+mod imp;
+
+/// A client of a jobserver
+///
+/// This structure is the main type exposed by this library, and is where
+/// interaction to a jobserver is configured through. Clients are either created
+/// from scratch in which case the internal semphore is initialied on the spot,
+/// or a client is created from the environment to connect to a jobserver
+/// already created.
+///
+/// Some usage examples can be found in the crate documentation for using a
+/// client.
+///
+/// Note that a `Client` implements the `Clone` trait, and all instances of a
+/// `Client` refer to the same jobserver instance.
+#[derive(Clone, Debug)]
+pub struct Client {
+    inner: Arc<imp::Client>,
+}
+
+/// An acquired token from a jobserver.
+///
+/// This token will be released back to the jobserver when it is dropped and
+/// otherwise represents the ability to spawn off another thread of work.
+#[derive(Debug)]
+pub struct Acquired {
+    client: Arc<imp::Client>,
+    data: imp::Acquired,
+    disabled: bool,
+}
+
+impl Acquired {
+    /// This drops the `Acquired` token without releasing the associated token.
+    ///
+    /// This is not generally useful, but can be helpful if you do not have the
+    /// ability to store an Acquired token but need to not yet release it.
+    ///
+    /// You'll typically want to follow this up with a call to `release_raw` or
+    /// similar to actually release the token later on.
+    pub fn drop_without_releasing(mut self) {
+        self.disabled = true;
+    }
+}
+
+#[derive(Default, Debug)]
+struct HelperState {
+    lock: Mutex<HelperInner>,
+    cvar: Condvar,
+}
+
+#[derive(Default, Debug)]
+struct HelperInner {
+    requests: usize,
+    producer_done: bool,
+    consumer_done: bool,
+}
+
+impl Client {
+    /// Creates a new jobserver initialized with the given parallelism limit.
+    ///
+    /// A client to the jobserver created will be returned. This client will
+    /// allow at most `limit` tokens to be acquired from it in parallel. More
+    /// calls to `acquire` will cause the calling thread to block.
+    ///
+    /// Note that the created `Client` is not automatically inherited into
+    /// spawned child processes from this program. Manual usage of the
+    /// `configure` function is required for a child process to have access to a
+    /// job server.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use jobserver::Client;
+    ///
+    /// let client = Client::new(4).expect("failed to create jobserver");
+    /// ```
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if any I/O error happens when attempting to create the
+    /// jobserver client.
+    pub fn new(limit: usize) -> io::Result<Client> {
+        Ok(Client {
+            inner: Arc::new(imp::Client::new(limit)?),
+        })
+    }
+
+    /// Attempts to connect to the jobserver specified in this process's
+    /// environment.
+    ///
+    /// When the a `make` executable calls a child process it will configure the
+    /// environment of the child to ensure that it has handles to the jobserver
+    /// it's passing down. This function will attempt to look for these details
+    /// and connect to the jobserver.
+    ///
+    /// Note that the created `Client` is not automatically inherited into
+    /// spawned child processes from this program. Manual usage of the
+    /// `configure` function is required for a child process to have access to a
+    /// job server.
+    ///
+    /// # Return value
+    ///
+    /// If a jobserver was found in the environment and it looks correct then
+    /// `Some` of the connected client will be returned. If no jobserver was
+    /// found then `None` will be returned.
+    ///
+    /// Note that on Unix the `Client` returned **takes ownership of the file
+    /// descriptors specified in the environment**. Jobservers on Unix are
+    /// implemented with `pipe` file descriptors, and they're inherited from
+    /// parent processes. This `Client` returned takes ownership of the file
+    /// descriptors for this process and will close the file descriptors after
+    /// this value is dropped.
+    ///
+    /// Additionally on Unix this function will configure the file descriptors
+    /// with `CLOEXEC` so they're not automatically inherited by spawned
+    /// children.
+    ///
+    /// # Safety
+    ///
+    /// This function is `unsafe` to call on Unix specifically as it
+    /// transitively requires usage of the `from_raw_fd` function, which is
+    /// itself unsafe in some circumstances.
+    ///
+    /// It's recommended to call this function very early in the lifetime of a
+    /// program before any other file descriptors are opened. That way you can
+    /// make sure to take ownership properly of the file descriptors passed
+    /// down, if any.
+    ///
+    /// It's generally unsafe to call this function twice in a program if the
+    /// previous invocation returned `Some`.
+    ///
+    /// Note, though, that on Windows it should be safe to call this function
+    /// any number of times.
+    pub unsafe fn from_env() -> Option<Client> {
+        let var = match env::var("CARGO_MAKEFLAGS")
+            .or_else(|_| env::var("MAKEFLAGS"))
+            .or_else(|_| env::var("MFLAGS"))
+        {
+            Ok(s) => s,
+            Err(_) => return None,
+        };
+        let mut arg = "--jobserver-fds=";
+        let pos = match var.find(arg) {
+            Some(i) => i,
+            None => {
+                arg = "--jobserver-auth=";
+                match var.find(arg) {
+                    Some(i) => i,
+                    None => return None,
+                }
+            }
+        };
+
+        let s = var[pos + arg.len()..].split(' ').next().unwrap();
+        imp::Client::open(s).map(|c| Client { inner: Arc::new(c) })
+    }
+
+    /// Acquires a token from this jobserver client.
+    ///
+    /// This function will block the calling thread until a new token can be
+    /// acquired from the jobserver.
+    ///
+    /// # Return value
+    ///
+    /// On successful acquisition of a token an instance of `Acquired` is
+    /// returned. This structure, when dropped, will release the token back to
+    /// the jobserver. It's recommended to avoid leaking this value.
+    ///
+    /// # Errors
+    ///
+    /// If an I/O error happens while acquiring a token then this function will
+    /// return immediately with the error. If an error is returned then a token
+    /// was not acquired.
+    pub fn acquire(&self) -> io::Result<Acquired> {
+        let data = self.inner.acquire()?;
+        Ok(Acquired {
+            client: self.inner.clone(),
+            data,
+            disabled: false,
+        })
+    }
+
+    /// Returns amount of tokens in the read-side pipe.
+    ///
+    /// # Return value
+    ///
+    /// Number of bytes available to be read from the jobserver pipe
+    ///
+    /// # Errors
+    ///
+    /// Underlying errors from the ioctl will be passed up.
+    pub fn available(&self) -> io::Result<usize> {
+        self.inner.available()
+    }
+
+    /// Configures a child process to have access to this client's jobserver as
+    /// well.
+    ///
+    /// This function is required to be called to ensure that a jobserver is
+    /// properly inherited to a child process. If this function is *not* called
+    /// then this `Client` will not be accessible in the child process. In other
+    /// words, if not called, then `Client::from_env` will return `None` in the
+    /// child process (or the equivalent of `Child::from_env` that `make` uses).
+    ///
+    /// ## Platform-specific behavior
+    ///
+    /// On Unix and Windows this will clobber the `CARGO_MAKEFLAGS` environment
+    /// variables for the child process, and on Unix this will also allow the
+    /// two file descriptors for this client to be inherited to the child.
+    ///
+    /// On platforms other than Unix and Windows this panics.
+    pub fn configure(&self, cmd: &mut Command) {
+        cmd.env("CARGO_MAKEFLAGS", &self.mflags_env());
+        self.inner.configure(cmd);
+    }
+
+    /// Configures a child process to have access to this client's jobserver as
+    /// well.
+    ///
+    /// This function is required to be called to ensure that a jobserver is
+    /// properly inherited to a child process. If this function is *not* called
+    /// then this `Client` will not be accessible in the child process. In other
+    /// words, if not called, then `Client::from_env` will return `None` in the
+    /// child process (or the equivalent of `Child::from_env` that `make` uses).
+    ///
+    /// ## Platform-specific behavior
+    ///
+    /// On Unix and Windows this will clobber the `CARGO_MAKEFLAGS`,
+    /// `MAKEFLAGS` and `MFLAGS` environment variables for the child process,
+    /// and on Unix this will also allow the two file descriptors for
+    /// this client to be inherited to the child.
+    ///
+    /// On platforms other than Unix and Windows this panics.
+    pub fn configure_make(&self, cmd: &mut Command) {
+        let value = self.mflags_env();
+        cmd.env("CARGO_MAKEFLAGS", &value);
+        cmd.env("MAKEFLAGS", &value);
+        cmd.env("MFLAGS", &value);
+        self.inner.configure(cmd);
+    }
+
+    fn mflags_env(&self) -> String {
+        let arg = self.inner.string_arg();
+        // Older implementations of make use `--jobserver-fds` and newer
+        // implementations use `--jobserver-auth`, pass both to try to catch
+        // both implementations.
+        format!("-j --jobserver-fds={0} --jobserver-auth={0}", arg)
+    }
+
+    /// Converts this `Client` into a helper thread to deal with a blocking
+    /// `acquire` function a little more easily.
+    ///
+    /// The fact that the `acquire` function on `Client` blocks isn't always
+    /// the easiest to work with. Typically you're using a jobserver to
+    /// manage running other events in parallel! This means that you need to
+    /// either (a) wait for an existing job to finish or (b) wait for a
+    /// new token to become available.
+    ///
+    /// Unfortunately the blocking in `acquire` happens at the implementation
+    /// layer of jobservers. On Unix this requires a blocking call to `read`
+    /// and on Windows this requires one of the `WaitFor*` functions. Both
+    /// of these situations aren't the easiest to deal with:
+    ///
+    /// * On Unix there's basically only one way to wake up a `read` early, and
+    ///   that's through a signal. This is what the `make` implementation
+    ///   itself uses, relying on `SIGCHLD` to wake up a blocking acquisition
+    ///   of a new job token. Unfortunately nonblocking I/O is not an option
+    ///   here, so it means that "waiting for one of two events" means that
+    ///   the latter event must generate a signal! This is not always the case
+    ///   on unix for all jobservers.
+    ///
+    /// * On Windows you'd have to basically use the `WaitForMultipleObjects`
+    ///   which means that you've got to canonicalize all your event sources
+    ///   into a `HANDLE` which also isn't the easiest thing to do
+    ///   unfortunately.
+    ///
+    /// This function essentially attempts to ease these limitations by
+    /// converting this `Client` into a helper thread spawned into this
+    /// process. The application can then request that the helper thread
+    /// acquires tokens and the provided closure will be invoked for each token
+    /// acquired.
+    ///
+    /// The intention is that this function can be used to translate the event
+    /// of a token acquisition into an arbitrary user-defined event.
+    ///
+    /// # Arguments
+    ///
+    /// This function will consume the `Client` provided to be transferred to
+    /// the helper thread that is spawned. Additionally a closure `f` is
+    /// provided to be invoked whenever a token is acquired.
+    ///
+    /// This closure is only invoked after calls to
+    /// `HelperThread::request_token` have been made and a token itself has
+    /// been acquired. If an error happens while acquiring the token then
+    /// an error will be yielded to the closure as well.
+    ///
+    /// # Return Value
+    ///
+    /// This function will return an instance of the `HelperThread` structure
+    /// which is used to manage the helper thread associated with this client.
+    /// Through the `HelperThread` you'll request that tokens are acquired.
+    /// When acquired, the closure provided here is invoked.
+    ///
+    /// When the `HelperThread` structure is returned it will be gracefully
+    /// torn down, and the calling thread will be blocked until the thread is
+    /// torn down (which should be prompt).
+    ///
+    /// # Errors
+    ///
+    /// This function may fail due to creation of the helper thread or
+    /// auxiliary I/O objects to manage the helper thread. In any of these
+    /// situations the error is propagated upwards.
+    ///
+    /// # Platform-specific behavior
+    ///
+    /// On Windows this function behaves pretty normally as expected, but on
+    /// Unix the implementation is... a little heinous. As mentioned above
+    /// we're forced into blocking I/O for token acquisition, namely a blocking
+    /// call to `read`. We must be able to unblock this, however, to tear down
+    /// the helper thread gracefully!
+    ///
+    /// Essentially what happens is that we'll send a signal to the helper
+    /// thread spawned and rely on `EINTR` being returned to wake up the helper
+    /// thread. This involves installing a global `SIGUSR1` handler that does
+    /// nothing along with sending signals to that thread. This may cause
+    /// odd behavior in some applications, so it's recommended to review and
+    /// test thoroughly before using this.
+    pub fn into_helper_thread<F>(self, f: F) -> io::Result<HelperThread>
+    where
+        F: FnMut(io::Result<Acquired>) + Send + 'static,
+    {
+        let state = Arc::new(HelperState::default());
+        Ok(HelperThread {
+            inner: Some(imp::spawn_helper(self, state.clone(), Box::new(f))?),
+            state,
+        })
+    }
+
+    /// Blocks the current thread until a token is acquired.
+    ///
+    /// This is the same as `acquire`, except that it doesn't return an RAII
+    /// helper. If successful the process will need to guarantee that
+    /// `release_raw` is called in the future.
+    pub fn acquire_raw(&self) -> io::Result<()> {
+        self.inner.acquire()?;
+        Ok(())
+    }
+
+    /// Releases a jobserver token back to the original jobserver.
+    ///
+    /// This is intended to be paired with `acquire_raw` if it was called, but
+    /// in some situations it could also be called to relinquish a process's
+    /// implicit token temporarily which is then re-acquired later.
+    pub fn release_raw(&self) -> io::Result<()> {
+        self.inner.release(None)?;
+        Ok(())
+    }
+}
+
+impl Drop for Acquired {
+    fn drop(&mut self) {
+        if !self.disabled {
+            drop(self.client.release(Some(&self.data)));
+        }
+    }
+}
+
+/// Structure returned from `Client::into_helper_thread` to manage the lifetime
+/// of the helper thread returned, see those associated docs for more info.
+#[derive(Debug)]
+pub struct HelperThread {
+    inner: Option<imp::Helper>,
+    state: Arc<HelperState>,
+}
+
+impl HelperThread {
+    /// Request that the helper thread acquires a token, eventually calling the
+    /// original closure with a token when it's available.
+    ///
+    /// For more information, see the docs on that function.
+    pub fn request_token(&self) {
+        // Indicate that there's one more request for a token and then wake up
+        // the helper thread if it's sleeping.
+        self.state.lock().requests += 1;
+        self.state.cvar.notify_one();
+    }
+}
+
+impl Drop for HelperThread {
+    fn drop(&mut self) {
+        // Flag that the producer half is done so the helper thread should exit
+        // quickly if it's waiting. Wake it up if it's actually waiting
+        self.state.lock().producer_done = true;
+        self.state.cvar.notify_one();
+
+        // ... and afterwards perform any thread cleanup logic
+        self.inner.take().unwrap().join();
+    }
+}
+
+impl HelperState {
+    fn lock(&self) -> MutexGuard<'_, HelperInner> {
+        self.lock.lock().unwrap_or_else(|e| e.into_inner())
+    }
+
+    /// Executes `f` for each request for a token, where `f` is expected to
+    /// block and then provide the original closure with a token once it's
+    /// acquired.
+    ///
+    /// This is an infinite loop until the helper thread is dropped, at which
+    /// point everything should get interrupted.
+    fn for_each_request(&self, mut f: impl FnMut(&HelperState)) {
+        let mut lock = self.lock();
+
+        // We only execute while we could receive requests, but as soon as
+        // that's `false` we're out of here.
+        while !lock.producer_done {
+            // If no one's requested a token then we wait for someone to
+            // request a token.
+            if lock.requests == 0 {
+                lock = self.cvar.wait(lock).unwrap_or_else(|e| e.into_inner());
+                continue;
+            }
+
+            // Consume the request for a token, and then actually acquire a
+            // token after unlocking our lock (not that acquisition happens in
+            // `f`). This ensures that we don't actually hold the lock if we
+            // wait for a long time for a token.
+            lock.requests -= 1;
+            drop(lock);
+            f(self);
+            lock = self.lock();
+        }
+        lock.consumer_done = true;
+        self.cvar.notify_one();
+    }
+
+    fn producer_done(&self) -> bool {
+        self.lock().producer_done
+    }
+}
+
+#[test]
+fn no_helper_deadlock() {
+    let x = crate::Client::new(32).unwrap();
+    let _y = x.clone();
+    std::mem::drop(x.into_helper_thread(|_| {}).unwrap());
+}
diff --git a/third_party/rust/jobserver/src/unix.rs b/third_party/rust/jobserver/src/unix.rs
new file mode 100644
index 0000000000..2474deefc5
--- /dev/null
+++ b/third_party/rust/jobserver/src/unix.rs
@@ -0,0 +1,368 @@
+use libc::c_int;
+
+use std::fs::File;
+use std::io::{self, Read, Write};
+use std::mem;
+use std::mem::MaybeUninit;
+use std::os::unix::prelude::*;
+use std::process::Command;
+use std::ptr;
+use std::sync::{Arc, Once};
+use std::thread::{self, Builder, JoinHandle};
+use std::time::Duration;
+
+#[derive(Debug)]
+pub struct Client {
+    read: File,
+    write: File,
+}
+
+#[derive(Debug)]
+pub struct Acquired {
+    byte: u8,
+}
+
+impl Client {
+    pub fn new(mut limit: usize) -> io::Result<Client> {
+        let client = unsafe { Client::mk()? };
+
+        // I don't think the character written here matters, but I could be
+        // wrong!
+        const BUFFER: [u8; 128] = [b'|'; 128];
+
+        set_nonblocking(client.write.as_raw_fd(), true)?;
+
+        while limit > 0 {
+            let n = limit.min(BUFFER.len());
+
+            (&client.write).write_all(&BUFFER[..n])?;
+            limit -= n;
+        }
+
+        set_nonblocking(client.write.as_raw_fd(), false)?;
+
+        Ok(client)
+    }
+
+    unsafe fn mk() -> io::Result<Client> {
+        let mut pipes = [0; 2];
+
+        // Attempt atomically-create-with-cloexec if we can on Linux,
+        // detected by using the `syscall` function in `libc` to try to work
+        // with as many kernels/glibc implementations as possible.
+        #[cfg(target_os = "linux")]
+        {
+            use std::sync::atomic::{AtomicBool, Ordering};
+
+            static PIPE2_AVAILABLE: AtomicBool = AtomicBool::new(true);
+            if PIPE2_AVAILABLE.load(Ordering::SeqCst) {
+                match libc::syscall(libc::SYS_pipe2, pipes.as_mut_ptr(), libc::O_CLOEXEC) {
+                    -1 => {
+                        let err = io::Error::last_os_error();
+                        if err.raw_os_error() == Some(libc::ENOSYS) {
+                            PIPE2_AVAILABLE.store(false, Ordering::SeqCst);
+                        } else {
+                            return Err(err);
+                        }
+                    }
+                    _ => return Ok(Client::from_fds(pipes[0], pipes[1])),
+                }
+            }
+        }
+
+        cvt(libc::pipe(pipes.as_mut_ptr()))?;
+        drop(set_cloexec(pipes[0], true));
+        drop(set_cloexec(pipes[1], true));
+        Ok(Client::from_fds(pipes[0], pipes[1]))
+    }
+
+    pub unsafe fn open(s: &str) -> Option<Client> {
+        let mut parts = s.splitn(2, ',');
+        let read = parts.next().unwrap();
+        let write = match parts.next() {
+            Some(s) => s,
+            None => return None,
+        };
+
+        let read = match read.parse() {
+            Ok(n) => n,
+            Err(_) => return None,
+        };
+        let write = match write.parse() {
+            Ok(n) => n,
+            Err(_) => return None,
+        };
+
+        // Ok so we've got two integers that look like file descriptors, but
+        // for extra sanity checking let's see if they actually look like
+        // instances of a pipe before we return the client.
+        //
+        // If we're called from `make` *without* the leading + on our rule
+        // then we'll have `MAKEFLAGS` env vars but won't actually have
+        // access to the file descriptors.
+        if is_valid_fd(read) && is_valid_fd(write) {
+            drop(set_cloexec(read, true));
+            drop(set_cloexec(write, true));
+            Some(Client::from_fds(read, write))
+        } else {
+            None
+        }
+    }
+
+    unsafe fn from_fds(read: c_int, write: c_int) -> Client {
+        Client {
+            read: File::from_raw_fd(read),
+            write: File::from_raw_fd(write),
+        }
+    }
+
+    pub fn acquire(&self) -> io::Result<Acquired> {
+        // Ignore interrupts and keep trying if that happens
+        loop {
+            if let Some(token) = self.acquire_allow_interrupts()? {
+                return Ok(token);
+            }
+        }
+    }
+
+    /// Block waiting for a token, returning `None` if we're interrupted with
+    /// EINTR.
+    fn acquire_allow_interrupts(&self) -> io::Result<Option<Acquired>> {
+        // We don't actually know if the file descriptor here is set in
+        // blocking or nonblocking mode. AFAIK all released versions of
+        // `make` use blocking fds for the jobserver, but the unreleased
+        // version of `make` doesn't. In the unreleased version jobserver
+        // fds are set to nonblocking and combined with `pselect`
+        // internally.
+        //
+        // Here we try to be compatible with both strategies. We optimistically
+        // try to read from the file descriptor which then may block, return
+        // a token or indicate that polling is needed.
+        // Blocking reads (if possible) allows the kernel to be more selective
+        // about which readers to wake up when a token is written to the pipe.
+        //
+        // We use `poll` here to block this thread waiting for read
+        // readiness, and then afterwards we perform the `read` itself. If
+        // the `read` returns that it would block then we start over and try
+        // again.
+        //
+        // Also note that we explicitly don't handle EINTR here. That's used
+        // to shut us down, so we otherwise punt all errors upwards.
+        unsafe {
+            let mut fd: libc::pollfd = mem::zeroed();
+            fd.fd = self.read.as_raw_fd();
+            fd.events = libc::POLLIN;
+            loop {
+                let mut buf = [0];
+                match (&self.read).read(&mut buf) {
+                    Ok(1) => return Ok(Some(Acquired { byte: buf[0] })),
+                    Ok(_) => {
+                        return Err(io::Error::new(
+                            io::ErrorKind::Other,
+                            "early EOF on jobserver pipe",
+                        ))
+                    }
+                    Err(e) => match e.kind() {
+                        io::ErrorKind::WouldBlock => { /* fall through to polling */ }
+                        io::ErrorKind::Interrupted => return Ok(None),
+                        _ => return Err(e),
+                    },
+                }
+
+                loop {
+                    fd.revents = 0;
+                    if libc::poll(&mut fd, 1, -1) == -1 {
+                        let e = io::Error::last_os_error();
+                        return match e.kind() {
+                            io::ErrorKind::Interrupted => Ok(None),
+                            _ => Err(e),
+                        };
+                    }
+                    if fd.revents != 0 {
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
+    pub fn release(&self, data: Option<&Acquired>) -> io::Result<()> {
+        // Note that the fd may be nonblocking but we're going to go ahead
+        // and assume that the writes here are always nonblocking (we can
+        // always quickly release a token). If that turns out to not be the
+        // case we'll get an error anyway!
+        let byte = data.map(|d| d.byte).unwrap_or(b'+');
+        match (&self.write).write(&[byte])? {
+            1 => Ok(()),
+            _ => Err(io::Error::new(
+                io::ErrorKind::Other,
+                "failed to write token back to jobserver",
+            )),
+        }
+    }
+
+    pub fn string_arg(&self) -> String {
+        format!("{},{}", self.read.as_raw_fd(), self.write.as_raw_fd())
+    }
+
+    pub fn available(&self) -> io::Result<usize> {
+        let mut len = MaybeUninit::<c_int>::uninit();
+        cvt(unsafe { libc::ioctl(self.read.as_raw_fd(), libc::FIONREAD, len.as_mut_ptr()) })?;
+        Ok(unsafe { len.assume_init() } as usize)
+    }
+
+    pub fn configure(&self, cmd: &mut Command) {
+        // Here we basically just want to say that in the child process
+        // we'll configure the read/write file descriptors to *not* be
+        // cloexec, so they're inherited across the exec and specified as
+        // integers through `string_arg` above.
+        let read = self.read.as_raw_fd();
+        let write = self.write.as_raw_fd();
+        unsafe {
+            cmd.pre_exec(move || {
+                set_cloexec(read, false)?;
+                set_cloexec(write, false)?;
+                Ok(())
+            });
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Helper {
+    thread: JoinHandle<()>,
+    state: Arc<super::HelperState>,
+}
+
+pub(crate) fn spawn_helper(
+    client: crate::Client,
+    state: Arc<super::HelperState>,
+    mut f: Box<dyn FnMut(io::Result<crate::Acquired>) + Send>,
+) -> io::Result<Helper> {
+    static USR1_INIT: Once = Once::new();
+    let mut err = None;
+    USR1_INIT.call_once(|| unsafe {
+        let mut new: libc::sigaction = mem::zeroed();
+        new.sa_sigaction = sigusr1_handler as usize;
+        new.sa_flags = libc::SA_SIGINFO as _;
+        if libc::sigaction(libc::SIGUSR1, &new, ptr::null_mut()) != 0 {
+            err = Some(io::Error::last_os_error());
+        }
+    });
+
+    if let Some(e) = err.take() {
+        return Err(e);
+    }
+
+    let state2 = state.clone();
+    let thread = Builder::new().spawn(move || {
+        state2.for_each_request(|helper| loop {
+            match client.inner.acquire_allow_interrupts() {
+                Ok(Some(data)) => {
+                    break f(Ok(crate::Acquired {
+                        client: client.inner.clone(),
+                        data,
+                        disabled: false,
+                    }))
+                }
+                Err(e) => break f(Err(e)),
+                Ok(None) if helper.producer_done() => break,
+                Ok(None) => {}
+            }
+        });
+    })?;
+
+    Ok(Helper { thread, state })
+}
+
+impl Helper {
+    pub fn join(self) {
+        let dur = Duration::from_millis(10);
+        let mut state = self.state.lock();
+        debug_assert!(state.producer_done);
+
+        // We need to join our helper thread, and it could be blocked in one
+        // of two locations. First is the wait for a request, but the
+        // initial drop of `HelperState` will take care of that. Otherwise
+        // it may be blocked in `client.acquire()`. We actually have no way
+        // of interrupting that, so resort to `pthread_kill` as a fallback.
+        // This signal should interrupt any blocking `read` call with
+        // `io::ErrorKind::Interrupt` and cause the thread to cleanly exit.
+        //
+        // Note that we don't do this forever though since there's a chance
+        // of bugs, so only do this opportunistically to make a best effort
+        // at clearing ourselves up.
+        for _ in 0..100 {
+            if state.consumer_done {
+                break;
+            }
+            unsafe {
+                // Ignore the return value here of `pthread_kill`,
+                // apparently on OSX if you kill a dead thread it will
+                // return an error, but on other platforms it may not. In
+                // that sense we don't actually know if this will succeed or
+                // not!
+                libc::pthread_kill(self.thread.as_pthread_t() as _, libc::SIGUSR1);
+            }
+            state = self
+                .state
+                .cvar
+                .wait_timeout(state, dur)
+                .unwrap_or_else(|e| e.into_inner())
+                .0;
+            thread::yield_now(); // we really want the other thread to run
+        }
+
+        // If we managed to actually see the consumer get done, then we can
+        // definitely wait for the thread. Otherwise it's... off in the ether
+        // I guess?
+        if state.consumer_done {
+            drop(self.thread.join());
+        }
+    }
+}
+
+fn is_valid_fd(fd: c_int) -> bool {
+    unsafe { libc::fcntl(fd, libc::F_GETFD) != -1 }
+}
+
+fn set_cloexec(fd: c_int, set: bool) -> io::Result<()> {
+    unsafe {
+        let previous = cvt(libc::fcntl(fd, libc::F_GETFD))?;
+        let new = if set {
+            previous | libc::FD_CLOEXEC
+        } else {
+            previous & !libc::FD_CLOEXEC
+        };
+        if new != previous {
+            cvt(libc::fcntl(fd, libc::F_SETFD, new))?;
+        }
+        Ok(())
+    }
+}
+
+fn set_nonblocking(fd: c_int, set: bool) -> io::Result<()> {
+    let status_flag = if set { libc::O_NONBLOCK } else { 0 };
+
+    unsafe {
+        cvt(libc::fcntl(fd, libc::F_SETFL, status_flag))?;
+    }
+
+    Ok(())
+}
+
+fn cvt(t: c_int) -> io::Result<c_int> {
+    if t == -1 {
+        Err(io::Error::last_os_error())
+    } else {
+        Ok(t)
+    }
+}
+
+extern "C" fn sigusr1_handler(
+    _signum: c_int,
+    _info: *mut libc::siginfo_t,
+    _ptr: *mut libc::c_void,
+) {
+    // nothing to do
+}
diff --git a/third_party/rust/jobserver/src/wasm.rs b/third_party/rust/jobserver/src/wasm.rs
new file mode 100644
index 0000000000..3793bd67cc
--- /dev/null
+++ b/third_party/rust/jobserver/src/wasm.rs
@@ -0,0 +1,95 @@
+use std::io;
+use std::process::Command;
+use std::sync::{Arc, Condvar, Mutex};
+use std::thread::{Builder, JoinHandle};
+
+#[derive(Debug)]
+pub struct Client {
+    inner: Arc<Inner>,
+}
+
+#[derive(Debug)]
+struct Inner {
+    count: Mutex<usize>,
+    cvar: Condvar,
+}
+
+#[derive(Debug)]
+pub struct Acquired(());
+
+impl Client {
+    pub fn new(limit: usize) -> io::Result<Client> {
+        Ok(Client {
+            inner: Arc::new(Inner {
+                count: Mutex::new(limit),
+                cvar: Condvar::new(),
+            }),
+        })
+    }
+
+    pub unsafe fn open(_s: &str) -> Option<Client> {
+        None
+    }
+
+    pub fn acquire(&self) -> io::Result<Acquired> {
+        let mut lock = self.inner.count.lock().unwrap_or_else(|e| e.into_inner());
+        while *lock == 0 {
+            lock = self
+                .inner
+                .cvar
+                .wait(lock)
+                .unwrap_or_else(|e| e.into_inner());
+        }
+        *lock -= 1;
+        Ok(Acquired(()))
+    }
+
+    pub fn release(&self, _data: Option<&Acquired>) -> io::Result<()> {
+        let mut lock = self.inner.count.lock().unwrap_or_else(|e| e.into_inner());
+        *lock += 1;
+        drop(lock);
+        self.inner.cvar.notify_one();
+        Ok(())
+    }
+
+    pub fn string_arg(&self) -> String {
+        panic!(
+            "On this platform there is no cross process jobserver support,
+             so Client::configure is not supported."
+        );
+    }
+
+    pub fn available(&self) -> io::Result<usize> {
+        let lock = self.inner.count.lock().unwrap_or_else(|e| e.into_inner());
+        Ok(*lock)
+    }
+
+    pub fn configure(&self, _cmd: &mut Command) {
+        unreachable!();
+    }
+}
+
+#[derive(Debug)]
+pub struct Helper {
+    thread: JoinHandle<()>,
+}
+
+pub(crate) fn spawn_helper(
+    client: crate::Client,
+    state: Arc<super::HelperState>,
+    mut f: Box<dyn FnMut(io::Result<crate::Acquired>) + Send>,
+) -> io::Result<Helper> {
+    let thread = Builder::new().spawn(move || {
+        state.for_each_request(|_| f(client.acquire()));
+    })?;
+
+    Ok(Helper { thread: thread })
+}
+
+impl Helper {
+    pub fn join(self) {
+        // TODO: this is not correct if the thread is blocked in
+        // `client.acquire()`.
+        drop(self.thread.join());
+    }
+}
diff --git a/third_party/rust/jobserver/src/windows.rs b/third_party/rust/jobserver/src/windows.rs
new file mode 100644
index 0000000000..6791efea4f
--- /dev/null
+++ b/third_party/rust/jobserver/src/windows.rs
@@ -0,0 +1,266 @@
+use std::ffi::CString;
+use std::io;
+use std::process::Command;
+use std::ptr;
+use std::sync::Arc;
+use std::thread::{Builder, JoinHandle};
+
+#[derive(Debug)]
+pub struct Client {
+    sem: Handle,
+    name: String,
+}
+
+#[derive(Debug)]
+pub struct Acquired;
+
+type BOOL = i32;
+type DWORD = u32;
+type HANDLE = *mut u8;
+type LONG = i32;
+
+const ERROR_ALREADY_EXISTS: DWORD = 183;
+const FALSE: BOOL = 0;
+const INFINITE: DWORD = 0xffffffff;
+const SEMAPHORE_MODIFY_STATE: DWORD = 0x2;
+const SYNCHRONIZE: DWORD = 0x00100000;
+const TRUE: BOOL = 1;
+const WAIT_OBJECT_0: DWORD = 0;
+
+extern "system" {
+    fn CloseHandle(handle: HANDLE) -> BOOL;
+    fn SetEvent(hEvent: HANDLE) -> BOOL;
+    fn WaitForMultipleObjects(
+        ncount: DWORD,
+        lpHandles: *const HANDLE,
+        bWaitAll: BOOL,
+        dwMilliseconds: DWORD,
+    ) -> DWORD;
+    fn CreateEventA(
+        lpEventAttributes: *mut u8,
+        bManualReset: BOOL,
+        bInitialState: BOOL,
+        lpName: *const i8,
+    ) -> HANDLE;
+    fn ReleaseSemaphore(
+        hSemaphore: HANDLE,
+        lReleaseCount: LONG,
+        lpPreviousCount: *mut LONG,
+    ) -> BOOL;
+    fn CreateSemaphoreA(
+        lpEventAttributes: *mut u8,
+        lInitialCount: LONG,
+        lMaximumCount: LONG,
+        lpName: *const i8,
+    ) -> HANDLE;
+    fn OpenSemaphoreA(dwDesiredAccess: DWORD, bInheritHandle: BOOL, lpName: *const i8) -> HANDLE;
+    fn WaitForSingleObject(hHandle: HANDLE, dwMilliseconds: DWORD) -> DWORD;
+    #[link_name = "SystemFunction036"]
+    fn RtlGenRandom(RandomBuffer: *mut u8, RandomBufferLength: u32) -> u8;
+}
+
+// Note that we ideally would use the `getrandom` crate, but unfortunately
+// that causes build issues when this crate is used in rust-lang/rust (see
+// rust-lang/rust#65014 for more information). As a result we just inline
+// the pretty simple Windows-specific implementation of generating
+// randomness.
+fn getrandom(dest: &mut [u8]) -> io::Result<()> {
+    // Prevent overflow of u32
+    for chunk in dest.chunks_mut(u32::max_value() as usize) {
+        let ret = unsafe { RtlGenRandom(chunk.as_mut_ptr(), chunk.len() as u32) };
+        if ret == 0 {
+            return Err(io::Error::new(
+                io::ErrorKind::Other,
+                "failed to generate random bytes",
+            ));
+        }
+    }
+    Ok(())
+}
+
+impl Client {
+    pub fn new(limit: usize) -> io::Result<Client> {
+        // Try a bunch of random semaphore names until we get a unique one,
+        // but don't try for too long.
+        //
+        // Note that `limit == 0` is a valid argument above but Windows
+        // won't let us create a semaphore with 0 slots available to it. Get
+        // `limit == 0` working by creating a semaphore instead with one
+        // slot and then immediately acquire it (without ever releaseing it
+        // back).
+        for _ in 0..100 {
+            let mut bytes = [0; 4];
+            getrandom(&mut bytes)?;
+            let mut name = format!("__rust_jobserver_semaphore_{}\0", u32::from_ne_bytes(bytes));
+            unsafe {
+                let create_limit = if limit == 0 { 1 } else { limit };
+                let r = CreateSemaphoreA(
+                    ptr::null_mut(),
+                    create_limit as LONG,
+                    create_limit as LONG,
+                    name.as_ptr() as *const _,
+                );
+                if r.is_null() {
+                    return Err(io::Error::last_os_error());
+                }
+                let handle = Handle(r);
+
+                let err = io::Error::last_os_error();
+                if err.raw_os_error() == Some(ERROR_ALREADY_EXISTS as i32) {
+                    continue;
+                }
+                name.pop(); // chop off the trailing nul
+                let client = Client {
+                    sem: handle,
+                    name: name,
+                };
+                if create_limit != limit {
+                    client.acquire()?;
+                }
+                return Ok(client);
+            }
+        }
+
+        Err(io::Error::new(
+            io::ErrorKind::Other,
+            "failed to find a unique name for a semaphore",
+        ))
+    }
+
+    pub unsafe fn open(s: &str) -> Option<Client> {
+        let name = match CString::new(s) {
+            Ok(s) => s,
+            Err(_) => return None,
+        };
+
+        let sem = OpenSemaphoreA(SYNCHRONIZE | SEMAPHORE_MODIFY_STATE, FALSE, name.as_ptr());
+        if sem.is_null() {
+            None
+        } else {
+            Some(Client {
+                sem: Handle(sem),
+                name: s.to_string(),
+            })
+        }
+    }
+
+    pub fn acquire(&self) -> io::Result<Acquired> {
+        unsafe {
+            let r = WaitForSingleObject(self.sem.0, INFINITE);
+            if r == WAIT_OBJECT_0 {
+                Ok(Acquired)
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+
+    pub fn release(&self, _data: Option<&Acquired>) -> io::Result<()> {
+        unsafe {
+            let r = ReleaseSemaphore(self.sem.0, 1, ptr::null_mut());
+            if r != 0 {
+                Ok(())
+            } else {
+                Err(io::Error::last_os_error())
+            }
+        }
+    }
+
+    pub fn string_arg(&self) -> String {
+        self.name.clone()
+    }
+
+    pub fn available(&self) -> io::Result<usize> {
+        // Can't read value of a semaphore on Windows, so
+        // try to acquire without sleeping, since we can find out the
+        // old value on release. If acquisiton fails, then available is 0.
+        unsafe {
+            let r = WaitForSingleObject(self.sem.0, 0);
+            if r != WAIT_OBJECT_0 {
+                Ok(0)
+            } else {
+                let mut prev: LONG = 0;
+                let r = ReleaseSemaphore(self.sem.0, 1, &mut prev);
+                if r != 0 {
+                    Ok(prev as usize + 1)
+                } else {
+                    Err(io::Error::last_os_error())
+                }
+            }
+        }
+    }
+
+    pub fn configure(&self, _cmd: &mut Command) {
+        // nothing to do here, we gave the name of our semaphore to the
+        // child above
+    }
+}
+
+#[derive(Debug)]
+struct Handle(HANDLE);
+// HANDLE is a raw ptr, but we're send/sync
+unsafe impl Sync for Handle {}
+unsafe impl Send for Handle {}
+
+impl Drop for Handle {
+    fn drop(&mut self) {
+        unsafe {
+            CloseHandle(self.0);
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Helper {
+    event: Arc<Handle>,
+    thread: JoinHandle<()>,
+}
+
+pub(crate) fn spawn_helper(
+    client: crate::Client,
+    state: Arc<super::HelperState>,
+    mut f: Box<dyn FnMut(io::Result<crate::Acquired>) + Send>,
+) -> io::Result<Helper> {
+    let event = unsafe {
+        let r = CreateEventA(ptr::null_mut(), TRUE, FALSE, ptr::null());
+        if r.is_null() {
+            return Err(io::Error::last_os_error());
+        } else {
+            Handle(r)
+        }
+    };
+    let event = Arc::new(event);
+    let event2 = event.clone();
+    let thread = Builder::new().spawn(move || {
+        let objects = [event2.0, client.inner.sem.0];
+        state.for_each_request(|_| {
+            const WAIT_OBJECT_1: u32 = WAIT_OBJECT_0 + 1;
+            match unsafe { WaitForMultipleObjects(2, objects.as_ptr(), FALSE, INFINITE) } {
+                WAIT_OBJECT_0 => return,
+                WAIT_OBJECT_1 => f(Ok(crate::Acquired {
+                    client: client.inner.clone(),
+                    data: Acquired,
+                    disabled: false,
+                })),
+                _ => f(Err(io::Error::last_os_error())),
+            }
+        });
+    })?;
+    Ok(Helper { thread, event })
+}
+
+impl Helper {
+    pub fn join(self) {
+        // Unlike unix this logic is much easier. If our thread was blocked
+        // in waiting for requests it should already be woken up and
+        // exiting. Otherwise it's waiting for a token, so we wake it up
+        // with a different event that it's also waiting on here. After
+        // these two we should be guaranteed the thread is on its way out,
+        // so we can safely `join`.
+        let r = unsafe { SetEvent(self.event.0) };
+        if r == 0 {
+            panic!("failed to set event: {}", io::Error::last_os_error());
+        }
+        drop(self.thread.join());
+    }
+}
diff --git a/third_party/rust/jobserver/tests/client-of-myself.rs b/third_party/rust/jobserver/tests/client-of-myself.rs
new file mode 100644
index 0000000000..45d57b0b58
--- /dev/null
+++ b/third_party/rust/jobserver/tests/client-of-myself.rs
@@ -0,0 +1,59 @@
+use std::env;
+use std::io::prelude::*;
+use std::io::BufReader;
+use std::process::{Command, Stdio};
+use std::sync::mpsc;
+use std::thread;
+
+use jobserver::Client;
+
+macro_rules! t {
+    ($e:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+        }
+    };
+}
+
+fn main() {
+    if env::var("I_AM_THE_CLIENT").is_ok() {
+        client();
+    } else {
+        server();
+    }
+}
+
+fn server() {
+    let me = t!(env::current_exe());
+    let client = t!(Client::new(1));
+    let mut cmd = Command::new(me);
+    cmd.env("I_AM_THE_CLIENT", "1").stdout(Stdio::piped());
+    client.configure(&mut cmd);
+    let acq = client.acquire().unwrap();
+    let mut child = t!(cmd.spawn());
+    let stdout = child.stdout.take().unwrap();
+    let (tx, rx) = mpsc::channel();
+    let t = thread::spawn(move || {
+        for line in BufReader::new(stdout).lines() {
+            tx.send(t!(line)).unwrap();
+        }
+    });
+
+    for _ in 0..100 {
+        assert!(rx.try_recv().is_err());
+    }
+
+    drop(acq);
+    assert_eq!(rx.recv().unwrap(), "hello!");
+    t.join().unwrap();
+    assert!(rx.recv().is_err());
+    client.acquire().unwrap();
+}
+
+fn client() {
+    let client = unsafe { Client::from_env().unwrap() };
+    let acq = client.acquire().unwrap();
+    println!("hello!");
+    drop(acq);
+}
diff --git a/third_party/rust/jobserver/tests/client.rs b/third_party/rust/jobserver/tests/client.rs
new file mode 100644
index 0000000000..2516b8ccf7
--- /dev/null
+++ b/third_party/rust/jobserver/tests/client.rs
@@ -0,0 +1,198 @@
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::process::Command;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::mpsc;
+use std::sync::Arc;
+use std::thread;
+
+use futures::future::{self, Future};
+use futures::stream::{self, Stream};
+use jobserver::Client;
+use tokio_core::reactor::Core;
+use tokio_process::CommandExt;
+
+macro_rules! t {
+    ($e:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+        }
+    };
+}
+
+struct Test {
+    name: &'static str,
+    f: &'static dyn Fn(),
+    make_args: &'static [&'static str],
+    rule: &'static dyn Fn(&str) -> String,
+}
+
+const TESTS: &[Test] = &[
+    Test {
+        name: "no j args",
+        make_args: &[],
+        rule: &|me| me.to_string(),
+        f: &|| {
+            assert!(unsafe { Client::from_env().is_none() });
+        },
+    },
+    Test {
+        name: "no j args with plus",
+        make_args: &[],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            assert!(unsafe { Client::from_env().is_none() });
+        },
+    },
+    Test {
+        name: "j args with plus",
+        make_args: &["-j2"],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            assert!(unsafe { Client::from_env().is_some() });
+        },
+    },
+    Test {
+        name: "acquire",
+        make_args: &["-j2"],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            let c = unsafe { Client::from_env().unwrap() };
+            drop(c.acquire().unwrap());
+            drop(c.acquire().unwrap());
+        },
+    },
+    Test {
+        name: "acquire3",
+        make_args: &["-j3"],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            let c = unsafe { Client::from_env().unwrap() };
+            let a = c.acquire().unwrap();
+            let b = c.acquire().unwrap();
+            drop((a, b));
+        },
+    },
+    Test {
+        name: "acquire blocks",
+        make_args: &["-j2"],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            let c = unsafe { Client::from_env().unwrap() };
+            let a = c.acquire().unwrap();
+            let hit = Arc::new(AtomicBool::new(false));
+            let hit2 = hit.clone();
+            let (tx, rx) = mpsc::channel();
+            let t = thread::spawn(move || {
+                tx.send(()).unwrap();
+                let _b = c.acquire().unwrap();
+                hit2.store(true, Ordering::SeqCst);
+            });
+            rx.recv().unwrap();
+            assert!(!hit.load(Ordering::SeqCst));
+            drop(a);
+            t.join().unwrap();
+            assert!(hit.load(Ordering::SeqCst));
+        },
+    },
+    Test {
+        name: "acquire_raw",
+        make_args: &["-j2"],
+        rule: &|me| format!("+{}", me),
+        f: &|| {
+            let c = unsafe { Client::from_env().unwrap() };
+            c.acquire_raw().unwrap();
+            c.release_raw().unwrap();
+        },
+    },
+];
+
+fn main() {
+    if let Ok(test) = env::var("TEST_TO_RUN") {
+        return (TESTS.iter().find(|t| t.name == test).unwrap().f)();
+    }
+
+    let me = t!(env::current_exe());
+    let me = me.to_str().unwrap();
+    let filter = env::args().nth(1);
+
+    let mut core = t!(Core::new());
+
+    let futures = TESTS
+        .iter()
+        .filter(|test| match filter {
+            Some(ref s) => test.name.contains(s),
+            None => true,
+        })
+        .map(|test| {
+            let td = t!(tempfile::tempdir());
+            let makefile = format!(
+                "\
+all: export TEST_TO_RUN={}
+all:
+\t{}
+",
+                test.name,
+                (test.rule)(me)
+            );
+            t!(t!(File::create(td.path().join("Makefile"))).write_all(makefile.as_bytes()));
+            let prog = env::var("MAKE").unwrap_or_else(|_| "make".to_string());
+            let mut cmd = Command::new(prog);
+            cmd.args(test.make_args);
+            cmd.current_dir(td.path());
+            future::lazy(move || {
+                cmd.output_async().map(move |e| {
+                    drop(td);
+                    (test, e)
+                })
+            })
+        })
+        .collect::<Vec<_>>();
+
+    println!("\nrunning {} tests\n", futures.len());
+
+    let stream = stream::iter(futures.into_iter().map(Ok)).buffer_unordered(num_cpus::get());
+
+    let mut failures = Vec::new();
+    t!(core.run(stream.for_each(|(test, output)| {
+        if output.status.success() {
+            println!("test {} ... ok", test.name);
+        } else {
+            println!("test {} ... FAIL", test.name);
+            failures.push((test, output));
+        }
+        Ok(())
+    })));
+
+    if failures.is_empty() {
+        println!("\ntest result: ok\n");
+        return;
+    }
+
+    println!("\n----------- failures");
+
+    for (test, output) in failures {
+        println!("test {}", test.name);
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        let stderr = String::from_utf8_lossy(&output.stderr);
+
+        println!("\texit status: {}", output.status);
+        if !stdout.is_empty() {
+            println!("\tstdout ===");
+            for line in stdout.lines() {
+                println!("\t\t{}", line);
+            }
+        }
+
+        if !stderr.is_empty() {
+            println!("\tstderr ===");
+            for line in stderr.lines() {
+                println!("\t\t{}", line);
+            }
+        }
+    }
+
+    std::process::exit(4);
+}
diff --git a/third_party/rust/jobserver/tests/helper.rs b/third_party/rust/jobserver/tests/helper.rs
new file mode 100644
index 0000000000..0b3ba88a70
--- /dev/null
+++ b/third_party/rust/jobserver/tests/helper.rs
@@ -0,0 +1,77 @@
+use jobserver::Client;
+use std::sync::atomic::*;
+use std::sync::mpsc;
+use std::sync::*;
+
+macro_rules! t {
+    ($e:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+        }
+    };
+}
+
+#[test]
+fn helper_smoke() {
+    let client = t!(Client::new(1));
+    drop(client.clone().into_helper_thread(|_| ()).unwrap());
+    drop(client.clone().into_helper_thread(|_| ()).unwrap());
+    drop(client.clone().into_helper_thread(|_| ()).unwrap());
+    drop(client.clone().into_helper_thread(|_| ()).unwrap());
+    drop(client.clone().into_helper_thread(|_| ()).unwrap());
+    drop(client.into_helper_thread(|_| ()).unwrap());
+}
+
+#[test]
+fn acquire() {
+    let (tx, rx) = mpsc::channel();
+    let client = t!(Client::new(1));
+    let helper = client
+        .into_helper_thread(move |a| drop(tx.send(a)))
+        .unwrap();
+    assert!(rx.try_recv().is_err());
+    helper.request_token();
+    rx.recv().unwrap().unwrap();
+    helper.request_token();
+    rx.recv().unwrap().unwrap();
+
+    helper.request_token();
+    helper.request_token();
+    rx.recv().unwrap().unwrap();
+    rx.recv().unwrap().unwrap();
+
+    helper.request_token();
+    helper.request_token();
+    drop(helper);
+}
+
+#[test]
+fn prompt_shutdown() {
+    for _ in 0..100 {
+        let client = jobserver::Client::new(4).unwrap();
+        let count = Arc::new(AtomicU32::new(0));
+        let count2 = count.clone();
+        let tokens = Arc::new(Mutex::new(Vec::new()));
+        let helper = client
+            .into_helper_thread(move |token| {
+                tokens.lock().unwrap().push(token);
+                count2.fetch_add(1, Ordering::SeqCst);
+            })
+            .unwrap();
+
+        // Request more tokens than what are available.
+        for _ in 0..5 {
+            helper.request_token();
+        }
+        // Wait for at least some of the requests to finish.
+        while count.load(Ordering::SeqCst) < 3 {
+            std::thread::yield_now();
+        }
+        // Drop helper
+        let t = std::time::Instant::now();
+        drop(helper);
+        let d = t.elapsed();
+        assert!(d.as_secs_f64() < 0.5);
+    }
+}
diff --git a/third_party/rust/jobserver/tests/make-as-a-client.rs b/third_party/rust/jobserver/tests/make-as-a-client.rs
new file mode 100644
index 0000000000..4faac5b880
--- /dev/null
+++ b/third_party/rust/jobserver/tests/make-as-a-client.rs
@@ -0,0 +1,81 @@
+use std::env;
+use std::fs::File;
+use std::io::prelude::*;
+use std::net::{TcpListener, TcpStream};
+use std::process::Command;
+
+use jobserver::Client;
+
+macro_rules! t {
+    ($e:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+        }
+    };
+}
+
+fn main() {
+    if env::var("_DO_THE_TEST").is_ok() {
+        std::process::exit(
+            Command::new(env::var_os("MAKE").unwrap())
+                .env("MAKEFLAGS", env::var_os("CARGO_MAKEFLAGS").unwrap())
+                .env_remove("_DO_THE_TEST")
+                .args(&env::args_os().skip(1).collect::<Vec<_>>())
+                .status()
+                .unwrap()
+                .code()
+                .unwrap_or(1),
+        );
+    }
+
+    if let Ok(s) = env::var("TEST_ADDR") {
+        let mut contents = Vec::new();
+        t!(t!(TcpStream::connect(&s)).read_to_end(&mut contents));
+        return;
+    }
+
+    let c = t!(Client::new(1));
+    let td = tempfile::tempdir().unwrap();
+
+    let prog = env::var("MAKE").unwrap_or_else(|_| "make".to_string());
+
+    let me = t!(env::current_exe());
+    let me = me.to_str().unwrap();
+
+    let mut cmd = Command::new(&me);
+    cmd.current_dir(td.path());
+    cmd.env("MAKE", prog);
+    cmd.env("_DO_THE_TEST", "1");
+
+    t!(t!(File::create(td.path().join("Makefile"))).write_all(
+        format!(
+            "\
+all: foo bar
+foo:
+\t{0}
+bar:
+\t{0}
+",
+            me
+        )
+        .as_bytes()
+    ));
+
+    // We're leaking one extra token to `make` sort of violating the makefile
+    // jobserver protocol. It has the desired effect though.
+    c.configure(&mut cmd);
+
+    let listener = t!(TcpListener::bind("127.0.0.1:0"));
+    let addr = t!(listener.local_addr());
+    cmd.env("TEST_ADDR", addr.to_string());
+    let mut child = t!(cmd.spawn());
+
+    // We should get both connections as the two programs should be run
+    // concurrently.
+    let a = t!(listener.accept());
+    let b = t!(listener.accept());
+    drop((a, b));
+
+    assert!(t!(child.wait()).success());
+}
diff --git a/third_party/rust/jobserver/tests/server.rs b/third_party/rust/jobserver/tests/server.rs
new file mode 100644
index 0000000000..70ea218fc7
--- /dev/null
+++ b/third_party/rust/jobserver/tests/server.rs
@@ -0,0 +1,181 @@
+use std::env;
+use std::fs::File;
+use std::io::prelude::*;
+use std::process::Command;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::mpsc;
+use std::sync::Arc;
+use std::thread;
+
+use jobserver::Client;
+
+macro_rules! t {
+    ($e:expr) => {
+        match $e {
+            Ok(e) => e,
+            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+        }
+    };
+}
+
+#[test]
+fn server_smoke() {
+    let c = t!(Client::new(1));
+    drop(c.acquire().unwrap());
+    drop(c.acquire().unwrap());
+}
+
+#[test]
+fn server_multiple() {
+    let c = t!(Client::new(2));
+    let a = c.acquire().unwrap();
+    let b = c.acquire().unwrap();
+    drop((a, b));
+}
+
+#[test]
+fn server_available() {
+    let c = t!(Client::new(10));
+    assert_eq!(c.available().unwrap(), 10);
+    let a = c.acquire().unwrap();
+    assert_eq!(c.available().unwrap(), 9);
+    drop(a);
+    assert_eq!(c.available().unwrap(), 10);
+}
+
+#[test]
+fn server_none_available() {
+    let c = t!(Client::new(2));
+    assert_eq!(c.available().unwrap(), 2);
+    let a = c.acquire().unwrap();
+    assert_eq!(c.available().unwrap(), 1);
+    let b = c.acquire().unwrap();
+    assert_eq!(c.available().unwrap(), 0);
+    drop(a);
+    assert_eq!(c.available().unwrap(), 1);
+    drop(b);
+    assert_eq!(c.available().unwrap(), 2);
+}
+
+#[test]
+fn server_blocks() {
+    let c = t!(Client::new(1));
+    let a = c.acquire().unwrap();
+    let hit = Arc::new(AtomicBool::new(false));
+    let hit2 = hit.clone();
+    let (tx, rx) = mpsc::channel();
+    let t = thread::spawn(move || {
+        tx.send(()).unwrap();
+        let _b = c.acquire().unwrap();
+        hit2.store(true, Ordering::SeqCst);
+    });
+    rx.recv().unwrap();
+    assert!(!hit.load(Ordering::SeqCst));
+    drop(a);
+    t.join().unwrap();
+    assert!(hit.load(Ordering::SeqCst));
+}
+
+#[test]
+fn make_as_a_single_thread_client() {
+    let c = t!(Client::new(1));
+    let td = tempfile::tempdir().unwrap();
+
+    let prog = env::var("MAKE").unwrap_or_else(|_| "make".to_string());
+    let mut cmd = Command::new(prog);
+    cmd.current_dir(td.path());
+
+    t!(t!(File::create(td.path().join("Makefile"))).write_all(
+        b"
+all: foo bar
+foo:
+\techo foo
+bar:
+\techo bar
+"
+    ));
+
+    // The jobserver protocol means that the `make` process itself "runs with a
+    // token", so we acquire our one token to drain the jobserver, and this
+    // should mean that `make` itself never has a second token available to it.
+    let _a = c.acquire();
+    c.configure(&mut cmd);
+    let output = t!(cmd.output());
+    println!(
+        "\n\t=== stderr\n\t\t{}",
+        String::from_utf8_lossy(&output.stderr).replace("\n", "\n\t\t")
+    );
+    println!(
+        "\t=== stdout\n\t\t{}",
+        String::from_utf8_lossy(&output.stdout).replace("\n", "\n\t\t")
+    );
+
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+
+    let stdout = String::from_utf8_lossy(&output.stdout).replace("\r\n", "\n");
+    let a = "\
+echo foo
+foo
+echo bar
+bar
+";
+    let b = "\
+echo bar
+bar
+echo foo
+foo
+";
+
+    assert!(stdout == a || stdout == b);
+}
+
+#[test]
+fn make_as_a_multi_thread_client() {
+    let c = t!(Client::new(1));
+    let td = tempfile::tempdir().unwrap();
+
+    let prog = env::var("MAKE").unwrap_or_else(|_| "make".to_string());
+    let mut cmd = Command::new(prog);
+    cmd.current_dir(td.path());
+
+    t!(t!(File::create(td.path().join("Makefile"))).write_all(
+        b"
+all: foo bar
+foo:
+\techo foo
+bar:
+\techo bar
+"
+    ));
+
+    // We're leaking one extra token to `make` sort of violating the makefile
+    // jobserver protocol. It has the desired effect though.
+    c.configure(&mut cmd);
+    let output = t!(cmd.output());
+    println!(
+        "\n\t=== stderr\n\t\t{}",
+        String::from_utf8_lossy(&output.stderr).replace("\n", "\n\t\t")
+    );
+    println!(
+        "\t=== stdout\n\t\t{}",
+        String::from_utf8_lossy(&output.stdout).replace("\n", "\n\t\t")
+    );
+
+    assert!(output.status.success());
+}
+
+#[test]
+fn zero_client() {
+    let client = t!(Client::new(0));
+    let (tx, rx) = mpsc::channel();
+    let helper = client
+        .into_helper_thread(move |a| drop(tx.send(a)))
+        .unwrap();
+    helper.request_token();
+    helper.request_token();
+
+    for _ in 0..1000 {
+        assert!(rx.try_recv().is_err());
+    }
+}