1 files changed, 221 insertions, 0 deletions
diff --git a/devtools/server/actors/utils/event-loop.js b/devtools/server/actors/utils/event-loop.js
new file mode 100644
index 0000000000..519d97ba7e
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+++ b/devtools/server/actors/utils/event-loop.js
@@ -0,0 +1,221 @@
+/* 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";
+
+const xpcInspector = require("xpcInspector");
+
+/**
+ * An object that represents a nested event loop. It is used as the nest
+ * requestor with nsIJSInspector instances.
+ *
+ * @param ThreadActor thread
+ *        The thread actor that is creating this nested event loop.
+ */
+class EventLoop {
+  constructor({ thread }) {
+    this._thread = thread;
+
+    // A flag which is true in between the two calls to enter() and exit().
+    this._entered = false;
+    // Another flag which is true only after having called exit().
+    // Note that this EventLoop may still be paused and its enter() method
+    // still be on hold, if another EventLoop paused about this one.
+    this._resolved = false;
+  }
+
+  /**
+   * This is meant for other thread actors, and is used by other thread actor's
+   * EventLoop's isTheLastPausedThreadActor()
+   */
+  get thread() {
+    return this._thread;
+  }
+  /**
+   * Similarly, it will be used by another thread actor's EventLoop's enter() method
+   */
+  get resolved() {
+    return this._resolved;
+  }
+
+  /**
+   * Tells if the last thread actor to have paused (i.e. last EventLoop on the stack)
+   * is the current one.
+   *
+   * We avoid trying to exit this event loop,
+   * if another thread actor pile up a more recent one.
+   * All the event loops will be effectively exited when
+   * the thread actor which piled up the most recent nested event loop resumes.
+   *
+   * For convenience for the callsite, this will return true if nothing paused.
+   */
+  isTheLastPausedThreadActor() {
+    if (xpcInspector.eventLoopNestLevel > 0) {
+      return xpcInspector.lastNestRequestor.thread === this._thread;
+    }
+    return true;
+  }
+
+  /**
+   * Enter a new nested event loop.
+   */
+  enter() {
+    if (this._entered) {
+      throw new Error(
+        "Can't enter an event loop that has already been entered!"
+      );
+    }
+
+    const preEnterData = this.preEnter();
+
+    this._entered = true;
+    // Note: next line will synchronously block the execution until exit() is being called.
+    //
+    // This enterNestedEventLoop is a bit magical and will break run-to-completion rule of JS.
+    // JS will become multi-threaded. Some other task may start running on change state
+    // while we are blocked on this enterNestedEventLoop function call.
+    // You may find valuable information about Tasks and Event Loops on:
+    // https://docs.google.com/document/d/1jTMd-H_BwH9_QNUDxPse80vq884_hMvd234lvE5gqY8/edit?usp=sharing
+    //
+    // Note #2: this will update xpcInspector.lastNestRequestor to this
+    xpcInspector.enterNestedEventLoop(this);
+
+    // If this code runs, it means that we just exited this event loop and lastNestRequestor is no longer equal to this.
+    //
+    // We will now "recursively" exit all the resolved EventLoops which are blocked on `enterNestedEventLoop`:
+    // - if the new lastNestRequestor is resolved, request to exit it as well
+    // - this lastNestRequestor is another EventLoop instance
+    // - exiting this EventLoop unblocks its "enter" method and moves lastNestRequestor to the next requestor (if any)
+    // - we go back to the first step, and attempt to exit the new lastNestRequestor if it is resolved, etc...
+    if (xpcInspector.eventLoopNestLevel > 0) {
+      const { resolved } = xpcInspector.lastNestRequestor;
+      if (resolved) {
+        xpcInspector.exitNestedEventLoop();
+      }
+    }
+
+    this.postExit(preEnterData);
+  }
+
+  /**
+   * Exit this nested event loop.
+   *
+   * @returns boolean
+   *          True if we exited this nested event loop because it was on top of
+   *          the stack, false if there is another nested event loop above this
+   *          one that hasn't exited yet.
+   */
+  exit() {
+    if (!this._entered) {
+      throw new Error("Can't exit an event loop before it has been entered!");
+    }
+    this._entered = false;
+    this._resolved = true;
+
+    // If another ThreadActor paused and spawn a new nested event loop after this one,
+    // let it resume the thread and ignore this call.
+    // The code calling exitNestedEventLoop from EventLoop.enter will resume execution,
+    // by seeing that resolved attribute that we just toggled is true.
+    //
+    // Note that ThreadActor.resume method avoids calling exit thanks to `isTheLastPausedThreadActor`
+    // So for all use requests to resume, the ThreadActor won't call exit until it is the last
+    // thread actor to have entered a nested EventLoop.
+    if (this === xpcInspector.lastNestRequestor) {
+      xpcInspector.exitNestedEventLoop();
+      return true;
+    }
+    return false;
+  }
+
+  /**
+   * Retrieve the list of all DOM Windows debugged by the current thread actor.
+   */
+  getAllWindowDebuggees() {
+    return this._thread.dbg
+      .getDebuggees()
+      .filter(debuggee => {
+        // Select only debuggee that relates to windows
+        // e.g. ignore sandboxes, jsm and such
+        return debuggee.class == "Window";
+      })
+      .map(debuggee => {
+        // Retrieve the JS reference for these windows
+        return debuggee.unsafeDereference();
+      })
+
+      .filter(window => {
+        // Ignore document which have already been nuked,
+        // so navigated to another location and removed from memory completely.
+        if (Cu.isDeadWrapper(window)) {
+          return false;
+        }
+        // Also ignore document which are closed, as trying to access window.parent or top would throw NS_ERROR_NOT_INITIALIZED
+        if (window.closed) {
+          return false;
+        }
+        // Ignore remote iframes, which will be debugged by another thread actor,
+        // running in the remote process
+        if (Cu.isRemoteProxy(window)) {
+          return false;
+        }
+        // Accept "top remote iframe document":
+        // document of iframe whose immediate parent is in another process.
+        if (Cu.isRemoteProxy(window.parent) && !Cu.isRemoteProxy(window)) {
+          return true;
+        }
+
+        // If EFT is enabled, accept any same process document (top-level or iframe).
+        if (this.thread.getParent().ignoreSubFrames) {
+          return true;
+        }
+
+        try {
+          // Ignore iframes running in the same process as their parent document,
+          // as they will be paused automatically when pausing their owner top level document
+          return window.top === window;
+        } catch (e) {
+          // Warn if this is throwing for an unknown reason, but suppress the
+          // exception regardless so that we can enter the nested event loop.
+          if (!/not initialized/.test(e)) {
+            console.warn(`Exception in getAllWindowDebuggees: ${e}`);
+          }
+          return false;
+        }
+      });
+  }
+
+  /**
+   * Prepare to enter a nested event loop by disabling debuggee events.
+   */
+  preEnter() {
+    const docShells = [];
+    // Disable events in all open windows.
+    for (const window of this.getAllWindowDebuggees()) {
+      const { windowUtils } = window;
+      windowUtils.suppressEventHandling(true);
+      windowUtils.suspendTimeouts();
+      docShells.push(window.docShell);
+    }
+    return docShells;
+  }
+
+  /**
+   * Prepare to exit a nested event loop by enabling debuggee events.
+   */
+  postExit(pausedDocShells) {
+    // Enable events in all window paused in preEnter
+    for (const docShell of pausedDocShells) {
+      // Do not try to resume documents which are in destruction
+      // as resume methods would throw
+      if (docShell.isBeingDestroyed()) {
+        continue;
+      }
+      const { windowUtils } = docShell.domWindow;
+      windowUtils.resumeTimeouts();
+      windowUtils.suppressEventHandling(false);
+    }
+  }
+}
+
+exports.EventLoop = EventLoop;