Bug 1275942 - Add option to wait for debugger on startup. r=jdescottes

Adds a `--wait-for-jsdebugger` CLI option which will spin the event loop until
the debugger connects to allow debugging _some_ startup code paths, such as the
beginning of the `browser.js` file.

Startup code paths that run before the DevTools CLI handler will be missed for
now.  This can be improved in the future if we move this to an earlier phase of
app startup.

MozReview-Commit-ID: J1A8lWLETGY

devtools/client/devtools-startup.js

@@ -13,7 +13,7 @@
 
 "use strict";
 
-const { interfaces: Ci, utils: Cu } = Components;
+const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
 const kDebuggerPrefs = [
   "devtools.debugger.remote-enabled",
   "devtools.chrome.enabled"
@@ -124,9 +124,28 @@ DevToolsStartup.prototype = {
     if (!this._isRemoteDebuggingEnabled()) {
       return;
     }
+
+    let devtoolsThreadResumed = false;
+    let pauseOnStartup = cmdLine.handleFlag("wait-for-jsdebugger", false);
+    if (pauseOnStartup) {
+      let observe = function (subject, topic, data) {
+        devtoolsThreadResumed = true;
+        Services.obs.removeObserver(observe, "devtools-thread-resumed");
+      };
+      Services.obs.addObserver(observe, "devtools-thread-resumed", false);
+    }
+
     const { BrowserToolboxProcess } = Cu.import("resource://devtools/client/framework/ToolboxProcess.jsm", {});
     BrowserToolboxProcess.init();
 
+    if (pauseOnStartup) {
+      // Spin the event loop until the debugger connects.
+      let thread = Cc["@mozilla.org/thread-manager;1"].getService().currentThread;
+      while (!devtoolsThreadResumed) {
+        thread.processNextEvent(true);
+      }
+    }
+
     if (cmdLine.state == Ci.nsICommandLine.STATE_REMOTE_AUTO) {
       cmdLine.preventDefault = true;
     }
@@ -203,6 +222,9 @@ DevToolsStartup.prototype = {
   /* eslint-disable max-len */
   helpInfo: "  --jsconsole        Open the Browser Console.\n" +
             "  --jsdebugger       Open the Browser Toolbox.\n" +
+            "  --wait-for-jsdebugger Spin event loop until JS debugger connects.\n" +
+            "                     Enables debugging (some) application startup code paths.\n" +
+            "                     Only has an effect when `--jsdebugger` is also supplied.\n" +
             "  --devtools         Open DevTools on initial load.\n" +
             "  --start-debugger-server [ws:][ <port> | <path> ] Start the debugger server on\n" +
             "                     a TCP port or Unix domain socket path. Defaults to TCP port\n" +

devtools/server/actors/chrome.js

@@ -50,7 +50,18 @@ function ChromeActor(connection) {
   if (!window) {
     window = Services.wm.getMostRecentWindow(null);
   }
-  // On xpcshell, there is no window/docshell
+
+  // We really want _some_ window at least, so fallback to the hidden window if
+  // there's nothing else (such as during early startup).
+  if (!window) {
+    try {
+      window = Services.appShell.hiddenDOMWindow;
+    } catch (e) {
+      // On XPCShell, the above line will throw.
+    }
+  }
+
+  // On XPCShell, there is no window/docshell
   let docShell = window ? window.QueryInterface(Ci.nsIInterfaceRequestor)
                                 .getInterface(Ci.nsIDocShell)
                         : null;

devtools/server/actors/script.js

@@ -1020,8 +1020,8 @@ const ThreadActor = ActorClassWithSpec(threadSpec, {
 
       let packet = this._resumed();
       this._popThreadPause();
-      // Tell anyone who cares of the resume (as of now, that's the xpcshell
-      // harness)
+      // Tell anyone who cares of the resume (as of now, that's the xpcshell harness and
+      // devtools-startup.js when handling the --wait-for-jsdebugger flag)
       if (Services.obs) {
         Services.obs.notifyObservers(this, "devtools-thread-resumed", null);
       }