Have content/ create and destroy its own threads.

content/public/browser/browser_main_parts.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_BROWSER_MAIN_PARTS_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_MAIN_PARTS_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "content/common/content_export.h"
+#include "content/public/browser/browser_thread.h"
 
 namespace content {
 
+class IOThreadDelegate;
+
 // This class contains different "stages" to be executed by |BrowserMain()|,
 // Each stage is represented by a single BrowserMainParts method, called from
 // the corresponding method in |BrowserMainLoop| (e.g., EarlyInitialization())
 // which does the following:
 //  - calls a method (e.g., "PreEarlyInitialization()") for each member of
 //    |parts_|. Parts will implement platform or tookit specific code for that
 //    stage.
 //  - calls various methods for things common to all platforms (for that stage).
 //  - calls a method (e.g., "PostEarlyInitialization()") for platform-specific
 //    code to be called after the common code.
@@ skipping 37 matching lines @@
 
   virtual void PostEarlyInitialization() = 0;
 
   virtual void PreMainMessageLoopStart() = 0;
 
   // Allows an embedder to do any extra toolkit initialization.
   virtual void ToolkitInitialized() = 0;
 
   virtual void PostMainMessageLoopStart() = 0;
 
-  virtual void PreMainMessageLoopRun() = 0;
+  // Child threads have not been created at this point.
+  //
+  // This method may return NULL, or an object implementing
+  // IOThreadDelegate.  If the latter, this object's Init method will
+  // be called as the first thing that happens on the "IO"
+  // BrowserThread, and its CleanUp method as the last.
+  //
+  // Implementations of BrowserMainParts must ensure that the returned
+  // object's lifetime extends beyond the call to
+  // PostStopThread(BrowserThread::IO).
+  //
+  // It is currently an error for more than one BrowserMainParts
+  // implementation to return a delegate.  This could be fixed to
+  // allow multiple delegates if needed.
+  virtual IOThreadDelegate* PreMainMessageLoopRun() = 0;

[jam, 2011/11/17 21:51:21]

it seems a little odd that this interface is given to content from here. this
interface should really be used for parts of the main message loop.

Stepping back, I don't understand why you needed to add a new interface for
this. In general, the fewer interfaces we have the better, so I'm curious if you
considered reusing Pre/PostStopThread in BrowserMainParts, along with the
addition of a new method like ThreadStarted(identifier). that way the embedder
can post tasks on init/cleanup to accomplish the same thing

[Jói, 2011/11/18 23:21:44]

I switched to just posting a message to initialize, before any other message can
get posted to the IO thread, and a message to tear down right before the thread
is stopped. This required a couple of not so huge changes around how IOThread is
constructed by BrowserProcessImpl, but otherwise seems harmless.

For cleanup, I just realized there might now be a race between others posting
messages to the thread, and the teardown message being posted; this was
definitely not a race previously as CleanUp is guaranteed to be the last thing
called on a thread AFAIU.  I wonder if this is important enough to bring back
the delegate approach.

If we do need to bring the delegate back, I think we should make it a generic
ThreadDelegate interface that just gets Init and CleanUp called from the thread
during the thread's Init and CleanUp.  I could look for a cleaner way to set up
the delegates, e.g. a separate delegate registration mechanism rather than
returning the delegate from BrowserMainParts.  Another alternatively might be,
instead of a separate delegate interface there could possibly be methods of
BrowserMainParts like ThreadInit and ThreadCleanUp that get called with a
BrowserThread::ID from each thread as it is set up and torn down.

+
+  // The various browser threads have been created at this point.
+  virtual void PreMainMessageLoopRunThreadsCreated() = 0;
 
   // Returns true if the message loop was run, false otherwise.
   // May set |result_code|, which will be returned by |BrowserMain()|.
   // If no BrowserMainParts implementations return true, the default
   // implementation will be run.
   virtual bool MainMessageLoopRun(int* result_code) = 0;
 
+  // This happens after the main message loop has stopped, but before
+  // threads are stopped.
   virtual void PostMainMessageLoopRun() = 0;
 
+  // Called once for each thread owned by the content framework just
+  // before and just after it is torn down. This is in reverse order
+  // of the threads' appearance in the BrowserThread::ID enumeration,
+  // i.e. the BrowserThread::DB thread goes last.
+  virtual void PreStopThread(BrowserThread::ID identifier) = 0;
+  virtual void PostStopThread(BrowserThread::ID identifier) = 0;
+
+  // Called as the very last part of shutdown.
+  virtual void FinalCleanup() = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(BrowserMainParts);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_MAIN_PARTS_H_