dbus: Fix a subtle butterfly-effect bug in handling incoming messages

dbus: Fix a subtle butterfly-effect bug in handling incoming messages

libdbus does bookkeeping of the number of bytes in the incoming message
queue implicitly, and asks client code (chrome) to stop monitoring the
underlying socket, as soon as it exceeds a certain number, which is set
to 63MB as of now.

This caused a DCHECK failure (and could break the D-Bus message
dispatching with Release builds) because the bookkeeping happned on the
UI thread via a seemingly harmless call to dbus_message_unref(), but we
cannot stop monitoring from UI thread.

This patch fixes (or works around) the problem by deleting incoming
messages on D-Bus thread, so the bookkeeping is done on D-Bus thread.

Note that we don't have to change exported_object.cc, as the method call
message is deleted on the D-Bus thread in
ExportedObject::OnMethodCompleted()

The following is a stacktrace of the DCHECK failure. Here,
dbus::Response (method reply) is deleted on UI thread, that results
in a call to dbus::Bus::OnToggleWatch, which should only be called on
the D-Bus thread, hence crashed as a DCHECK failure


Backtrace:
	base::debug::StackTrace::StackTrace() [0x517972]
	logging::LogMessage::~LogMessage() [0x4b3a57] <- crashing because we are not
	base::ThreadRestrictions::AssertIOAllowed() [0x4f0b35]
	dbus::Bus::AssertOnDBusThread() [0x45ceb6] <- checking if we are on the right thread
	dbus::Bus::OnToggleWatch() [0x45d0c1]
	dbus::Bus::OnToggleWatchThunk() [0x45d45d]  <-- the change is notified.
	_dbus_watch_list_toggle_watch [0x7f35e0a15245]
	protected_change_watch [0x7f35e09f2eef]
	_dbus_connection_toggle_watch_unlocked [0x7f35e09f302e]
	check_read_watch [0x7f35e0a1332d]   <-- what? why checking socket status here??
	socket_live_messages_changed [0x7f35e0a1436c]
	live_messages_size_notify [0x7f35e0a11996]
	_dbus_counter_adjust [0x7f35e0a0c098]
	free_size_counter [0x7f35e0a04423]
	_dbus_list_foreach [0x7f35e0a180d9]
	dbus_message_cache_or_finalize [0x7f35e0a0446b]
	dbus_message_unref [0x7f35e0a05e7e]   <-- releasing a message
	dbus::Message::~Message() [0x46abbb]
	dbus::Response::~Response() [0x470478]
	scoped_ptr<>::~scoped_ptr() [0x41e99f]
	dbus::ObjectProxy::RunResponseCallback() [0x472095]

BUG=126217
TEST=added unit tests

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=140165

[satorux1, 2012-06-01 23:20:00]

I guess this isn't the culprit of crbug.com/126217, but it's good to fix a bug
anyway. If we are very lucky, this may be the culprit.

[stevenjb, 2012-06-01 23:50:56]

Wow, that's subtle. This LGTM. Hope it does the trick!

[hashimoto, 2012-06-02 01:07:57]

lgtm
Hope these are the only libdbus function calls we should move from UI thread to
D-Bus thread.

[commit-bot: I haz the power, 2012-06-02 01:16:06]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/satorux@chromium.org/10492005/1

[commit-bot: I haz the power, 2012-06-02 02:53:24]

Change committed as 140165