[PATCHES] D-Bus C++ binding fixes

[Olivier Hochreutiner]

Hi there,

After several tests on D-Bus C++ binding I found several bugs, and I
made fixes for some of them.
So here is what I found:


1. When a proxy is instantiated several times for a given connection,
only one instance receive signals. Patch:
fix-signals-multiple-proxies.patch

***

2. When two processes (both connected to the same bus) have an adaptor
for the same object path and interface, when a method call is done on
one of them, it is also done on the other one. For instance if you
have two 'echo-server' running (supposing they have different bus
names) and you call the method Hello of
org.freedesktop.DBus.Example.Echo on one of them (with e.g.
dbus-send), the method will also be called on the other one. Patch:
fix-method-call.patch

***

3. Threading in libdbus-c++ is never compiled due to incorrect #ifdefs
and an invalid type. Patch: pthread.patch

***

4. Even after applying patch (3), having multiple threads is not
working correctly. The problem can be easily reproduced with the
'echo-client-mt' example of D-Bus C++ binding. For recall this program
creates 16 threads. Each thread does the same method call 100 times in
a loop (method calls are done with
dbus_connection_send_with_reply_and_block()), and the main thread is
the dispatcher thread. When echo-client-mt is run, two things can
occur:

(1) It crashes with an out-of-memory error (that is, an exception is
raised because dbus_connection_send_with_reply_and_block() returns an
OOM error). I will explain this issue in another post because I
believe the problem is in libdbus-1, not libdbus-c++.

or

(2) One of the threads hangs in poll() waiting for the method reply
(see bt-thread-poll.txt), while the other threads are waiting to
acquire the io path mutex (see bt-wait-io.txt). The dispatcher thread
is usually also waiting in poll() (see bt-dispatcher-poll.txt), but it
sometimes happens that it is waiting for the io path mutex.
In this situation when the poll timeout is reached the program
continues normally until it hangs again, or crashes as in situation
(1).
I believe this problem is caused by the fact that two threads (namely
the dispatcher thread and one of the method-calling threads) poll the
same fd at the same time. The method-calling thread owns the io path
mutex while poll()'ing, but the dispatcher thread doesn't (it calls
poll() on the fd without locking anything). Sometimes the
method-calling thread returns from poll() before the dispatcher
thread, and it goes as expected, but sometimes the dispatcher thread
is notified first, resulting in the method-calling thread having to
wait until poll times out, and only then it returns from
dbus_connection_send_with_reply_and_block(). Of course all the other
threads have to wait for the io path mutex.
A workaround for this issue could be to make the dispatcher thread
leave poll() before calling
dbus_connection_send_with_reply_and_block() or
dbus_bus_{add,remove}_match(). I guess this can be done by creating a
pipe, adding its read fd to the list of fds the dispatcher polls, and
writing something to the pipe's write fd before calling the
above-mentionned functions. Probably a mutex or a condition variable
would be needed too, to tell the dispatcher when the method call is
finished.
Is this solution acceptable ? Should the issue rather be fixed somehow
in libdbus-1 ? What do you think ?

***

Beside these issues there are two things that I would like to ask:

- I think D-Bus C++ sources should be moved out of Wengo's SVN. I
believe it was already discussed on this list that it can be moved to
git.freedesktop.org, using
http://www.kernel.org/pub/software/scm/git/docs/git-svnimport.html. Is
it still planned ?

- What about relicensing D-Bus C++ with the same license as dbus-1
(i.e. MIT/X11) ?

Well I think that's all. Merry Christmas and Happy New Year to everybody.

Olivier Hochreutiner
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fix-method-call.patch
Type: text/x-patch
Size: 947 bytes
Desc: not available
Url : http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: pthread.patch
Type: text/x-patch
Size: 1273 bytes
Desc: not available
Url : http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment-0001.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fix-signals-multiple-proxies.patch
Type: text/x-patch
Size: 506 bytes
Desc: not available
Url : http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment-0002.bin 
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: bt-thread-poll.txt
Url: http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment.txt 
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: bt-wait-io-path.txt
Url: http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment-0001.txt 
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: bt-dispatcher-poll.txt
Url: http://lists.freedesktop.org/archives/dbus/attachments/20071228/4303a128/attachment-0002.txt