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gdbusconnection: Fix double unref on timeout/cancel sending a message

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This appears to fix an intermittent failure seen when sending a D-Bus
message with either of a cancellable or a timeout set.

In particular, I can reliably reproduce it with:
```
meson test gdbus-test-codegen-min-required-2-64 --repeat 10000
```

It can be caught easily with asan when reproduced. Tracking down the
location of the refcount mismatch was a little tricky, but was
simplified by replacing a load of `g_object_ref (message)` calls with
`g_dbus_message_copy (message, NULL)` to switch `GDBusMessage` handling
to using copy semantics. This allowed asan to home in on where the
refcount mismatch was happening.

The problem was that `send_message_data_deliver_error()` takes ownership
of the `GTask` passed to it, but the
`send_message_with_replace_cancelled_idle_cb()` and
`send_message_with_reply_timeout_cb()` functions which were calling it,
were not passing in a strong reference as they should have.

Another approach to fixing this would have been to change the transfer
semantics of `send_message_data_deliver_error()` so it was `(transfer
none)` on its `GTask`. That would probably have resulted in cleaner
code, but would have been a lot harder to verify/review the fix, and
easier to inadvertently introduce new bugs.

The fact that the bug was only triggered by the cancellation and timeout
callbacks explains why it was intermittent: these code paths are
typically never hit, but the timeout path may sometimes be hit on a very
slow test run.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #1264
This commit is contained in:
Philip Withnall 2023-02-22 02:40:35 +00:00
parent 88fd3f0a76
commit 4900ea5215
1 changed files with 7 additions and 5 deletions
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12
gio/gdbusconnection.c
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@ -1822,7 +1822,7 @@ send_message_data_deliver_reply_unlocked (GTask *task,
;
}
/* Called from a user thread, lock is not held */
/* Called from a user thread, lock is not held; @task is (transfer full) */
static void
send_message_data_deliver_error (GTask *task,
GQuark domain,
@ -1849,13 +1849,14 @@ send_message_data_deliver_error (GTask *task,
/* ---------------------------------------------------------------------------------------------------- */
/* Called from a user thread, lock is not held; @task is (transfer full) */
/* Called from a user thread, lock is not held; @task is (transfer none) */
static gboolean
send_message_with_reply_cancelled_idle_cb (gpointer user_data)
{
GTask *task = user_data;
send_message_data_deliver_error (task, G_IO_ERROR, G_IO_ERROR_CANCELLED,
g_object_ref (task);
send_message_data_deliver_error (g_steal_pointer (&task), G_IO_ERROR, G_IO_ERROR_CANCELLED,
_("Operation was cancelled"));
return FALSE;
}
@ -1879,13 +1880,14 @@ send_message_with_reply_cancelled_cb (GCancellable *cancellable,
/* ---------------------------------------------------------------------------------------------------- */
/* Called from a user thread, lock is not held; @task is (transfer full) */
/* Called from a user thread, lock is not held; @task is (transfer none) */
static gboolean
send_message_with_reply_timeout_cb (gpointer user_data)
{
GTask *task = user_data;
send_message_data_deliver_error (task, G_IO_ERROR, G_IO_ERROR_TIMED_OUT,
g_object_ref (task);
send_message_data_deliver_error (g_steal_pointer (&task), G_IO_ERROR, G_IO_ERROR_TIMED_OUT,
_("Timeout was reached"));
return FALSE;
}