Revert "qemu-char: do not operate on sources from finalize callbacks"

This reverts commit 2b316774f6.

After 038b421788 ("Revert "chardev: use a child source for qio input
source"") we've been observing the "iwp->src == NULL" assertion
triggering periodically during the initial capabilities querying by
libvirtd. One of possible backtraces:

Thread 1 (Thread 0x7f16cd4f0700 (LWP 43858)):
0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
1  0x00007f16c6c21e65 in __GI_abort () at abort.c:79
2  0x00007f16c6c21d39 in __assert_fail_base  at assert.c:92
3  0x00007f16c6c46e86 in __GI___assert_fail (assertion=assertion@entry=0x562e9bcdaadd "iwp->src == NULL", file=file@entry=0x562e9bcdaac8 "../chardev/char-io.c", line=line@entry=99, function=function@entry=0x562e9bcdab10 <__PRETTY_FUNCTION__.20549> "io_watch_poll_finalize") at assert.c:101
4  0x0000562e9ba20c2c in io_watch_poll_finalize (source=<optimized out>) at ../chardev/char-io.c:99
5  io_watch_poll_finalize (source=<optimized out>) at ../chardev/char-io.c:88
6  0x00007f16c904aae0 in g_source_unref_internal () from /lib64/libglib-2.0.so.0
7  0x00007f16c904baf9 in g_source_destroy_internal () from /lib64/libglib-2.0.so.0
8  0x0000562e9ba20db0 in io_remove_watch_poll (source=0x562e9d6720b0) at ../chardev/char-io.c:147
9  remove_fd_in_watch (chr=chr@entry=0x562e9d5f3800) at ../chardev/char-io.c:153
10 0x0000562e9ba23ffb in update_ioc_handlers (s=0x562e9d5f3800) at ../chardev/char-socket.c:592
11 0x0000562e9ba2072f in qemu_chr_fe_set_handlers_full at ../chardev/char-fe.c:279
12 0x0000562e9ba207a9 in qemu_chr_fe_set_handlers at ../chardev/char-fe.c:304
13 0x0000562e9ba2ca75 in monitor_qmp_setup_handlers_bh (opaque=0x562e9d4c2c60) at ../monitor/qmp.c:509
14 0x0000562e9bb6222e in aio_bh_poll (ctx=ctx@entry=0x562e9d4c2f20) at ../util/async.c:216
15 0x0000562e9bb4de0a in aio_poll (ctx=0x562e9d4c2f20, blocking=blocking@entry=true) at ../util/aio-posix.c:722
16 0x0000562e9b99dfaa in iothread_run (opaque=0x562e9d4c26f0) at ../iothread.c:63
17 0x0000562e9bb505a4 in qemu_thread_start (args=0x562e9d4c7ea0) at ../util/qemu-thread-posix.c:543
18 0x00007f16c70081ca in start_thread (arg=<optimized out>) at pthread_create.c:479
19 0x00007f16c6c398d3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

io_remove_watch_poll(), which makes sure that iwp->src is NULL, calls
g_source_destroy() which finds that iwp->src is not NULL in the finalize
callback. This can only happen if another thread has managed to trigger
io_watch_poll_prepare() callback in the meantime.

Move iwp->src destruction back to the finalize callback to prevent the
described race, and also remove the stale comment. The deadlock glib bug
was fixed back in 2010 by b35820285668 ("gmain: move finalization of
GSource outside of context lock").

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sergey Dyasli <sergey.dyasli@nutanix.com>
Link: https://lore.kernel.org/r/20240712092659.216206-1-sergey.dyasli@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

chardev/char-io.c

@@ -87,16 +87,12 @@ static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback,
 
 static void io_watch_poll_finalize(GSource *source)
 {
-    /*
-     * Due to a glib bug, removing the last reference to a source
-     * inside a finalize callback causes recursive locking (and a
-     * deadlock).  This is not a problem inside other callbacks,
-     * including dispatch callbacks, so we call io_remove_watch_poll
-     * to remove this source.  At this point, iwp->src must
-     * be NULL, or we would leak it.
-     */
     IOWatchPoll *iwp = io_watch_poll_from_source(source);
-    assert(iwp->src == NULL);
+    if (iwp->src) {
+        g_source_destroy(iwp->src);
+        g_source_unref(iwp->src);
+        iwp->src = NULL;
+    }
 }
 
 static GSourceFuncs io_watch_poll_funcs = {
@@ -139,11 +135,6 @@ static void io_remove_watch_poll(GSource *source)
     IOWatchPoll *iwp;
 
     iwp = io_watch_poll_from_source(source);
-    if (iwp->src) {
-        g_source_destroy(iwp->src);
-        g_source_unref(iwp->src);
-        iwp->src = NULL;
-    }
     g_source_destroy(&iwp->parent);
 }