iface-modem: fix crash in wait_for_final_state_context_complete_and_free

When wait_for_final_state_context_complete_and_free is invoked, if the callback
associated with the GSimpleAsyncResult object of the WaitForFinalStateContext
object updates the modem state via mm_iface_modem_update_state, state_changed
is invoked, which causes wait_for_final_state_context_complete_and_free to be
invoked again on the same WaitForFinalStateContext object. That leads to the
following crash, which is observed sometimes when a modem is being disabled.

Thread 0 *CRASHED* ( SIGSEGV @ 0x00000000 )

0x7fcb7728f202   [libgobject-2.0.so.0.3400.3] - gobject.c:2916              g_object_unref
0x7fcb7743205c   [ModemManager]               - mm-broadband-modem.c:8034   disabling_context_complete_and_free
0x7fcb77434d64   [ModemManager]               - mm-broadband-modem.c:8130   disabling_wait_for_final_state_ready
0x7fcb770c0b86   [libgio-2.0.so.0.3400.3]     - gsimpleasyncresult.c:775    g_simple_async_result_complete
0x7fcb7740cdbc   [ModemManager]               - mm-iface-modem.c:101]       wait_for_final_state_context_complete_and_free
0x7fcb7740ce19   [ModemManager]               - mm-iface-modem.c:128]       state_changed_wait_expired
0x7fcb76f78c33   [libglib-2.0.so.0.3400.3]    - gmain.c:4026]               g_timeout_dispatch
0x7fcb76f78087   [libglib-2.0.so.0.3400.3]    - gmain.c:2715]               g_main_context_dispatch
0x7fcb76f78437   [libglib-2.0.so.0.3400.3]    - gmain.c:3290]               g_main_context_iterate
0x7fcb76f78891   [libglib-2.0.so.0.3400.3]    - gmain.c:3484]               g_main_loop_run
0x7fcb773f4d55   [ModemManager]               - main.c:142]                 main
0x7fcb7698a9c6   [libc-2.15.so]               - libc-start.c:234]           __libc_start_main
0x7fcb773f48b8   [ModemManager]               + 0x000218b8]

1 files changed, 6 insertions, 1 deletions

diff --git a/src/mm-iface-modem.c b/src/mm-iface-modem.c
index befb0e22..868df81c 100644
--- a/src/mm-iface-modem.c
+++ b/src/mm-iface-modem.c
@@ -98,9 +98,14 @@ typedef struct {
 static void
 wait_for_final_state_context_complete_and_free (WaitForFinalStateContext *ctx)
 {
+    /* The callback associated with 'ctx->result' may update the modem state.
+     * Disconnect the signal handler for modem state changes before completing
+     * 'ctx->result' in order to prevent state_changed from being invoked, which
+     * invokes wait_for_final_state_context_complete_and_free (ctx) again. */
+    g_signal_handler_disconnect (ctx->self, ctx->state_changed_id);
+
     g_simple_async_result_complete (ctx->result);
     g_object_unref (ctx->result);
-    g_signal_handler_disconnect (ctx->self, ctx->state_changed_id);
     g_source_remove (ctx->state_changed_wait_id);
     g_object_unref (ctx->self);
     g_slice_free (WaitForFinalStateContext, ctx);