ges: discoverer-manager: Fix race when freeing unused discoverer

When a new discoverer was created for a thread so discovery could
recurse we could end up removing the wrong discoverer info from the
cache leading to freeing it while it was still discovering URIS, which
lead to the following assertion:

``` bt
Thread 1 (Thread 0x7fcc2e1a5840 (LWP 1855496)):
 #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
 #1  0x00007fcc2e9d98a3 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at pthread_kill.c:78
 #2  0x00007fcc2e9878ee in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
 #3  0x00007fcc2e96f8ff in __GI_abort () at abort.c:79
 #4  0x00007fcc2ed80056 in g_assertion_message (domain=domain@entry=0x7fcc2f2c19f9 "GES", file=file@entry=0x7fcc2f2dfd68 "../subprojects/gst-editing-services/ges/ges-discoverer-manager.c", line=line@entry=20, func=func@entry=0x7fcc2f2e0030 <__func__.7> "ges_discoverer_data_free", message=message@entry=0x12dab70 "assertion failed: (data->n_uri == 0)") at ../glib/gtestutils.c:3497
 #5  0x00007fcc2ede1d87 in g_assertion_message_expr (domain=domain@entry=0x7fcc2f2c19f9 "GES", file=file@entry=0x7fcc2f2dfd68 "../subprojects/gst-editing-services/ges/ges-discoverer-manager.c", line=line@entry=20, func=func@entry=0x7fcc2f2e0030 <__func__.7> "ges_discoverer_data_free", expr=expr@entry=0x7fcc2f2dfcf1 "data->n_uri == 0") at ../glib/gtestutils.c:3523
 #6  0x00007fcc2f2bd5c5 in ges_discoverer_data_free (data=0x160e430) at ../subprojects/gst-editing-services/ges/ges-discoverer-manager.c:20
 #7  0x00007fcc2ed8509d in g_atomic_rc_box_release_full (clear_func=0x7fcc2f2bd4f0 <ges_discoverer_data_free>, mem_block=0x160e430) at ../glib/garcbox.c:355
 #8  g_atomic_rc_box_release_full (mem_block=0x160e430, clear_func=0x7fcc2f2bd4f0 <ges_discoverer_data_free>) at ../glib/garcbox.c:338
 #9  0x00007fcc2eda6809 in g_hash_table_remove_internal (notify=1, key=0x10448a0, hash_table=0x12e0be0) at ../glib/ghash.c:1776
 #10 g_hash_table_remove (hash_table=0x12e0be0, key=0x10448a0) at ../glib/ghash.c:1804
 #11 0x00007fcc2f2bd95f in cleanup_discoverer_cb (discoverer_data=discoverer_data@entry=0x13e7000) at ../subprojects/gst-editing-services/ges/ges-discoverer-manager.c:379
 #12 0x00007fcc2edbc759 in g_timeout_dispatch (source=0x15a6060, callback=0x7fcc2f2bd910 <cleanup_discoverer_cb>, user_data=0x13e7000) at ../glib/gmain.c:5121
 #13 0x00007fcc2edbbe1c in g_main_dispatch (context=0x1044700) at ../glib/gmain.c:3476
 #14 g_main_context_dispatch_unlocked (context=0x1044700) at ../glib/gmain.c:4284
 #15 0x00007fcc2ee16d78 in g_main_context_iterate_unlocked.isra.0 (context=0x1044700, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:4349
 #16 0x00007fcc2edbd407 in g_main_loop_run (loop=0x12ccbd0) at ../glib/gmain.c:4551
 #17 0x00007fcc2f285791 in ges_uri_clip_asset_request_sync (uri=uri@entry=0x12d7980 "file:///var/home/phil/gstreamer/build/subprojects/gst-integration-testsuites/logs/ges/scenarios/check_seek_on_very_deeply_nested_timeline/nested_timeline_depth6.xges", error=error@entry=0x7fff499015a8) at ../subprojects/gst-editing-services/ges/ges-uri-asset.c:688
 #18 0x00007fcc2f28949b in ges_project_create_asset_sync (project=0x12c1c70, id=id@entry=0x12d7980 "file:///var/home/phil/gstreamer/build/subprojects/gst-integration-testsuites/logs/ges/scenarios/check_seek_on_very_deeply_nested_timeline/nested_timeline_depth6.xges", extractable_type=extractable_type@entry=Python Exception <class 'gdb.error'>: value has been optimized out , error=error@entry=0x7fff499015a8) at ../subprojects/gst-editing-services/ges/ges-project.c:959
 #19 0x00007fcc2f2ba484 in _ges_get_asset_from_timeline (timeline=timeline@entry=0x12bdc80, type=type@entry=Python Exception <class 'gdb.error'>: value has been optimized out , id=id@entry=0x12d7980 "file:///var/home/phil/gstreamer/build/subprojects/gst-integration-testsuites/logs/ges/scenarios/check_seek_on_very_deeply_nested_timeline/nested_timeline_depth6.xges", error=error@entry=0x7fff49901728) at ../subprojects/gst-editing-services/ges/ges-structured-interface.c:540
 #20 0x00007fcc2f2ba9b2 in _ges_add_clip_from_struct (timeline=0x12bdc80, structure=0x157f690, error=0x7fff49901728) at ../subprojects/gst-editing-services/ges/ges-structured-interface.c:697
 #21 0x00007fcc2f2b6a9d in _validate_action_execute (scenario=0x15f7620, action=0x157f500) at ../subprojects/gst-editing-services/ges/ges-validate.c:922
 #22 0x00007fcc2eef5c9c in gst_validate_execute_action (action=0x157f500, action_type=0x13e0500) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2541
 #23 gst_validate_execute_action (action_type=0x13e0500, action=0x157f500) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2507
 #24 0x00007fcc2eef8ce3 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2782
 #25 0x00007fcc2eef9dee in _action_set_done (action=0x157efb0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #26 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157efb0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #27 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157efb0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #28 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #29 0x00007fcc2eef9dee in _action_set_done (action=0x157ea60) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #30 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157ea60, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #31 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157ea60) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #32 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #33 0x00007fcc2eef9dee in _action_set_done (action=0x157e510) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #34 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157e510, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #35 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157e510) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #36 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #37 0x00007fcc2eef9dee in _action_set_done (action=0x157df10) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #38 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157df10, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #39 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157df10) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #40 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #41 0x00007fcc2eef9dee in _action_set_done (action=0x157d9e0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #42 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157d9e0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #43 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157d9e0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #44 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #45 0x00007fcc2eef9dee in _action_set_done (action=0x157d3e0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #46 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157d3e0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #47 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157d3e0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #48 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #49 0x00007fcc2eef9dee in _action_set_done (action=0x157cf70) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #50 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157cf70, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #51 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157cf70) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #52 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #53 0x00007fcc2eef9dee in _action_set_done (action=0x157cb00) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #54 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157cb00, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #55 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157cb00) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #56 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #57 0x00007fcc2eef9dee in _action_set_done (action=0x157c690) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #58 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157c690, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #59 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157c690) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #60 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #61 0x00007fcc2eef9dee in _action_set_done (action=0x157c220) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #62 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x157c220, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #63 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x157c220) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #64 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #65 0x00007fcc2eef9dee in _action_set_done (action=0x15233c0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #66 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x15233c0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #67 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x15233c0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #68 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #69 0x00007fcc2eef9dee in _action_set_done (action=0x1522f80) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #70 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x1522f80, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #71 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x1522f80) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #72 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #73 0x00007fcc2eef9dee in _action_set_done (action=0x1522ae0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #74 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x1522ae0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #75 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x1522ae0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #76 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #77 0x00007fcc2eef9dee in _action_set_done (action=0x1522190) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #78 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x1522190, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #79 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x1522190) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #80 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #81 0x00007fcc2eef9dee in _action_set_done (action=0x1520ea0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6368
 #82 0x00007fcc2edbd26d in g_main_context_invoke_full (context=0x1044700, priority=200, function=0x7fcc2eef9ab0 <_action_set_done>, data=0x1520ea0, notify=0x7fcc2eeea5d0 <gst_validate_action_unref>) at ../glib/gmain.c:6533
 #83 0x00007fcc2eef6cf2 in gst_validate_action_set_done (action=0x1520ea0) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:6411
 #84 0x00007fcc2eef9018 in execute_next_action_full (scenario=<optimized out>, message=<optimized out>) at ../subprojects/gst-devtools/validate/gst/validate/gst-validate-scenario.c:2803
 #85 0x00007fcc2edbc759 in g_timeout_dispatch (source=0x14b6340, callback=0x7fcc2eef99c0 <execute_next_action>, user_data=0x15f7620) at ../glib/gmain.c:5121
 #86 0x00007fcc2edbbe1c in g_main_dispatch (context=0x1044700) at ../glib/gmain.c:3476
 #87 g_main_context_dispatch_unlocked (context=0x1044700) at ../glib/gmain.c:4284
 #88 0x00007fcc2ee16d78 in g_main_context_iterate_unlocked.isra.0 (context=context@entry=0x1044700, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:4349
 #89 0x00007fcc2edb9a93 in g_main_context_iteration (context=context@entry=0x1044700, may_block=may_block@entry=1) at ../glib/gmain.c:4414
 #90 0x00007fcc2ec14c3d in g_application_run (application=application@entry=0x1042ab0, argc=argc@entry=4, argv=argv@entry=0x7fff499031e8) at ../gio/gapplication.c:2577
 #91 0x0000000000405dfd in real_main (argv=0x7fff499031e8, argc=4) at ../subprojects/gst-editing-services/tools/ges-launch.c:38
 #92 main (argc=4, argv=0x7fff499031e8) at ../subprojects/gst-editing-services/tools/ges-launch.c:56

```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5619>

subprojects/gst-editing-services/ges/ges-discoverer-manager.c

@@ -125,7 +125,6 @@ ges_discoverer_manager_finalize (GObject * object)
   if (!context)
     context = g_main_context_default ();
 
-
   g_mutex_lock (&self->lock);
   g_hash_table_iter_init (&iter, self->discoverers);
   while (g_hash_table_iter_next (&iter, NULL, (gpointer *) & discoverer_data)) {
@@ -367,16 +366,23 @@ cleanup_discoverer_cb (GESDiscovererData * discoverer_data)
     return G_SOURCE_REMOVE;
   }
 
+  g_mutex_lock (&self->lock);
   if (discoverer_data->n_uri > 0) {
     GST_DEBUG_OBJECT (self, "Discoverer still has %d uris to discover",
         discoverer_data->n_uri);
     goto done;
   }
 
-  g_mutex_lock (&self->lock);
-  res = G_SOURCE_REMOVE;
   GST_DEBUG_OBJECT (self, "Removing unused discoverer");
-  g_hash_table_remove (self->discoverers, discoverer_data->thread);
+
+  // Remove the discoverer if the one is use for that thread is still the
+  // one we have been asked to free, otherwise this one will be destroyed anyway
+  // once this source is removed
+  res = G_SOURCE_REMOVE;
+  if (g_hash_table_lookup (self->discoverers,
+          discoverer_data->thread) == discoverer_data) {
+    g_hash_table_remove (self->discoverers, discoverer_data->thread);
+  }
 
 done:
   g_mutex_unlock (&self->lock);