Draw: Avoid hang when depsgraph update happens during draw

This should not happen and any failure here should be considered a bug.
But for end users better not to hang Blender, and to have a better
diagnostic for developers in bug reports.

Ref #82483

Pull Request: https://projects.blender.org/blender/blender/pulls/123023

source/blender/blenlib/BLI_threads.h

@@ -144,6 +144,7 @@ typedef struct TicketMutex TicketMutex;
 TicketMutex *BLI_ticket_mutex_alloc(void);
 void BLI_ticket_mutex_free(TicketMutex *ticket);
 void BLI_ticket_mutex_lock(TicketMutex *ticket);
+bool BLI_ticket_mutex_lock_check_recursive(TicketMutex *ticket);
 void BLI_ticket_mutex_unlock(TicketMutex *ticket);
 
 /* Condition */

source/blender/blenlib/intern/threads.cc

@@ -504,6 +504,8 @@ struct TicketMutex {
   pthread_cond_t cond;
   pthread_mutex_t mutex;
   uint queue_head, queue_tail;
+  pthread_t owner;
+  bool has_owner;
 };
 
 TicketMutex *BLI_ticket_mutex_alloc()
@@ -524,24 +526,46 @@ void BLI_ticket_mutex_free(TicketMutex *ticket)
   MEM_freeN(ticket);
 }
 
-void BLI_ticket_mutex_lock(TicketMutex *ticket)
+static bool ticket_mutex_lock(TicketMutex *ticket, const bool check_recursive)
 {
   uint queue_me;
 
   pthread_mutex_lock(&ticket->mutex);
+
+  /* Check for recursive locks, for debugging only. */
+  if (check_recursive && ticket->has_owner && pthread_equal(pthread_self(), ticket->owner)) {
+    pthread_mutex_unlock(&ticket->mutex);
+    return false;
+  }
+
   queue_me = ticket->queue_tail++;
 
   while (queue_me != ticket->queue_head) {
     pthread_cond_wait(&ticket->cond, &ticket->mutex);
   }
 
+  ticket->owner = pthread_self();
+  ticket->has_owner = true;
+
   pthread_mutex_unlock(&ticket->mutex);
+  return true;
+}
+
+void BLI_ticket_mutex_lock(TicketMutex *ticket)
+{
+  ticket_mutex_lock(ticket, false);
+}
+
+bool BLI_ticket_mutex_lock_check_recursive(TicketMutex *ticket)
+{
+  return ticket_mutex_lock(ticket, true);
 }
 
 void BLI_ticket_mutex_unlock(TicketMutex *ticket)
 {
   pthread_mutex_lock(&ticket->mutex);
   ticket->queue_head++;
+  ticket->has_owner = false;
   pthread_cond_broadcast(&ticket->cond);
   pthread_mutex_unlock(&ticket->mutex);
 }

source/blender/draw/intern/draw_manager_c.cc

@@ -8,6 +8,8 @@
 
 #include <cstdio>
 
+#include "CLG_log.h"
+
 #include "BLI_alloca.h"
 #include "BLI_listbase.h"
 #include "BLI_memblock.h"
@@ -103,6 +105,8 @@
 
 #include "DRW_select_buffer.hh"
 
+static CLG_LogRef LOG = {"draw.manager"};
+
 /** Render State: No persistent data between draw calls. */
 DRWManager DST = {nullptr};
 
@@ -1327,10 +1331,17 @@ void DRW_notify_view_update(const DRWUpdateContext *update_ctx)
 
   const bool gpencil_engine_needed = drw_gpencil_engine_needed(depsgraph, v3d);
 
-  /* XXX Really nasty locking. But else this could
-   * be executed by the material previews thread
-   * while rendering a viewport. */
-  BLI_ticket_mutex_lock(DST.system_gpu_context_mutex);
+  /* XXX Really nasty locking. But else this could be executed by the
+   * material previews thread while rendering a viewport.
+   *
+   * Check for recursive lock which can deadlock. This should not
+   * happen, but in case there is a bug where depsgraph update is called
+   * during drawing we try not to hang Blender. */
+  if (!BLI_ticket_mutex_lock_check_recursive(DST.system_gpu_context_mutex)) {
+    CLOG_ERROR(&LOG, "GPU context already bound");
+    BLI_assert_unreachable();
+    return;
+  }
 
   /* Reset before using it. */
   drw_state_prepare_clean_for_draw(&DST);