client API: send MPV_EVENT_SHUTDOWN only once

Before this change, mpv_wait_event() could inconsistently return
multiple MPV_EVENT_SHUTDOWN events to a single mpv_handle, up to the
point of spamming the event queue under certain circumstances. Change
this and just send it exactly once to each mpv_handle.

Some client API users might have weird requirements about destroying
their state asynchronously (and not reacting immediately to the SHUTDOWN
event). This change will help a bit to make this less weird and
surprising.

DOCS/client-api-changes.rst

@@ -40,6 +40,8 @@ API changes
         - rename mpv_detach_destroy() to mpv_destroy() (the old function will
           remain valid as deprecated alias)
         - add mpv_create_weak_client(), which makes use of above changes
+        - MPV_EVENT_SHUTDOWN is now returned exactly once if a mpv_handle
+          should terminate, instead of spamming the event queue with this event
  1.28   - deprecate the render opengl_cb API, and replace it with render.h
           and render_gl.h. The goal is allowing support for APIs other than
           OpenGL. The old API is emulated with the new API.

libmpv/client.h

@@ -1166,9 +1166,8 @@ typedef enum mpv_event_id {
     /**
      * Happens when the player quits. The player enters a state where it tries
      * to disconnect all clients. Most requests to the player will fail, and
-     * mpv_wait_event() will always return instantly (returning new shutdown
-     * events if no other events are queued). The client should react to this
-     * and quit with mpv_destroy() as soon as possible.
+     * the client should react to this and quit with mpv_destroy() as soon as
+     * possible.
      */
     MPV_EVENT_SHUTDOWN          = 1,
     /**

player/client.c

@@ -634,6 +634,8 @@ static int append_event(struct mpv_handle *ctx, struct mpv_event event, bool cop
     ctx->events[(ctx->first_event + ctx->num_events) % ctx->max_events] = event;
     ctx->num_events++;
     wakeup_client(ctx);
+    if (event.event_id == MPV_EVENT_SHUTDOWN)
+        ctx->event_mask &= ctx->event_mask & ~(1ULL << MPV_EVENT_SHUTDOWN);
     return 0;
 }