client API: provide ways to finish property changes on file changes

When the current file changes (or rather, when starting/finishing playback of a playlist entry), clients tend to have the problem that it's hard to tell whether a property change notification (via mpv_observe_property() and mechanisms layered on top of it) is from the previous or new playlist entry. The previous commit probably helps, but all the asynchronity is still a bit unhelpful. Try to make this better by adding new hooks, that are run before/after playback init/deinit. This is similar to the existing hooks, except they're outside of "initialized" playback, which excludes that you might accidentally get an overlap between the current and the previous/next playlist entry. That still doesn't seem quite enough, since normally, property change notifications come after the hook event. So basically a client would have to explicitly "drain" the event queue within the hook, and make the hook continue only after that is done. Knowing when property notifications are done is another asynchronous nightmare (how exactly it works keeps changing within client.c, and an API user probably can't tell anymore when all pending properties are truly done). So introduce another guarantee: properties that were changed before the hook happens will be returned before the hook event is returned. That means the client will have received all pending property notifications from the previous playlist entry (or whatever) before the hook is entered. As another minor complication, we shouldn't just keep the hook pending until _all_ property notifications are done, since the client's hook could produce new ones. (Or just consider things like the demuxer thread hammering the client with cache update events, while the "on_preloaded" hook is run.) So there is some extra untested, fragile logic in client.c to handle this (the waiting_for_hook flag). This probably works, but was barely tested. Not sure if this helps anyone, but I think it's fine for my own purposes. (I really hated this aspect of the API whenever I used it myself.)

wm4 committed 6 years ago

Commit ba70b151 parent 8a4f8124 files · +104 −5Browse files

Unified · Split

4 files

DOCS/man/input.rst +16 −0
TOOLS/lua/test-hooks.lua +32 −0
player/client.c +47 −3
player/loadfile.c +9 −2

DOCS/man/input.rst+16−0

@@ -1253,6 +1253,12 @@ the player freeze randomly. Basically, nobody should use this API. · 1252 unmodified lines
1253	The C API is described in the header files. The Lua API is described in the	1253	The C API is described in the header files. The Lua API is described in the
1254	Lua section.	1254	Lua section.
1255		1255
		1256	Before a hook is actually invoked on an API clients, it will attempt to return
		1257	new values for all observed properties that were changed before the hook. This
		1258	may make it easier for an application to set defined "barriers" between property
		1259	change notifications by registering hooks. (That means these hooks will have an
		1260	effect, even if you do nothing and make them continue immediately.)
		1261
1256	The following hooks are currently defined:	1262	The following hooks are currently defined:
1257		1263
1258	``on_load``	1264	``on_load``
@@ -1286,6 +1292,16 @@ The following hooks are currently defined: · 27 unmodified lines
1286	Run before closing a file, and before actually uninitializing	1292	Run before closing a file, and before actually uninitializing
1287	everything. It's not possible to resume playback in this state.	1293	everything. It's not possible to resume playback in this state.
1288		1294
		1295	``on_before_start_file``
		1296	Run before a ``start-file`` event is sent. (If any client changes the
		1297	current playlist entry, or sends a quit command to the player, the
		1298	corresponding event will not actually happen after the hook returns.)
		1299	Useful to drain property changes before a new file is loaded.
		1300
		1301	``on_after_end_file``
		1302	Run after an ``end-file`` event. Useful to drain property changes after a
		1303	file has finished.
		1304
1289	Input Command Prefixes	1305	Input Command Prefixes
1290	----------------------	1306	----------------------
1291		1307

TOOLS/lua/test-hooks.lua+32−0

@@ -0,0 +1,32 @@
	1	local utils = require("mp.utils")
	2
	3	function hardsleep()
	4	os.execute("sleep 1s")
	5	end
	6
	7	local hooks = {"on_before_start_file", "on_load", "on_load_fail",
	8	"on_preloaded", "on_unload", "on_after_end_file"}
	9
	10	for _, name in ipairs(hooks) do
	11	mp.add_hook(name, 0, function()
	12	print("--- hook: " .. name)
	13	hardsleep()
	14	print(" ... continue")
	15	end)
	16	end
	17
	18	local events = {"start-file", "end-file", "file-loaded", "seek",
	19	"playback-restart", "idle", "shutdown"}
	20	for _, name in ipairs(events) do
	21	mp.register_event(name, function()
	22	print("--- event: " .. name)
	23	end)
	24	end
	25
	26	local props = {"path", "metadata"}
	27	for _, name in ipairs(props) do
	28	mp.observe_property(name, "native", function(name, val)
	29	print("property '" .. name .. "' changed to '" ..
	30	utils.to_string(val) .. "'")
	31	end)
	32	end

player/client.c+47−3

     union m_option_value value;
     uint64_t value_ret_ts;  // logical timestamp of value returned to user
     union m_option_value value_ret;
+    bool waiting_for_hook;  // flag for draining old property changes on a hook
 };
 struct mpv_handle {
     size_t async_counter;   // pending other async events
     bool choked;            // recovering from queue overflow
     bool destroying;        // pending destruction; no API accesses allowed
+    bool hook_pending;      // hook events are returned after draining properties
     struct observe_property **properties;
     int num_properties;
     return 0;
 }
+// Set waiting_for_hook==true for all possibly pending properties.
+static void set_wait_for_hook_flags(mpv_handle *ctx)
+{
+    for (int n = 0; n < ctx->num_properties; n++) {
+        struct observe_property *prop = ctx->properties[n];
+        if (prop->value_ret_ts != prop->change_ts)
+            prop->waiting_for_hook = true;
+    }
+}
+// Return whether any property still has waiting_for_hook set.
+static bool check_for_for_hook_flags(mpv_handle *ctx)
+{
+    for (int n = 0; n < ctx->num_properties; n++) {
+        if (ctx->properties[n]->waiting_for_hook)
+            return true;
+    }
+    return false;
+}
 mpv_event *mpv_wait_event(mpv_handle *ctx, double timeout)
 {
     mpv_event *event = ctx->cur_event;
             event->event_id = MPV_EVENT_QUEUE_OVERFLOW;
             break;
         }
-        if (ctx->num_events) {
+        struct mpv_event *ev =
-            *event = ctx->events[ctx->first_event];
+            ctx->num_events ? &ctx->events[ctx->first_event] : NULL;
+        if (ev && ev->event_id == MPV_EVENT_HOOK) {
+            // Give old property notifications priority over hooks. This is a
+            // guarantee given to clients to simplify their logic. New property
+            // changes after this are treated normally, so
+            if (!ctx->hook_pending) {
+                ctx->hook_pending = true;
+                set_wait_for_hook_flags(ctx);
+            }
+            if (check_for_for_hook_flags(ctx)) {
+                ev = NULL; // delay
+            } else {
+                ctx->hook_pending = false;
+            }
+        }
+        if (ev) {
+            *event = *ev;
             ctx->first_event = (ctx->first_event + 1) % ctx->max_events;
             ctx->num_events--;
             talloc_steal(event, event->data);
             ctx->async_counter -= 1;
             prop_unref(prop);
-            // Set of observed properties was changed or something similar
+            // Set if observed properties was changed or something similar
             // => start over, retry next time.
             if (cur_ts != ctx->properties_change_ts || ctx->destroying) {
                 m_option_free(type, &val);
             changed = true;
         }
+        if (prop->waiting_for_hook)
+            ctx->new_property_events = true; // make sure to wakeup
         // Avoid retriggering the change event if the property didn't change,
         // and the previous value was actually returned to the client.
         if (!changed && prop->value_ret_ts == prop->value_ts) {
             prop->value_ret_ts = prop->change_ts; // no change => no event
+            prop->waiting_for_hook = false;
         } else {
             ctx->new_property_events = true;
         }
             prop->value_ret_ts != prop->value_ts)   // other value than last time?
         {
             prop->value_ret_ts = prop->value_ts;
+            prop->waiting_for_hook = false;
             prop_unref(ctx->cur_property);
             ctx->cur_property = prop;
             prop->refcount += 1;

player/loadfile.c+9−2

     while (!mp_hook_test_completion(mpctx, name)) {
         mp_idle(mpctx);
-        // We have no idea what blocks a hook, so just do a full abort.
+        // We have no idea what blocks a hook, so just do a full abort. This
+        // does nothing for hooks that happen outside of playback.
         if (mpctx->stop_play)
             mp_abort_playback_async(mpctx);
     }
     double playback_start = -1e100;
     assert(mpctx->stop_play);
+    mpctx->stop_play = 0;
+    process_hooks(mpctx, "on_before_start_file");
+    if (mpctx->stop_play)
+        return;
     mp_notify(mpctx, MPV_EVENT_START_FILE, NULL);
     mp_cancel_reset(mpctx->playback_abort);
     mpctx->error_playing = MPV_ERROR_LOADING_FAILED;
-    mpctx->stop_play = 0;
     mpctx->filename = NULL;
     mpctx->shown_aframes = 0;
     mpctx->shown_vframes = 0;
     }
     assert(mpctx->stop_play);
+    process_hooks(mpctx, "on_after_end_file");
 }
 // Determine the next file to play. Note that if this function returns non-NULL,

client API: provide ways to finish property changes on file changes

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