player: fix --lavfi-complex freeze

Commit 0e0b87b6f3 fixed that dropped packets did not trigger further
work correctly. But it also made trivial --lavfi-complex freeze. The
reason is that the meaning if DATA_AGAIN was overloaded: the decoders
meant that they should be called again, while lavfi.c meant that other
outputs needed to be checked again. Rename the latter meaning to
DATA_STARVE, which means that the current input will deliver no more
data, until "other" work has been done (like reading other outputs, or
feeding input).

The decoders never return DATA_STARVE, because they don't get input from
the player core (instead, they get it from the demuxer directly, which
is why they still can return DATA_WAIT).

Also document the DATA_* semantics in the enum.

Fixes #4746.

common/common.h

@@ -59,10 +59,11 @@ enum stream_type {
 };
 
 enum {
-    DATA_OK     = 1,
+    DATA_OK     = 1,        // data is actually being returned
-    DATA_WAIT   = 0,
+    DATA_WAIT   = 0,        // async wait: check state again after next wakeup
-    DATA_AGAIN  = -1,
+    DATA_AGAIN  = -2,       // repeat request (internal progress was made)
-    DATA_EOF    = -2,
+    DATA_STARVE = -1,       // need input (might require to drain other outputs)
+    DATA_EOF    = -3,       // no more data available
 };
 
 extern const char mpv_version[];

player/audio.c

@@ -51,6 +51,7 @@ enum {
     AD_NEW_FMT = -3,
     AD_WAIT = -4,
     AD_NO_PROGRESS = -5,
+    AD_STARVE = -6,
 };
 
 // Use pitch correction only for speed adjustments by the user, not minor sync
@@ -846,6 +847,7 @@ static int decode_new_frame(struct ao_chain *ao_c)
     case DATA_OK:       return AD_OK;
     case DATA_WAIT:     return AD_WAIT;
     case DATA_AGAIN:    return AD_NO_PROGRESS;
+    case DATA_STARVE:   return AD_STARVE;
     case DATA_EOF:      return AD_EOF;
     default:            abort();
     }
@@ -880,7 +882,7 @@ static int filter_audio(struct MPContext *mpctx, struct mp_audio_buffer *outbuf,
         res = decode_new_frame(ao_c);
         if (res == AD_NO_PROGRESS)
             continue;
-        if (res == AD_WAIT)
+        if (res == AD_WAIT || res == AD_STARVE)
             break;
         if (res < 0) {
             // drain filters first (especially for true EOF case)

player/lavfi.c

@@ -716,12 +716,12 @@ static int lavfi_request_frame(struct lavfi_pad *pad)
     } else if (pad->main->all_waiting) {
         return DATA_WAIT;
     }
-    return DATA_AGAIN;
+    return DATA_STARVE;
 }
 
 // Try to read a new frame from an output pad. Returns one of the following:
 //      DATA_OK: a frame is returned
-//      DATA_AGAIN: needs more input data
+//      DATA_STARVE: needs more input data
 //      DATA_WAIT: needs more input data, and all inputs in LAVFI_WAIT state
 //      DATA_EOF: no more data
 int lavfi_request_frame_a(struct lavfi_pad *pad, struct mp_audio **out_aframe)
@@ -750,7 +750,7 @@ bool lavfi_needs_input(struct lavfi_pad *pad)
 
 // A filter user is supposed to call lavfi_needs_input(), and if that returns
 // true, send either a new status or a frame. A status can be one of:
-//      DATA_AGAIN: a new frame/status will come, caller will retry
+//      DATA_STARVE: a new frame/status will come, caller will retry
 //      DATA_WAIT: a new frame/status will come, but caller goes to sleep
 //      DATA_EOF: no more input possible (in near time)
 // If you have a new frame, use lavfi_send_frame_ instead.
@@ -764,7 +764,7 @@ void lavfi_send_status(struct lavfi_pad *pad, int status)
     assert(!pad->pending_v && !pad->pending_a);
 
     pad->input_waiting = status == DATA_WAIT || status == DATA_EOF;
-    pad->input_again = status == DATA_AGAIN;
+    pad->input_again = status == DATA_STARVE;
     pad->input_eof = status == DATA_EOF;
 }
 

player/video.c

@@ -617,6 +617,7 @@ static int decode_image(struct MPContext *mpctx)
     switch (res) {
     case DATA_WAIT:     return VD_WAIT;
     case DATA_OK:
+    case DATA_STARVE:
     case DATA_AGAIN:    return VD_PROGRESS;
     case DATA_EOF:      return VD_EOF;
     default:            abort();