Implement buffering

To minimize latency (at the cost of jitter), scrcpy always displays a
frame as soon as it available, without waiting.

However, when recording (--record), it still writes the captured
timestamps to the output file, so that the recorded file can be played
correctly without jitter.

Some real-time use cases might benefit from adding a small latency to
compensate for jitter too. For example, few tens of seconds of latency
for live-streaming are not important, but jitter is noticeable.

Therefore, implement a buffering mechanism (disabled by default) to add
a configurable latency delay.

PR #2417 <https://github.com/Genymobile/scrcpy/issues/2417>

app/src/video_buffer.c

@@ -1,35 +1,44 @@
 #include "video_buffer.h"
 
 #include <assert.h>
+#include <stdlib.h>
+
 #include <libavutil/avutil.h>
 #include <libavformat/avformat.h>
 
 #include "util/log.h"
 
-bool
-sc_video_buffer_init(struct sc_video_buffer *vb,
-                     const struct sc_video_buffer_callbacks *cbs,
-                     void *cbs_userdata) {
-    bool ok = sc_frame_buffer_init(&vb->fb);
-    if (!ok) {
-        return false;
+static struct sc_video_buffer_frame *
+sc_video_buffer_frame_new(const AVFrame *frame) {
+    struct sc_video_buffer_frame *vb_frame = malloc(sizeof(*vb_frame));
+    if (!vb_frame) {
+        return NULL;
     }
 
-    assert(cbs);
-    assert(cbs->on_new_frame);
+    vb_frame->frame = av_frame_alloc();
+    if (!vb_frame->frame) {
+        free(vb_frame);
+        return NULL;
+    }
 
-    vb->cbs = cbs;
-    vb->cbs_userdata = cbs_userdata;
-    return true;
+    if (av_frame_ref(vb_frame->frame, frame)) {
+        av_frame_free(&vb_frame->frame);
+        free(vb_frame);
+        return NULL;
+    }
+
+    return vb_frame;
 }
 
-void
-sc_video_buffer_destroy(struct sc_video_buffer *vb) {
-    sc_frame_buffer_destroy(&vb->fb);
+static void
+sc_video_buffer_frame_delete(struct sc_video_buffer_frame *vb_frame) {
+    av_frame_unref(vb_frame->frame);
+    av_frame_free(&vb_frame->frame);
+    free(vb_frame);
 }
 
-bool
-sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame) {
+static bool
+sc_video_buffer_offer(struct sc_video_buffer *vb, const AVFrame *frame) {
     bool previous_skipped;
     bool ok = sc_frame_buffer_push(&vb->fb, frame, &previous_skipped);
     if (!ok) {
@@ -40,6 +49,196 @@ sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame) {
     return true;
 }
 
+static int
+run_buffering(void *data) {
+    struct sc_video_buffer *vb = data;
+
+    assert(vb->buffering_time > 0);
+
+    for (;;) {
+        sc_mutex_lock(&vb->b.mutex);
+
+        while (!vb->b.stopped && sc_queue_is_empty(&vb->b.queue)) {
+            sc_cond_wait(&vb->b.queue_cond, &vb->b.mutex);
+        }
+
+        if (vb->b.stopped) {
+            sc_mutex_unlock(&vb->b.mutex);
+            goto stopped;
+        }
+
+        struct sc_video_buffer_frame *vb_frame;
+        sc_queue_take(&vb->b.queue, next, &vb_frame);
+
+        sc_tick max_deadline = sc_tick_now() + vb->buffering_time;
+        // PTS (written by the server) are expressed in microseconds
+        sc_tick pts = SC_TICK_TO_US(vb_frame->frame->pts);
+
+        bool timed_out = false;
+        while (!vb->b.stopped && !timed_out) {
+            sc_tick deadline = sc_clock_to_system_time(&vb->b.clock, pts)
+                             + vb->buffering_time;
+            if (deadline > max_deadline) {
+                deadline = max_deadline;
+            }
+
+            timed_out =
+                !sc_cond_timedwait(&vb->b.wait_cond, &vb->b.mutex, deadline);
+        }
+
+        if (vb->b.stopped) {
+            sc_video_buffer_frame_delete(vb_frame);
+            sc_mutex_unlock(&vb->b.mutex);
+            goto stopped;
+        }
+
+        sc_mutex_unlock(&vb->b.mutex);
+
+        sc_video_buffer_offer(vb, vb_frame->frame);
+
+        sc_video_buffer_frame_delete(vb_frame);
+    }
+
+stopped:
+    // Flush queue
+    while (!sc_queue_is_empty(&vb->b.queue)) {
+        struct sc_video_buffer_frame *vb_frame;
+        sc_queue_take(&vb->b.queue, next, &vb_frame);
+        sc_video_buffer_frame_delete(vb_frame);
+    }
+
+    LOGD("Buffering thread ended");
+
+    return 0;
+}
+
+bool
+sc_video_buffer_init(struct sc_video_buffer *vb, sc_tick buffering_time,
+                     const struct sc_video_buffer_callbacks *cbs,
+                     void *cbs_userdata) {
+    bool ok = sc_frame_buffer_init(&vb->fb);
+    if (!ok) {
+        return false;
+    }
+
+    assert(buffering_time >= 0);
+    if (buffering_time) {
+        ok = sc_mutex_init(&vb->b.mutex);
+        if (!ok) {
+            LOGC("Could not create mutex");
+            sc_frame_buffer_destroy(&vb->fb);
+            return false;
+        }
+
+        ok = sc_cond_init(&vb->b.queue_cond);
+        if (!ok) {
+            LOGC("Could not create cond");
+            sc_mutex_destroy(&vb->b.mutex);
+            sc_frame_buffer_destroy(&vb->fb);
+            return false;
+        }
+
+        ok = sc_cond_init(&vb->b.wait_cond);
+        if (!ok) {
+            LOGC("Could not create wait cond");
+            sc_cond_destroy(&vb->b.queue_cond);
+            sc_mutex_destroy(&vb->b.mutex);
+            sc_frame_buffer_destroy(&vb->fb);
+            return false;
+        }
+
+        sc_clock_init(&vb->b.clock);
+        sc_queue_init(&vb->b.queue);
+    }
+
+    assert(cbs);
+    assert(cbs->on_new_frame);
+
+    vb->buffering_time = buffering_time;
+    vb->cbs = cbs;
+    vb->cbs_userdata = cbs_userdata;
+    return true;
+}
+
+bool
+sc_video_buffer_start(struct sc_video_buffer *vb) {
+    if (vb->buffering_time) {
+        bool ok =
+            sc_thread_create(&vb->b.thread, run_buffering, "buffering", vb);
+        if (!ok) {
+            LOGE("Could not start buffering thread");
+            return false;
+        }
+    }
+
+    return true;
+}
+
+void
+sc_video_buffer_stop(struct sc_video_buffer *vb) {
+    if (vb->buffering_time) {
+        sc_mutex_lock(&vb->b.mutex);
+        vb->b.stopped = true;
+        sc_cond_signal(&vb->b.queue_cond);
+        sc_cond_signal(&vb->b.wait_cond);
+        sc_mutex_unlock(&vb->b.mutex);
+    }
+}
+
+void
+sc_video_buffer_join(struct sc_video_buffer *vb) {
+    if (vb->buffering_time) {
+        sc_thread_join(&vb->b.thread, NULL);
+    }
+}
+
+void
+sc_video_buffer_destroy(struct sc_video_buffer *vb) {
+    sc_frame_buffer_destroy(&vb->fb);
+    if (vb->buffering_time) {
+        sc_cond_destroy(&vb->b.wait_cond);
+        sc_cond_destroy(&vb->b.queue_cond);
+        sc_mutex_destroy(&vb->b.mutex);
+    }
+}
+
+bool
+sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame) {
+    if (!vb->buffering_time) {
+        // No buffering
+        return sc_video_buffer_offer(vb, frame);
+    }
+
+    sc_mutex_lock(&vb->b.mutex);
+
+    sc_tick pts = SC_TICK_FROM_US(frame->pts);
+    sc_clock_update(&vb->b.clock, sc_tick_now(), pts);
+    sc_cond_signal(&vb->b.wait_cond);
+
+    if (vb->b.clock.count == 1) {
+        sc_mutex_unlock(&vb->b.mutex);
+        // First frame, offer it immediately, for two reasons:
+        //  - not to delay the opening of the scrcpy window
+        //  - the buffering estimation needs at least two clock points, so it
+        //  could not handle the first frame
+        return sc_video_buffer_offer(vb, frame);
+    }
+
+    struct sc_video_buffer_frame *vb_frame = sc_video_buffer_frame_new(frame);
+    if (!vb_frame) {
+        sc_mutex_unlock(&vb->b.mutex);
+        LOGE("Could not allocate frame");
+        return false;
+    }
+
+    sc_queue_push(&vb->b.queue, next, vb_frame);
+    sc_cond_signal(&vb->b.queue_cond);
+
+    sc_mutex_unlock(&vb->b.mutex);
+
+    return true;
+}
+
 void
 sc_video_buffer_consume(struct sc_video_buffer *vb, AVFrame *dst) {
     sc_frame_buffer_consume(&vb->fb, dst);