Extract current video_buffer to frame_buffer

The current video buffer only stores one pending frame.

In order to add a new buffering feature, move this part to a separate
"frame buffer". Keep the video_buffer, which currently delegates all its
calls to the frame_buffer.

app/meson.build

@@ -10,6 +10,7 @@ src = [
     'src/event_converter.c',
     'src/file_handler.c',
     'src/fps_counter.c',
+    'src/frame_buffer.c',
     'src/input_manager.c',
     'src/opengl.c',
     'src/receiver.c',

app/src/frame_buffer.c

@@ -0,0 +1,88 @@
+#include "frame_buffer.h"
+
+#include <assert.h>
+#include <libavutil/avutil.h>
+#include <libavformat/avformat.h>
+
+#include "util/log.h"
+
+bool
+sc_frame_buffer_init(struct sc_frame_buffer *fb) {
+    fb->pending_frame = av_frame_alloc();
+    if (!fb->pending_frame) {
+        return false;
+    }
+
+    fb->tmp_frame = av_frame_alloc();
+    if (!fb->tmp_frame) {
+        av_frame_free(&fb->pending_frame);
+        return false;
+    }
+
+    bool ok = sc_mutex_init(&fb->mutex);
+    if (!ok) {
+        av_frame_free(&fb->pending_frame);
+        av_frame_free(&fb->tmp_frame);
+        return false;
+    }
+
+    // there is initially no frame, so consider it has already been consumed
+    fb->pending_frame_consumed = true;
+
+    return true;
+}
+
+void
+sc_frame_buffer_destroy(struct sc_frame_buffer *fb) {
+    sc_mutex_destroy(&fb->mutex);
+    av_frame_free(&fb->pending_frame);
+    av_frame_free(&fb->tmp_frame);
+}
+
+static inline void
+swap_frames(AVFrame **lhs, AVFrame **rhs) {
+    AVFrame *tmp = *lhs;
+    *lhs = *rhs;
+    *rhs = tmp;
+}
+
+bool
+sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
+                  bool *previous_frame_skipped) {
+    sc_mutex_lock(&fb->mutex);
+
+    // Use a temporary frame to preserve pending_frame in case of error.
+    // tmp_frame is an empty frame, no need to call av_frame_unref() beforehand.
+    int r = av_frame_ref(fb->tmp_frame, frame);
+    if (r) {
+        LOGE("Could not ref frame: %d", r);
+        return false;
+    }
+
+    // Now that av_frame_ref() succeeded, we can replace the previous
+    // pending_frame
+    swap_frames(&fb->pending_frame, &fb->tmp_frame);
+    av_frame_unref(fb->tmp_frame);
+
+    if (previous_frame_skipped) {
+        *previous_frame_skipped = !fb->pending_frame_consumed;
+    }
+    fb->pending_frame_consumed = false;
+
+    sc_mutex_unlock(&fb->mutex);
+
+    return true;
+}
+
+void
+sc_frame_buffer_consume(struct sc_frame_buffer *fb, AVFrame *dst) {
+    sc_mutex_lock(&fb->mutex);
+    assert(!fb->pending_frame_consumed);
+    fb->pending_frame_consumed = true;
+
+    av_frame_move_ref(dst, fb->pending_frame);
+    // av_frame_move_ref() resets its source frame, so no need to call
+    // av_frame_unref()
+
+    sc_mutex_unlock(&fb->mutex);
+}

app/src/frame_buffer.h

@@ -0,0 +1,44 @@
+#ifndef SC_FRAME_BUFFER_H
+#define SC_FRAME_BUFFER_H
+
+#include "common.h"
+
+#include <stdbool.h>
+
+#include "util/thread.h"
+
+// forward declarations
+typedef struct AVFrame AVFrame;
+
+/**
+ * A frame buffer holds 1 pending frame, which is the last frame received from
+ * the producer (typically, the decoder).
+ *
+ * If a pending frame has not been consumed when the producer pushes a new
+ * frame, then it is lost. The intent is to always provide access to the very
+ * last frame to minimize latency.
+ */
+
+struct sc_frame_buffer {
+    AVFrame *pending_frame;
+    AVFrame *tmp_frame; // To preserve the pending frame on error
+
+    sc_mutex mutex;
+
+    bool pending_frame_consumed;
+};
+
+bool
+sc_frame_buffer_init(struct sc_frame_buffer *fb);
+
+void
+sc_frame_buffer_destroy(struct sc_frame_buffer *fb);
+
+bool
+sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
+                     bool *skipped);
+
+void
+sc_frame_buffer_consume(struct sc_frame_buffer *fb, AVFrame *dst);
+
+#endif

app/src/video_buffer.c

@@ -8,81 +8,21 @@
 
 bool
 sc_video_buffer_init(struct sc_video_buffer *vb) {
-    vb->pending_frame = av_frame_alloc();
-    if (!vb->pending_frame) {
-        return false;
-    }
-
-    vb->tmp_frame = av_frame_alloc();
-    if (!vb->tmp_frame) {
-        av_frame_free(&vb->pending_frame);
-        return false;
-    }
-
-    bool ok = sc_mutex_init(&vb->mutex);
-    if (!ok) {
-        av_frame_free(&vb->pending_frame);
-        av_frame_free(&vb->tmp_frame);
-        return false;
-    }
-
-    // there is initially no frame, so consider it has already been consumed
-    vb->pending_frame_consumed = true;
-
-    return true;
+    return sc_frame_buffer_init(&vb->fb);
 }
 
 void
 sc_video_buffer_destroy(struct sc_video_buffer *vb) {
-    sc_mutex_destroy(&vb->mutex);
-    av_frame_free(&vb->pending_frame);
-    av_frame_free(&vb->tmp_frame);
-}
-
-static inline void
-swap_frames(AVFrame **lhs, AVFrame **rhs) {
-    AVFrame *tmp = *lhs;
-    *lhs = *rhs;
-    *rhs = tmp;
+    sc_frame_buffer_destroy(&vb->fb);
 }
 
 bool
 sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame,
                   bool *previous_frame_skipped) {
-    sc_mutex_lock(&vb->mutex);
-
-    // Use a temporary frame to preserve pending_frame in case of error.
-    // tmp_frame is an empty frame, no need to call av_frame_unref() beforehand.
-    int r = av_frame_ref(vb->tmp_frame, frame);
-    if (r) {
-        LOGE("Could not ref frame: %d", r);
-        return false;
-    }
-
-    // Now that av_frame_ref() succeeded, we can replace the previous
-    // pending_frame
-    swap_frames(&vb->pending_frame, &vb->tmp_frame);
-    av_frame_unref(vb->tmp_frame);
-
-    if (previous_frame_skipped) {
-        *previous_frame_skipped = !vb->pending_frame_consumed;
-    }
-    vb->pending_frame_consumed = false;
-
-    sc_mutex_unlock(&vb->mutex);
-
-    return true;
+    return sc_frame_buffer_push(&vb->fb, frame, previous_frame_skipped);
 }
 
 void
 sc_video_buffer_consume(struct sc_video_buffer *vb, AVFrame *dst) {
-    sc_mutex_lock(&vb->mutex);
-    assert(!vb->pending_frame_consumed);
-    vb->pending_frame_consumed = true;
-
-    av_frame_move_ref(dst, vb->pending_frame);
-    // av_frame_move_ref() resets its source frame, so no need to call
-    // av_frame_unref()
-
-    sc_mutex_unlock(&vb->mutex);
+    sc_frame_buffer_consume(&vb->fb, dst);
 }

app/src/video_buffer.h

@@ -5,27 +5,13 @@
 
 #include <stdbool.h>
 
-#include "util/thread.h"
+#include "frame_buffer.h"
 
 // forward declarations
 typedef struct AVFrame AVFrame;
 
-/**
- * A video buffer holds 1 pending frame, which is the last frame received from
- * the producer (typically, the decoder).
- *
- * If a pending frame has not been consumed when the producer pushes a new
- * frame, then it is lost. The intent is to always provide access to the very
- * last frame to minimize latency.
- */
-
 struct sc_video_buffer {
-    AVFrame *pending_frame;
-    AVFrame *tmp_frame; // To preserve the pending frame on error
-
-    sc_mutex mutex;
-
-    bool pending_frame_consumed;
+    struct sc_frame_buffer fb;
 };
 
 bool