Fix round-robin frame drawing behavior

Before this commit, every time redraw() was called, it would block
and wait for a new frame. This works when only one output is being
drawn; but when multiple outputs are drawn, this means only one
output at a time is given the latest frame, instead of all of them.

This change makes redraw() non-blocking,† and uses mpv_render_context's
set_update_callback method to request redraws from for all outputs
when a new frame is available. This requires a) tracking the lifetime
of all frame callbacks, to ensure frames are not rendered too quickly
(before the frame callback has returned) and b) extending the main
loop so that it can be woken up via self-pipe trick after the mpv
update callback is invoked.

† eglSwapBuffers calls wl_display_roundtrip(), so technically redraw()
  ends up waiting for the compositor to respond. This can introduce
  some extra lag when drawing onto many outputs at once. It's possible
  to avoid this problem by managing the buffers onto which OpenGL
  draws ourselves and committing the output surfaces as a batch.
  However, doing so would be quite complicated.

src/main.c

@@ -1,8 +1,11 @@
+#include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <getopt.h>
 #include <unistd.h>
+#include <poll.h>
 #include <pthread.h>
 #include <time.h>
 #include <stdbool.h>
@@ -51,6 +54,9 @@ struct display_output {
     uint32_t scale;
 
     struct wl_list link;
+
+    struct wl_callback *frame_callback;
+    bool redraw_needed;
 };
 
 static EGLConfig egl_config;
@@ -59,6 +65,7 @@ static EGLContext *egl_context;
 
 static mpv_handle *mpv;
 static mpv_render_context *mpv_glcontext;
+static int wakeup_pipe[2];
 static char *video_path;
 static char *mpv_options;
 
@@ -128,6 +135,9 @@ static void render(struct display_output *output) {
         }},
         // Flip rendering (needed due to flipped GL coordinate system).
         {MPV_RENDER_PARAM_FLIP_Y, &(int){1}},
+        // Do not wait for a fresh frame to render
+        {MPV_RENDER_PARAM_BLOCK_FOR_TARGET_TIME, &(int){0}},
+        {MPV_RENDER_PARAM_INVALID, NULL},
     };
 
     if (!eglMakeCurrent(egl_display, output->egl_surface, output->egl_surface, egl_context)) {
@@ -139,8 +149,9 @@ static void render(struct display_output *output) {
     mpv_render_context_render(mpv_glcontext, render_params);
 
     // Callback new frame
-    struct wl_callback *callback = wl_surface_frame(output->surface);
-    wl_callback_add_listener(callback, &wl_surface_frame_listener, output);
+    output->frame_callback = wl_surface_frame(output->surface);
+    wl_callback_add_listener(output->frame_callback, &wl_surface_frame_listener, output);
+    output->redraw_needed = false;
 
     // Display frame
     if (!eglSwapBuffers(egl_display, output->egl_surface))
@@ -149,6 +160,8 @@ static void render(struct display_output *output) {
 
 static void frame_handle_done(void *data, struct wl_callback *callback, uint32_t frame_time) {
     (void) frame_time;
+    struct display_output *output = data;
+    output->frame_callback = NULL;
     wl_callback_destroy(callback);
 
     // Reset deadman switch timer
@@ -169,9 +182,10 @@ static void frame_handle_done(void *data, struct wl_callback *callback, uint32_t
     }
 
     // Render next frame
-    if (!halt_info.kill_render_loop)
-        render(data);
-    else {
+    if (!halt_info.kill_render_loop) {
+        if (output->redraw_needed)
+            render(output);
+    } else {
         halt_info.kill_render_loop = 0;
         exit_mpvpaper(0);
     }
@@ -488,6 +502,12 @@ static void *get_proc_address_mpv(void *ctx, const char *name){
     return eglGetProcAddress(name);
 }
 
+static void render_update_callback(void *callback_ctx) {
+    (void)callback_ctx;
+    uint8_t tmp = 0;
+    write(wakeup_pipe[1], &tmp, 1);
+}
+
 static void init_mpv(struct display_output *output) {
 
     mpv = mpv_create();
@@ -518,6 +538,7 @@ static void init_mpv(struct display_output *output) {
         {MPV_RENDER_PARAM_OPENGL_INIT_PARAMS, &(mpv_opengl_init_params){
             .get_proc_address = get_proc_address_mpv,
         }},
+        {MPV_RENDER_PARAM_INVALID, NULL},
     };
     if (mpv_render_context_create(&mpv_glcontext, mpv, params) < 0)
         cflp_error("Failed to initialize mpv GL context");
@@ -552,6 +573,8 @@ static void init_mpv(struct display_output *output) {
 
     // mpv must never idle
     mpv_command(mpv, (const char*[]) {"set", "idle", "no", NULL});
+
+    mpv_render_context_set_update_callback(mpv_glcontext, render_update_callback, NULL);
 }
 
 static void init_egl(struct wl_state *state) {
@@ -1010,6 +1033,13 @@ int main(int argc, char **argv) {
         }
     }
 
+    if (pipe(wakeup_pipe) == -1) {
+        cflp_error("Creating a self-pipe failed.");
+        return EXIT_FAILURE;
+    }
+    fcntl(wakeup_pipe[0], F_SETFD, FD_CLOEXEC);
+    fcntl(wakeup_pipe[1], F_SETFD, FD_CLOEXEC);
+
     state.display = wl_display_connect(NULL);
     if (!state.display) {
         cflp_error("Unable to connect to the compositor. "
@@ -1043,8 +1073,39 @@ int main(int argc, char **argv) {
     }
 
     state.run_display = 1;
-    while (wl_display_dispatch(state.display) != -1) {
-        // NOP
+    while (true) {
+        if (wl_display_flush(state.display) == -1 && errno != EAGAIN)
+            break;
+
+        struct pollfd fds[2];
+        fds[0].fd = wl_display_get_fd(state.display);
+        fds[0].events = POLLIN;
+        fds[1].fd = wakeup_pipe[0];
+        fds[1].events = POLLIN;
+
+        if (poll(fds, sizeof(fds) / sizeof(fds[0]), -1) == -1 && errno != EINTR)
+            break;
+
+        if (fds[0].revents & POLLIN) {
+            if (wl_display_dispatch(state.display) == -1)
+                break;
+        }
+
+        if (fds[1].revents & POLLIN) {
+            // Empty the pipe
+            char tmp[64];
+            read(wakeup_pipe[0], tmp, sizeof(tmp));
+
+            mpv_render_context_update(mpv_glcontext);
+
+            wl_list_for_each(output, &state.outputs, link) {
+                // Redraw immediately if not waiting for frame callback
+                if (output->frame_callback == NULL)
+                    render(output);
+                else
+                    output->redraw_needed = true;
+            }
+        }
     }
 
     struct display_output *tmp_output;