output: add wlr_output_preferred_read_format()

The read format is dependent on the output, so we first need to make it
current. This fixes a race condition in wlr-screencopy-v1 where a dmabuf
client would cause EGL_NO_SURFACE to be bound at the time when
screencopy needs to query for the preferred format, causing GL errors.

include/wlr/render/wlr_renderer.h

@@ -96,11 +96,6 @@ int wlr_renderer_get_dmabuf_formats(struct wlr_renderer *renderer,
  */
 int wlr_renderer_get_dmabuf_modifiers(struct wlr_renderer *renderer, int format,
 	uint64_t **modifiers);
-/**
- * Get the preferred format for reading pixels.
- */
-bool wlr_renderer_preferred_read_format(struct wlr_renderer *renderer,
-	enum wl_shm_format *fmt);
 /**
  * Reads out of pixels of the currently bound surface into data. `stride` is in
  * bytes.

include/wlr/types/wlr_output.h

@@ -192,6 +192,12 @@ void wlr_output_effective_resolution(struct wlr_output *output,
  * unknown. This is useful for damage tracking.
  */
 bool wlr_output_make_current(struct wlr_output *output, int *buffer_age);
+/**
+ * Get the preferred format for reading pixels.
+ * This function might change the current rendering context.
+ */
+bool wlr_output_preferred_read_format(struct wlr_output *output,
+	enum wl_shm_format *fmt);
 /**
  * Swaps the output buffers. If the time of the frame isn't known, set `when` to
  * NULL. If the compositor doesn't support damage tracking, set `damage` to

render/wlr_renderer.c

@@ -139,15 +139,6 @@ int wlr_renderer_get_dmabuf_modifiers(struct wlr_renderer *r, int format,
 	return r->impl->get_dmabuf_modifiers(r, format, modifiers);
 }
 
-bool wlr_renderer_preferred_read_format(struct wlr_renderer *r,
-		enum wl_shm_format *fmt) {
-	if (!r->impl->preferred_read_format || !r->impl->read_pixels) {
-		return false;
-	}
-	*fmt = r->impl->preferred_read_format(r);
-	return true;
-}
-
 bool wlr_renderer_read_pixels(struct wlr_renderer *r, enum wl_shm_format fmt,
 		uint32_t *flags, uint32_t stride, uint32_t width, uint32_t height,
 		uint32_t src_x, uint32_t src_y, uint32_t dst_x, uint32_t dst_y,

types/wlr_output.c

@@ -6,6 +6,7 @@
 #include <time.h>
 #include <wayland-server.h>
 #include <wlr/interfaces/wlr_output.h>
+#include <wlr/render/interface.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/types/wlr_box.h>
 #include <wlr/types/wlr_matrix.h>
@@ -339,6 +340,20 @@ bool wlr_output_make_current(struct wlr_output *output, int *buffer_age) {
 	return output->impl->make_current(output, buffer_age);
 }
 
+bool wlr_output_preferred_read_format(struct wlr_output *output,
+		enum wl_shm_format *fmt) {
+	if (!wlr_output_make_current(output, NULL)) {
+		return false;
+	}
+
+	struct wlr_renderer *renderer = wlr_backend_get_renderer(output->backend);
+	if (!renderer->impl->preferred_read_format || !renderer->impl->read_pixels) {
+		return false;
+	}
+	*fmt = renderer->impl->preferred_read_format(renderer);
+	return true;
+}
+
 bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
 		pixman_region32_t *damage) {
 	if (output->frame_pending) {

types/wlr_screencopy_v1.c

@@ -219,7 +219,7 @@ static void capture_output(struct wl_client *client,
 	struct wlr_renderer *renderer = wlr_backend_get_renderer(output->backend);
 	assert(renderer);
 
-	if (!wlr_renderer_preferred_read_format(renderer, &frame->format)) {
+	if (!wlr_output_preferred_read_format(frame->output, &frame->format)) {
 		wlr_log(WLR_ERROR,
 			"Failed to capture output: no read format supported by renderer");
 		goto error;