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screen-cast/monitor-stream: Immediately record scanout frames

Georges Basile Stavracas Neto requested to merge gbsneto/scanout-screencast into main

When the before-paint function is executed, it's only purpose is to check if there's any scanout queue, and immediately record it if any.

However, since [1], we regressed in this specific case with the introduction of an idle callback in the before-paint function. The regression only happens when the PipeWire stream is using DMA-BUF buffers, and it would operate as follows:

  1. In before-paint, when there's a scanout available, we queue an idle callback to capture the monitor. The idle callback (almost always) executes after the scanout is pulled from the stage view

  2. meta_screen_cast_stream_src_maybe_record_frame() is called by the idle callback. In the DMA-BUF case, it then runs meta_screen_cast_monitor_stream_src_record_to_framebuffer()

  3. In meta_screen_cast_monitor_stream_src_record_to_framebuffer(), because the stage view doesn't have a scanout anymore, it ends up calling cogl_blit_framebuffer() with the stage view framebuffer. This is the regression bug.

This regression presents itself in the form of the screencast stream showing the desktop when there's an unredirected fullscreen application window running.

Revert before-paint - and only that - back to immediately capturing any available scanout. Only record these frames when the target buffer is a DMA-BUF handle. Nothing is captured on before-paint if the stream is not using DMA-BUF, since the regular paint routine will handle these frames regularly post-paint.

[1] !1914 (merged)

Edited by Georges Basile Stavracas Neto

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