Windows: FlutterDesktopPixelBuffer: Add optional release callback

[jnschulze]

This PR adds an optional callback to FlutterDesktopPixelBuffer that gets invoked when it's safe to release the backing buffer (or unlock a mutex protecting that buffer etc.).

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[jnschulze]

@chinmaygarde

[chinmaygarde]

I think this is a fine addition. But can we a test for this? I don't see an existing test for this so I am unable to provide any guidance here. Perhaps @cbracken might know.

[cbracken]

Will take a quick look -- I'm pretty sure I added a test for something similar a couple months back.

[chinmaygarde]

Chris may be busy with other things. To test this, I think we can just add a latch that is signalled in the unregistration callback and wait for it after calling FlutterEngineUnregisterExternalTexture. If the test times out, the watchdog should terminate the harness anyway. Since the test seems easy-ish to wire up, let's attempt to write one before landing this.

[jnschulze]

@chinmaygarde
I've mocked all gl* functions used by ExternalTextureGL and added a test for ExternalTextureGL::PopulateTexture (which invokes the newly introduced callback after the pixel buffer has been copied).
So we not only test the new callback but also the texture population itself.

[chinmaygarde]

Thanks!

[cbracken]

lgtm! Thanks!

[stuartmorgan-g]

Adding these caused undefined behavior for existing code (unless the code was specifically written to defend against the possibility of new values by zero-initializing the entire memory block).

If we edit this again we should add a struct size to do safe reads, as we do with embedder.h.

[MindStudioOfficial]

I have set up the Following buffer update function:

void Frame::Update(uint8_t *buffer, int32_t width, int32_t height)
{
    const std::lock_guard<std::mutex> lock(mutex_);
    flutter_pixel_buffer_.buffer = buffer;
    flutter_pixel_buffer_.width = width;
    flutter_pixel_buffer_.height = height;
    flutter_pixel_buffer_.release_context = &buffer;
    flutter_pixel_buffer_.release_callback = [](void *user_data)
    {
        uint8_t *old_buffer = reinterpret_cast<uint8_t *>(user_data);
        if (old_buffer != nullptr)
            free(old_buffer);
    };
    texture_registrar_->MarkTextureFrameAvailable(texture_id_);
}

This gets invoked by dart throught the method channel

However when I try to free(old_buffer)

I get the following error:

Debug Assertion Failed!
...
Expression: _CrtlsValidHeapPointer(block)
...

The original buffer is a uint8_t * from dart ffi.

How and where am I supposed to free the buffer?