Output management on the gbm-kms platform is racy

[mattkae]

This issue was uncovered while investigating #3729.

Issue

The crux of the issue is in src/platforms/gbm-kms/server/kms/display.cpp.

Display::configure_locked (called by Display::configure) is responsible for associating incoming outptus with their DisplaySink. A DisplaySink is only created for an output when the output meets a number of conditions, one of which is being connected. Each DisplaySink has a shared_ptr reference to a RealKMSOutput.

When the configuration is requested by Display::configuration(), we update the properties of RealKMSOutputs to reflect its current status (e.g. the output may now be disconnected). The problem is, however, that an output that currently has a display sink may have been updated such that it should no longer have a display sink. However, the renderer thread is still running and submitting buffers to this sink which is associated with an invalid output.

Expected

A DisplaySink cannot be submitted for an output that does not deserve a sink. Or - perhaps even better! - the DisplaySink should be invalidated such that it will not be used by the renderer while the output is in an invalid state.

[mattkae]

GBMSurfaceImpl constructor is created with a zero width and height.