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[Public WebGL] Issues with sharing resources across contexts



So ........... I've been playing around with sharing resources across WebGL contexts and I'm running into issues and looking for solutions.

The biggest issue is that GL is command buffer driven and so calling some GL function doesn't mean it's actually been executed. You have to call glFlush. This raises lots of issues of where a program might work on some browser / driver / platform combo but not others if the users forgets to call flush.  

For example, assume I have 2 contexts sharing resources. gl1, and gl2


  var vs = gl1.createShader(gl1.VERTEX_SHADER);
  var fs = gl1.createShader(gl1.FRAGMENT_SHADER);
  //...
  // assume shaders are validly compiled
  // ...
  var p = gl1.createProgram();
  gl1.attachShader(p, vs);
  gl1.attachShader(p, fs);
  gl2.linkProgram(p);

I attached on gl1 but linked on gl2. There's is no guarantee in GL that that link will succeed because the 2 attach commands may have only been queued and not excuted in which case the linkProgram call will fail with "missing shaders". The correct code is

  var p = gl1.createProgram();
  gl1.attachShader(p, vs);
  gl1.attachShader(p, fs);
  gl1.flush(); // <--- important 
  gl2.linkProgram(p);

That seems unacceptable. 

2 Approaches off the of my head

1) Try to make it just work without the flush

One solution might be for the implementation to track which context last had a call. Any call to a different context causes an automatic implicit flush

2) Try to make it fail when not done correctly

This solution would be to try to track of an object is "dirty" (state has been changed) and no flush has been issued since then for that object. When an object is used, if it's dirty then either a flush is issued (which is solution #1) or a error is generated. ("called function on dirty object, did you forget to call flush?")

Neither solution seems ideal. Worse, whatever solution is chosen also has issues if we ever get WebGL in Workers.

A 3rd solution is just to leave it with the flush required and forgetting means you get random success / failure. No such a great prospect. :-(

Thoughts?