gpu: Allow BufferData size=0 for pixel transfer buffer objects.

gpu/command_buffer/client/gles2_implementation.cc

Index: gpu/command_buffer/client/gles2_implementation.cc
diff --git a/gpu/command_buffer/client/gles2_implementation.cc b/gpu/command_buffer/client/gles2_implementation.cc
index 30389fa281a582810a0a090650730048c1a4c846..794d46fcf1eb3f37035554aa556583ac5cda17a6 100644
--- a/gpu/command_buffer/client/gles2_implementation.cc
+++ b/gpu/command_buffer/client/gles2_implementation.cc
@@ -1197,10 +1197,6 @@ void GLES2Implementation::ShaderSource(
 
 void GLES2Implementation::BufferDataHelper(
     GLenum target, GLsizeiptr size, const void* data, GLenum usage) {
-  if (size == 0) {

[greggman, 2012/11/30 09:13:15]

this removal I understand

-    return;
-  }
-
   if (size < 0) {
     SetGLError(GL_INVALID_VALUE, "glBufferData", "size < 0");
     return;
@@ -1222,11 +1218,17 @@ void GLES2Implementation::BufferDataHelper(
       buffer_tracker_->RemoveBuffer(buffer_id);
     }
 
-    // Create new buffer.
-    buffer = buffer_tracker_->CreateBuffer(buffer_id, size);
-    GPU_DCHECK(buffer);
-    if (data)
-      memcpy(buffer->address(), data, size);
+    if (size) {

[greggman, 2012/11/30 09:13:15]

this addition I don't understand. It seems like you'd still want to call
CreateBuffer here and it will just create a zero sized buffer.

That way everything *should* just work.

   GLsizei someSizeThatHappensToBe0
   glBindBuffer(GL_PIXEL_UNPACK..., buf);
   glBufferData(GL_PIXEL_UNPACK..., 0, ptr, usage);
   // this should return a valid pointer just like
   // malloc(0) does
   glMapBufferCHROMIUM(GL_PIXEL_UNPACK,...);
   glTexImage2D(..., widthOf0, heightOf0, ....);
 
etc.

If you skip creating zero sized buffers then several other checks will fail if
called with a zero sized request. 

Right? No?

[reveman, 2012/11/30 09:30:33]

Yes, I think you're right. I might need to update the BufferTracker for this to
work. I'll look at what's needed.

+      // Create new buffer.
+      buffer = buffer_tracker_->CreateBuffer(buffer_id, size);
+      GPU_DCHECK(buffer);
+      if (data)
+        memcpy(buffer->address(), data, size);
+    }
+    return;
+  }
+
+  if (size == 0) {
     return;
   }