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

gpu/command_buffer/client/gles2_implementation.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // A class to emulate GLES2 over command buffers.
 
 #include "../client/gles2_implementation.h"
 
 #include <algorithm>
 #include <map>
@@ skipping 1179 matching lines @@
   }
 
   GPU_DCHECK_EQ(total_size, offset);
 
   helper_->ShaderSourceBucket(shader, kResultBucketId);
   helper_->SetBucketSize(kResultBucketId, 0);
 }
 
 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;
   }
 
   if (target == GL_PIXEL_UNPACK_TRANSFER_BUFFER_CHROMIUM) {
     GLuint buffer_id = bound_pixel_unpack_transfer_buffer_id_;
     if (!buffer_id) {
       SetGLError(GL_INVALID_VALUE, "glBufferData", "unknown buffer");
       return;
     }
 
     BufferTracker::Buffer* buffer = buffer_tracker_->GetBuffer(buffer_id);
     if (buffer) {
       // Free buffer memory, pending the passage of a token.
       buffer_tracker_->FreePendingToken(buffer, helper_->InsertToken());
 
       // Remove old buffer.
       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;
+  }
+
   // If there is no data just send BufferData
   if (!data) {
     helper_->BufferData(target, size, 0, 0, usage);
     return;
   }
 
   // See if we can send all at once.
   ScopedTransferBufferPtr buffer(size, helper_, transfer_buffer_);
   if (!buffer.valid()) {
     return;
@@ skipping 1957 matching lines @@
   return true;
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "../client/gles2_implementation_impl_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu