Bug 13978

Created attachment 13602 [details]
test case

seems this can fix this issue:

--- a/src/mesa/main/buffers.c   2008-01-09 09:54:06.000000000 +0800
+++ b/src/mesa/main/buffers.c   2008-01-09 09:56:15.000000000 +0800
@@ -540,10 +540,12 @@
       for (buf = 0; buf < n; buf++ ) {
          if (destMask[buf]) {
             fb->_ColorDrawBufferIndexes[buf] = _mesa_ffs(destMask[buf]) - 1;
+            fb->ColorDrawBuffer[buf] = buffers[buf];
             count = buf + 1;
          }
          else {
             fb->_ColorDrawBufferIndexes[buf] = -1;
+            fb->ColorDrawBuffer[buf] = GL_NONE;
          }
       }
       /* set remaining outputs to -1 (GL_NONE) */

Your patch is the correct fix.  I'm committing it to git.

verified 

Created attachment 91486 [details]
Simple Demonstration

GL_DRAW_BUFFER0 returns 1029 instead of GL_BACK_LEFT

(In reply to Benjamin Botwin from comment #6)
> GL_DRAW_BUFFER0 returns 1029 instead of GL_BACK_LEFT

I test it and I have not found anything wrong in Mesa. The initial value for double-buffered contexts is GL_BACK.
   * See: https://www.opengl.org/sdk/docs/man4/html/glDrawBuffer.xhtml
   * Also see: https://www.opengl.org/sdk/docs/man2/xhtml/glGet.xml Search for GL_DRAW_BUFFER.

Furthermore, glClearBuffer() doc says the following:
  "If buffer is GL_COLOR, a particular draw buffer GL_DRAW_BUFFERi
   is specified by passing i as drawBuffer."

As a second parameter of glClearbuffer(), you are passing the value got from glGetIntegeriv() which is wrong. For GL_DRAW_BUFFER0, you should pass a 0. If you do that, the window is cleared with red color once we swap buffers.
   * See https://www.opengl.org/sdk/docs/man3/xhtml/glClearBuffer.xml

I tested as well, GL_BACK is returned as specified, therefore resolving as fixed