mesa: Make single-buffered GLES representation internally consistent

There are a few places in the code where clearing and reading are done on
incorrect buffers for GLES contexts.  See comments for details.  This
fixes 75 GLES3 dEQP tests on the surfaceless platform with no regressions.

v2: Corrected unclear comment
v3: Make the change in context.c instead of get.c
v4: Removed whitespace

Reviewed-by: Stéphane Marchesin <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index e8aedde8777..86696b80c87 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -173,12 +173,22 @@ draw_buffer_enum_to_bitmask(const struct gl_context *ctx, GLenum buffer)
  * return -1 for an invalid buffer.
  */
 static gl_buffer_index
-read_buffer_enum_to_index(GLenum buffer)
+read_buffer_enum_to_index(const struct gl_context *ctx, GLenum buffer)
 {
    switch (buffer) {
       case GL_FRONT:
          return BUFFER_FRONT_LEFT;
       case GL_BACK:
+         if (_mesa_is_gles(ctx)) {
+            /* In draw_buffer_enum_to_bitmask, when GLES contexts draw to
+             * GL_BACK with a single-buffered configuration, we actually end
+             * up drawing to the sole front buffer in our internal
+             * representation.  For consistency, we must read from that
+             * front left buffer too.
+             */
+            if (!ctx->DrawBuffer->Visual.doubleBufferMode)
+               return BUFFER_FRONT_LEFT;
+         }
          return BUFFER_BACK_LEFT;
       case GL_RIGHT:
          return BUFFER_FRONT_RIGHT;
@@ -724,7 +734,7 @@ read_buffer(struct gl_context *ctx, struct gl_framebuffer *fb,
       if (_mesa_is_gles3(ctx) && !is_legal_es3_readbuffer_enum(buffer))
          srcBuffer = -1;
       else
-         srcBuffer = read_buffer_enum_to_index(buffer);
+         srcBuffer = read_buffer_enum_to_index(ctx, buffer);
 
       if (srcBuffer == -1) {
          _mesa_error(ctx, GL_INVALID_ENUM,