mesa: Cope with the degenerate case of zero length ranges.

Fixes assertion failures on glean's bufferObject test.

src/mesa/state_tracker/st_cb_bufferobjects.c

@@ -103,6 +103,17 @@ st_bufferobj_subdata(GLcontext *ctx,
    ASSERT(size >= 0);
    ASSERT(offset + size <= obj->Size);
 
+   if (!size)
+      return;
+
+   /*
+    * According to ARB_vertex_buffer_object specification, if data is null,
+    * then the contents of the buffer object's data store is undefined. We just
+    * ignore, and leave it unchanged.
+    */
+   if (!data)
+      return;
+
    st_cond_flush_pipe_buffer_write(st_context(ctx), st_obj->buffer,
 				   offset, size, data);
 }
@@ -125,6 +136,9 @@ st_bufferobj_get_subdata(GLcontext *ctx,
    ASSERT(size >= 0);
    ASSERT(offset + size <= obj->Size);
 
+   if (!size)
+      return;
+
    st_cond_flush_pipe_buffer_read(st_context(ctx), st_obj->buffer,
 				  offset, size, data);
 }
@@ -222,6 +236,13 @@ st_bufferobj_map(GLcontext *ctx, GLenum target, GLenum access,
 }
 
 
+/**
+ * Dummy data whose's pointer is used for zero length ranges.
+ */
+static long
+st_bufferobj_zero_length_range = 0;
+
+
 /**
  * Called via glMapBufferRange().
  */
@@ -257,14 +278,26 @@ st_bufferobj_map_range(GLcontext *ctx, GLenum target,
    assert(offset < obj->Size);
    assert(offset + length <= obj->Size);
 
-   obj->Pointer = pipe_buffer_map_range(pipe->screen, st_obj->buffer, offset, length, flags);
+   /*
+    * We go out of way here to hide the degenerate yet valid case of zero
+    * length range from the pipe driver.
+    */
+   if (!length) {
+      obj->Pointer = &st_bufferobj_zero_length_range;
+   }
+   else {
+      obj->Pointer = pipe_buffer_map_range(pipe->screen, st_obj->buffer, offset, length, flags);
+      if (obj->Pointer) {
+         obj->Pointer = (ubyte *) obj->Pointer + offset;
+      }
+   }
+   
    if (obj->Pointer) {
-      obj->Pointer = (ubyte *) obj->Pointer + offset;
       obj->Offset = offset;
       obj->Length = length;
       obj->AccessFlags = access;
    }
-   
+
    return obj->Pointer;
 }
 
@@ -282,6 +315,9 @@ st_bufferobj_flush_mapped_range(GLcontext *ctx, GLenum target,
    assert(length >= 0);
    assert(offset + length <= obj->Length);
    
+   if (!length)
+      return;
+
    pipe_buffer_flush_mapped_range(pipe->screen, st_obj->buffer, 
                                   obj->Offset + offset, length);
 }
@@ -296,7 +332,9 @@ st_bufferobj_unmap(GLcontext *ctx, GLenum target, struct gl_buffer_object *obj)
    struct pipe_context *pipe = st_context(ctx)->pipe;
    struct st_buffer_object *st_obj = st_buffer_object(obj);
 
-   pipe_buffer_unmap(pipe->screen, st_obj->buffer);
+   if(obj->Length)
+      pipe_buffer_unmap(pipe->screen, st_obj->buffer);
+
    obj->Pointer = NULL;
    obj->Offset = 0;
    obj->Length = 0;
@@ -319,6 +357,9 @@ st_copy_buffer_subdata(GLcontext *ctx,
    struct st_buffer_object *dstObj = st_buffer_object(dst);
    ubyte *srcPtr, *dstPtr;
 
+   if(!size)
+      return;
+
    /* buffer should not already be mapped */
    assert(!src->Pointer);
    assert(!dst->Pointer);