cogl: Fallback to set_data when mapping a buffer to fill it

In the journal code and when generating the stroke path the vertices
are generated on the fly and stored in a CoglBuffer using
cogl_buffer_map. However cogl_buffer_map is allowed to fail but it
wasn't checking for a NULL return value. In particular on GLES it will
always fail because glMapBuffer is only provided by an extension. This
adds a new pair of internal functions called
_cogl_buffer_{un,}map_for_fill_or_fallback which wrap
cogl_buffer_map. If the map fails then it will instead return a
pointer into a GByteArray attached to the context. When the buffer is
unmapped the array is copied into the buffer using
cogl_buffer_set_data.

cogl/cogl-buffer-private.h

@@ -56,9 +56,10 @@ struct _CoglBufferVtable
 
 typedef enum _CoglBufferFlags
 {
-  COGL_BUFFER_FLAG_NONE          = 0,
-  COGL_BUFFER_FLAG_BUFFER_OBJECT = 1UL << 0,  /* real openGL buffer object */
-  COGL_BUFFER_FLAG_MAPPED        = 1UL << 1
+  COGL_BUFFER_FLAG_NONE            = 0,
+  COGL_BUFFER_FLAG_BUFFER_OBJECT   = 1UL << 0,  /* real openGL buffer object */
+  COGL_BUFFER_FLAG_MAPPED          = 1UL << 1,
+  COGL_BUFFER_FLAG_MAPPED_FALLBACK = 1UL << 2
 } CoglBufferFlags;
 
 typedef enum {
@@ -145,6 +146,18 @@ _cogl_buffer_immutable_ref (CoglBuffer *buffer);
 void
 _cogl_buffer_immutable_unref (CoglBuffer *buffer);
 
+/* This is a wrapper around cogl_buffer_map for internal use when we
+   want to map the buffer for write only to replace the entire
+   contents. If the map fails then it will fallback to writing to a
+   temporary buffer. When _cogl_buffer_unmap_for_fill_or_fallback is
+   called the temporary buffer will be copied into the array. Note
+   that these calls share a global array so they can not be nested. */
+void *
+_cogl_buffer_map_for_fill_or_fallback (CoglBuffer *buffer);
+
+void
+_cogl_buffer_unmap_for_fill_or_fallback (CoglBuffer *buffer);
+
 G_END_DECLS
 
 #endif /* __COGL_BUFFER_PRIVATE_H__ */

cogl/cogl-buffer.c

@@ -474,6 +474,57 @@ cogl_buffer_unmap (CoglBuffer *buffer)
   buffer->vtable.unmap (buffer);
 }
 
+void *
+_cogl_buffer_map_for_fill_or_fallback (CoglBuffer *buffer)
+{
+  void *ret;
+
+  _COGL_GET_CONTEXT (ctx, NULL);
+
+  g_return_val_if_fail (!ctx->buffer_map_fallback_in_use, NULL);
+
+  ctx->buffer_map_fallback_in_use = TRUE;
+
+  ret = cogl_buffer_map (buffer,
+                         COGL_BUFFER_ACCESS_WRITE,
+                         COGL_BUFFER_MAP_HINT_DISCARD);
+
+  if (ret)
+    return ret;
+  else
+    {
+      /* If the map fails then we'll use a temporary buffer to fill
+         the data and then upload it using cogl_buffer_set_data when
+         the buffer is unmapped. The temporary buffer is shared to
+         avoid reallocating it every time */
+      g_byte_array_set_size (ctx->buffer_map_fallback_array, buffer->size);
+
+      buffer->flags |= COGL_BUFFER_FLAG_MAPPED_FALLBACK;
+
+      return ctx->buffer_map_fallback_array->data;
+    }
+}
+
+void
+_cogl_buffer_unmap_for_fill_or_fallback (CoglBuffer *buffer)
+{
+  _COGL_GET_CONTEXT (ctx, NO_RETVAL);
+
+  g_return_if_fail (ctx->buffer_map_fallback_in_use);
+
+  ctx->buffer_map_fallback_in_use = FALSE;
+
+  if ((buffer->flags & COGL_BUFFER_FLAG_MAPPED_FALLBACK))
+    {
+      cogl_buffer_set_data (buffer, 0,
+                            ctx->buffer_map_fallback_array->data,
+                            buffer->size);
+      buffer->flags &= ~COGL_BUFFER_FLAG_MAPPED_FALLBACK;
+    }
+  else
+    cogl_buffer_unmap (buffer);
+}
+
 gboolean
 cogl_buffer_set_data (CoglBuffer   *buffer,
                       gsize         offset,

cogl/cogl-context.c

@@ -287,6 +287,9 @@ cogl_create_context (void)
 
   _context->atlases = NULL;
 
+  _context->buffer_map_fallback_array = g_byte_array_new ();
+  _context->buffer_map_fallback_in_use = FALSE;
+
   /* As far as I can tell, GL_POINT_SPRITE doesn't have any effect
      unless GL_COORD_REPLACE is enabled for an individual
      layer. Therefore it seems like it should be ok to just leave it
@@ -385,6 +388,8 @@ _cogl_destroy_context (void)
   g_hash_table_unref (_context->arbfp_cache);
 #endif
 
+  g_byte_array_free (_context->buffer_map_fallback_array, TRUE);
+
   g_free (_context);
 }
 

cogl/cogl-context.h

@@ -229,6 +229,12 @@ typedef struct
      stencil buffer */
   gboolean          current_clip_stack_uses_stencil;
 
+  /* This is used as a temporary buffer to fill a CoglBuffer when
+     cogl_buffer_map fails and we only want to map to fill it with new
+     data */
+  GByteArray       *buffer_map_fallback_array;
+  gboolean          buffer_map_fallback_in_use;
+
   CoglContextDriver drv;
   CoglContextWinsys winsys;
 } CoglContext;

cogl/cogl-journal.c

@@ -1058,8 +1058,7 @@ upload_vertices (const CoglJournalEntry *entries,
   buffer = COGL_BUFFER (array);
   cogl_buffer_set_update_hint (buffer, COGL_BUFFER_UPDATE_HINT_STATIC);
 
-  vout = cogl_buffer_map (buffer, COGL_BUFFER_ACCESS_WRITE,
-                          COGL_BUFFER_MAP_HINT_DISCARD);
+  vout = _cogl_buffer_map_for_fill_or_fallback (buffer);
   vin = &g_array_index (vertices, float, 0);
 
   /* Expand the number of vertices from 2 to 4 while uploading */
@@ -1128,7 +1127,7 @@ upload_vertices (const CoglJournalEntry *entries,
       vout += vb_stride * 4;
     }
 
-  cogl_buffer_unmap (buffer);
+  _cogl_buffer_unmap_for_fill_or_fallback (buffer);
 
   return array;
 }

cogl/cogl2-path.c

@@ -1484,9 +1484,8 @@ _cogl_path_build_stroke_vbo (CoglPath *path)
                                             sizeof (floatVec2),
                                             NULL);
 
-  vbo_p = cogl_buffer_map (COGL_BUFFER (data->stroke_vbo),
-                           COGL_BUFFER_ACCESS_WRITE,
-                           COGL_BUFFER_MAP_HINT_DISCARD);
+  vbo_p =
+    _cogl_buffer_map_for_fill_or_fallback (COGL_BUFFER (data->stroke_vbo));
 
   /* Copy the vertices in and count the number of sub paths. Each sub
      path will form a separate attribute so we can paint the disjoint
@@ -1506,7 +1505,7 @@ _cogl_path_build_stroke_vbo (CoglPath *path)
       n_attributes++;
     }
 
-  cogl_buffer_unmap (COGL_BUFFER (data->stroke_vbo));
+  _cogl_buffer_unmap_for_fill_or_fallback (COGL_BUFFER (data->stroke_vbo));
 
   data->stroke_vbo_attributes = g_new (CoglVertexAttribute *, n_attributes);