obu: ensure that the frame header reference is writable

src/obu.c

@@ -1267,6 +1267,8 @@ int dav1d_parse_obus(Dav1dContext *const c, Dav1dData *const in, int global) {
             c->frame_hdr_ref = dav1d_ref_create(sizeof(Dav1dFrameHeader));
             if (!c->frame_hdr_ref) return -ENOMEM;
         }
+        // ensure that the reference is writable
+        assert(dav1d_ref_is_writable(c->frame_hdr_ref));
         c->frame_hdr = c->frame_hdr_ref->data;
         memset(c->frame_hdr, 0, sizeof(*c->frame_hdr));
         c->frame_hdr->temporal_id = temporal_id;

src/ref.c

@@ -84,3 +84,7 @@ void dav1d_ref_dec(Dav1dRef **const pref) {
     }
     *pref = NULL;
 }
+
+int dav1d_ref_is_writable(Dav1dRef *const ref) {
+    return atomic_load(&ref->ref_cnt) == 1;
+}

src/ref.h

@@ -48,4 +48,6 @@ Dav1dRef *dav1d_ref_wrap(const uint8_t *ptr,
 void dav1d_ref_inc(Dav1dRef *ref);
 void dav1d_ref_dec(Dav1dRef **ref);
 
+int dav1d_ref_is_writable(Dav1dRef *ref);
+
 #endif /* __DAV1D_SRC_REF_H__ */