ofpbuf: Add helper method to truncate the buffer.

Add helper to truncate the buffer to certain size which might be
useful if some earlier part of the buffer can be reused multiple
times without copying the whole buffer.

Signed-off-by: Ales Musil <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>

include/openvswitch/ofpbuf.h

@@ -292,6 +292,13 @@ static inline bool ofpbuf_oversized(const struct ofpbuf *ofpacts)
     return (char *)ofpbuf_tail(ofpacts) - (char *)ofpacts->header > UINT16_MAX;
 }
 
+/* Truncates the buffer to 'new_size' bytes from the tail end of 'b'. */
+static inline void ofpbuf_truncate(struct ofpbuf *b, size_t new_size)
+{
+    ovs_assert(b->size >= new_size);
+    b->size = new_size;
+}
+
 #ifdef  __cplusplus
 }
 #endif

lib/ofp-actions.c

@@ -9674,7 +9674,7 @@ ofpacts_parse(char *str, const struct ofpact_parse_params *pp,
     uint32_t orig_size = pp->ofpacts->size;
     char *error = ofpacts_parse__(str, pp, allow_instructions, outer_action);
     if (error) {
-        pp->ofpacts->size = orig_size;
+        ofpbuf_truncate(pp->ofpacts, orig_size);
     }
     CONST_CAST(struct ofpact_parse_params *, pp)->depth--;
     return error;

tests/test-ofpbuf.c

@@ -24,6 +24,7 @@
 #define BUF_SIZE 100
 #define HDR_OFS 10
 #define MSG_OFS 50
+#define DATA_SIZE 16
 
 static void
 test_ofpbuf_main(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
@@ -59,6 +60,15 @@ test_ofpbuf_main(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
     ovs_assert(buf->header == (char *) buf->base + BUF_SIZE + HDR_OFS);
     ovs_assert(buf->msg == (char *) buf->base + BUF_SIZE + MSG_OFS);
 
+    size_t prev_size = buf->size;
+    ofpbuf_put_uninit(buf, DATA_SIZE);
+    ofpbuf_truncate(buf, prev_size);
+    /* Check that everything else is unchanged after truncate. */
+    ovs_assert(!buf->size);
+    ovs_assert((char *) buf->base + BUF_SIZE == buf->data);
+    ovs_assert(buf->header == (char *) buf->base + BUF_SIZE + HDR_OFS);
+    ovs_assert(buf->msg == (char *) buf->base + BUF_SIZE + MSG_OFS);
+
     ofpbuf_delete(buf);
     exit(exit_code);
 }