ofpbuf: Add ofpbuf_new_with_headroom(), ofpbuf_clone_with_headroom().

These new functions simplify an increasingly common usage pattern.

Suggested-by: Jesse Gross <jesse@nicira.com>

lib/dpif-linux.c

@@ -478,8 +478,7 @@ dpif_linux_recv(struct dpif *dpif_, struct ofpbuf **bufp)
     int retval;
     int error;
 
-    buf = ofpbuf_new(65536 + DPIF_RECV_MSG_PADDING);
+    buf = ofpbuf_new_with_headroom(65536, DPIF_RECV_MSG_PADDING);
-    ofpbuf_reserve(buf, DPIF_RECV_MSG_PADDING);
     retval = read(dpif->fd, ofpbuf_tail(buf), ofpbuf_tailroom(buf));
     if (retval < 0) {
         error = errno;

lib/dpif-netdev.c

@@ -1262,8 +1262,7 @@ dp_netdev_output_control(struct dp_netdev *dp, const struct ofpbuf *packet,
     }
 
     msg_size = sizeof *header + packet->size;
-    msg = ofpbuf_new(msg_size + DPIF_RECV_MSG_PADDING);
+    msg = ofpbuf_new_with_headroom(msg_size, DPIF_RECV_MSG_PADDING);
-    ofpbuf_reserve(msg, DPIF_RECV_MSG_PADDING);
     header = ofpbuf_put_uninit(msg, sizeof *header);
     header->type = queue_no;
     header->length = msg_size;

lib/ofpbuf.c

@@ -75,12 +75,32 @@ ofpbuf_new(size_t size)
     return b;
 }
 
+/* Creates and returns a new ofpbuf with an initial capacity of 'size +
+ * headroom' bytes, reserving the first 'headroom' bytes as headroom. */
+struct ofpbuf *
+ofpbuf_new_with_headroom(size_t size, size_t headroom)
+{
+    struct ofpbuf *b = ofpbuf_new(size + headroom);
+    ofpbuf_reserve(b, headroom);
+    return b;
+}
+
 struct ofpbuf *
 ofpbuf_clone(const struct ofpbuf *buffer)
 {
     return ofpbuf_clone_data(buffer->data, buffer->size);
 }
 
+/* Creates and returns a new ofpbuf whose data are copied from 'buffer'.   The
+ * returned ofpbuf will additionally have 'headroom' bytes of headroom. */
+struct ofpbuf *
+ofpbuf_clone_with_headroom(const struct ofpbuf *buffer, size_t headroom)
+{
+    struct ofpbuf *b = ofpbuf_new_with_headroom(buffer->size, headroom);
+    ofpbuf_put(b, buffer->data, buffer->size);
+    return b;
+}
+
 struct ofpbuf *
 ofpbuf_clone_data(const void *data, size_t size)
 {

lib/ofpbuf.h

@@ -48,7 +48,10 @@ void ofpbuf_uninit(struct ofpbuf *);
 void ofpbuf_reinit(struct ofpbuf *, size_t);
 
 struct ofpbuf *ofpbuf_new(size_t);
+struct ofpbuf *ofpbuf_new_with_headroom(size_t, size_t headroom);
 struct ofpbuf *ofpbuf_clone(const struct ofpbuf *);
+struct ofpbuf *ofpbuf_clone_with_headroom(const struct ofpbuf *,
+                                          size_t headroom);
 struct ofpbuf *ofpbuf_clone_data(const void *, size_t);
 void ofpbuf_delete(struct ofpbuf *);
 

ofproto/pktbuf.c

@@ -112,8 +112,8 @@ pktbuf_save(struct pktbuf *pb, struct ofpbuf *buffer, uint16_t in_port)
     if (++p->cookie >= COOKIE_MAX) {
         p->cookie = 0;
     }
-    p->buffer = ofpbuf_new(sizeof(struct ofp_packet_in) + buffer->size);
+    p->buffer = ofpbuf_new_with_headroom(buffer->size,
-    ofpbuf_reserve(p->buffer, sizeof(struct ofp_packet_in));
+                                         sizeof(struct ofp_packet_in));
     ofpbuf_put(p->buffer, buffer->data, buffer->size);
     p->timeout = time_msec() + OVERWRITE_MSECS;
     p->in_port = in_port;