ofpbuf: Enable ofpbuf_push() to expand headroom.

Until now, ofpbuf_put() has been able to reallocate an ofpbuf to
expand the tailroom, but ofpbuf_push() has not been able to expand
the headroom in an analogous way.  This omission is simply because
it has never been useful to do this.  However, an upcoming change
will make the amount of headroom in an ofpbuf passed up from a
dpif to ofproto harder to predict.  It seems that we might as well
simply implement expanding headroom, so this commit does that.

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

diff --git a/lib/ofpbuf.c b/lib/ofpbuf.c
index bb8855bd619d81654a3e3c799a22cb538082dc1c..8191c87b7ef47b269df227e62ee0473b5f5dfc08 100644 (file)
--- a/lib/ofpbuf.c
+++ b/lib/ofpbuf.c
@@ -175,47 +175,45 @@ ofpbuf_tailroom(const struct ofpbuf *b)
     return (char*)ofpbuf_end(b) - (char*)ofpbuf_tail(b);
 }
 
-/* Changes 'b->base' to 'new_base' and adjusts all of 'b''s internal pointers
- * to reflect the change. */
 static void
-ofpbuf_rebase__(struct ofpbuf *b, void *new_base)
+ofpbuf_copy__(struct ofpbuf *b, uint8_t *new_base,
+              size_t new_headroom, size_t new_tailroom)
 {
-    if (b->base != new_base) {
-        uintptr_t base_delta = (char*)new_base - (char*)b->base;
-        b->base = new_base;
-        b->data = (char*)b->data + base_delta;
-        if (b->l2) {
-            b->l2 = (char*)b->l2 + base_delta;
-        }
-        if (b->l3) {
-            b->l3 = (char*)b->l3 + base_delta;
-        }
-        if (b->l4) {
-            b->l4 = (char*)b->l4 + base_delta;
-        }
-        if (b->l7) {
-            b->l7 = (char*)b->l7 + base_delta;
-        }
-    }
+    const uint8_t *old_base = b->base;
+    size_t old_headroom = ofpbuf_headroom(b);
+    size_t old_tailroom = ofpbuf_tailroom(b);
+    size_t copy_headroom = MIN(old_headroom, new_headroom);
+    size_t copy_tailroom = MIN(old_tailroom, new_tailroom);
+
+    memcpy(&new_base[new_headroom - copy_headroom],
+           &old_base[old_headroom - copy_headroom],
+           copy_headroom + b->size + copy_tailroom);
 }
 
-/* Reallocates 'b' so that it has exactly 'new_tailroom' bytes of tailroom. */
+/* Reallocates 'b' so that it has exactly 'new_headroom' and 'new_tailroom'
+ * bytes of headroom and tailroom, respectively. */
 static void
-ofpbuf_resize_tailroom__(struct ofpbuf *b, size_t new_tailroom)
+ofpbuf_resize__(struct ofpbuf *b, size_t new_headroom, size_t new_tailroom)
 {
+    void *new_base, *new_data;
     size_t new_allocated;
-    void *new_base;
 
-    new_allocated = ofpbuf_headroom(b) + b->size + new_tailroom;
+    new_allocated = new_headroom + b->size + new_tailroom;
 
     switch (b->source) {
     case OFPBUF_MALLOC:
-        new_base = xrealloc(b->base, new_allocated);
+        if (new_headroom == ofpbuf_headroom(b)) {
+            new_base = xrealloc(b->base, new_allocated);
+        } else {
+            new_base = xmalloc(new_allocated);
+            ofpbuf_copy__(b, new_base, new_headroom, new_tailroom);
+            free(b->base);
+        }
         break;
 
     case OFPBUF_STACK:
         new_base = xmalloc(new_allocated);
-        memcpy(new_base, b->base, MIN(new_allocated, b->allocated));
+        ofpbuf_copy__(b, new_base, new_headroom, new_tailroom);
         b->source = OFPBUF_MALLOC;
         break;
 
@@ -227,7 +225,25 @@ ofpbuf_resize_tailroom__(struct ofpbuf *b, size_t new_tailroom)
     }
 
     b->allocated = new_allocated;
-    ofpbuf_rebase__(b, new_base);
+    b->base = new_base;
+
+    new_data = (char *) new_base + new_headroom;
+    if (b->data != new_data) {
+        uintptr_t data_delta = (char *) new_data - (char *) b->data;
+        b->data = new_data;
+        if (b->l2) {
+            b->l2 = (char *) b->l2 + data_delta;
+        }
+        if (b->l3) {
+            b->l3 = (char *) b->l3 + data_delta;
+        }
+        if (b->l4) {
+            b->l4 = (char *) b->l4 + data_delta;
+        }
+        if (b->l7) {
+            b->l7 = (char *) b->l7 + data_delta;
+        }
+    }
 }
 
 /* Ensures that 'b' has room for at least 'size' bytes at its tail end,
@@ -237,14 +253,19 @@ void
 ofpbuf_prealloc_tailroom(struct ofpbuf *b, size_t size)
 {
     if (size > ofpbuf_tailroom(b)) {
-        ofpbuf_resize_tailroom__(b, MAX(size, 64));
+        ofpbuf_resize__(b, ofpbuf_headroom(b), MAX(size, 64));
     }
 }
 
+/* Ensures that 'b' has room for at least 'size' bytes at its head,
+ * reallocating and copying its data if necessary.  Its tailroom, if any, is
+ * preserved. */
 void
 ofpbuf_prealloc_headroom(struct ofpbuf *b, size_t size)
 {
-    assert(size <= ofpbuf_headroom(b));
+    if (size > ofpbuf_headroom(b)) {
+        ofpbuf_resize__(b, MAX(size, 64), ofpbuf_tailroom(b));
+    }
 }
 
 /* Trims the size of 'b' to fit its actual content, reducing its tailroom to
@@ -255,8 +276,9 @@ ofpbuf_prealloc_headroom(struct ofpbuf *b, size_t size)
 void
 ofpbuf_trim(struct ofpbuf *b)
 {
-    if (b->source == OFPBUF_MALLOC && ofpbuf_tailroom(b) > 0) {
-        ofpbuf_resize_tailroom__(b, 0);
+    if (b->source == OFPBUF_MALLOC
+        && (ofpbuf_headroom(b) || ofpbuf_tailroom(b))) {
+        ofpbuf_resize__(b, 0, 0);
     }
 }
 
@@ -332,6 +354,9 @@ ofpbuf_reserve(struct ofpbuf *b, size_t size)
     b->data = (char*)b->data + size;
 }
 
+/* Prefixes 'size' bytes to the head end of 'b', reallocating and copying its
+ * data if necessary.  Returns a pointer to the first byte of the data's
+ * location in the ofpbuf.  The new data is left uninitialized. */
 void *
 ofpbuf_push_uninit(struct ofpbuf *b, size_t size)
 {
@@ -341,9 +366,9 @@ ofpbuf_push_uninit(struct ofpbuf *b, size_t size)
     return b->data;
 }
 
-/* Prefixes 'size' zeroed bytes to the head end of 'b'.  'b' must have at least
- * 'size' bytes of headroom.  Returns a pointer to the first byte of the data's
- * location in the ofpbuf. */
+/* Prefixes 'size' zeroed bytes to the head end of 'b', reallocating and
+ * copying its data if necessary.  Returns a pointer to the first byte of the
+ * data's location in the ofpbuf. */
 void *
 ofpbuf_push_zeros(struct ofpbuf *b, size_t size)
 {
@@ -352,6 +377,9 @@ ofpbuf_push_zeros(struct ofpbuf *b, size_t size)
     return dst;
 }
 
+/* Copies the 'size' bytes starting at 'p' to the head end of 'b', reallocating
+ * and copying its data if necessary.  Returns a pointer to the first byte of
+ * the data's location in the ofpbuf. */
 void *
 ofpbuf_push(struct ofpbuf *b, const void *p, size_t size)
 {