diff --git a/doc/internals/buffer-api.txt b/doc/internals/buffer-api.txt
index d4e26dbd99..a163e387ae 100644
--- a/doc/internals/buffer-api.txt
+++ b/doc/internals/buffer-api.txt
@@ -547,13 +547,12 @@ buffer_almost_full  | const buffer *buf| returns true if the buffer is not null
                     | ret: int         | and at least 3/4 of the buffer's space
                     |                  | are used. A waiting buffer will match.
 --------------------+------------------+---------------------------------------
-b_alloc             | buffer *buf      | allocates a buffer and assigns it to
-                    | ret: buffer *    | *buf. If no memory is available, (1)
+b_alloc             | buffer *buf      | ensures that <buf> is allocated or
+                    | ret: buffer *    | allocates a buffer and assigns it to
+                    |                  | *buf. If no memory is available, (1)
                     |                  | is assigned instead with a zero size.
-                    |                  | No control is made to check if *buf
-                    |                  | already pointed to another buffer. The
-                    |                  | allocated buffer is returned, or NULL
-                    |                  | in case no memory is available
+                    |                  | The allocated buffer is returned, or
+                    |                  | NULL in case no memory is available
 --------------------+------------------+---------------------------------------
 b_alloc_fast        | buffer *buf      | allocates a buffer and assigns it to
                     | ret: buffer *    | *buf. If no memory is available, (1)
diff --git a/include/haproxy/dynbuf.h b/include/haproxy/dynbuf.h
index c31b83aaac..00e5fff508 100644
--- a/include/haproxy/dynbuf.h
+++ b/include/haproxy/dynbuf.h
@@ -56,15 +56,17 @@ static inline int buffer_almost_full(const struct buffer *buf)
 /* Functions below are used for buffer allocation */
 /**************************************************/
 
-/* Allocates a buffer and assigns it to *buf. If no memory is available,
- * ((char *)1) is assigned instead with a zero size. No control is made to
- * check if *buf already pointed to another buffer. The allocated buffer is
+/* Ensures that <buf> is allocated, or allocates it. If no memory is available,
+ * ((char *)1) is assigned instead with a zero size. The allocated buffer is
  * returned, or NULL in case no memory is available.
  */
 static inline struct buffer *b_alloc(struct buffer *buf)
 {
 	char *area;
 
+	if (buf->size)
+		return buf;
+
 	*buf = BUF_WANTED;
 	area = pool_alloc_dirty(pool_head_buffer);
 	if (unlikely(!area)) {