MINOR: buf: Add function to insert a string at an absolute offset in a buffer

The b_insert_blk() function may now be used to insert a string, given a pointer
and the string length, at an absolute offset in a buffer, moving data between
this offset and the buffer's tail just after the end of the inserted string. The
buffer's length is automatically updated. This function supports wrapping. All
the string is copied or nothing. So it returns 0 if there are not enough space
to perform the copy. Otherwise, the number of bytes copied is returned.

include/common/buf.h

@@ -673,6 +673,35 @@ static inline int b_rep_blk(struct buffer *b, char *pos, char *end, const char *
 	return delta;
 }
 
+/* b_insert_blk(): inserts the block <blk> at the absolute offset <off> moving
+ * data between this offset and the buffer's tail just after the end of the copy
+ * of <blk>. The buffer's length is automatically updated. It Supports
+ * wrapping. If there are not enough space to perform the copy, 0 is
+ * returned. Otherwise, the number of bytes copied is returned
+*/
+static inline int b_insert_blk(struct buffer *b, size_t off, const char *blk, size_t len)
+{
+	size_t pos;
+
+	if (!len || len > b_room(b))
+		return 0; /* nothing to copy or not enough space left */
+
+	pos = b_peek_ofs(b, off);
+	if (pos == b_tail_ofs(b))
+		__b_putblk(b, blk, len);
+	else {
+		size_t delta = b_data(b) - off;
+
+		/* first, protect the end of the buffer */
+		b_move(b, pos, delta, len);
+
+		/* change the amount of data in the buffer during the copy */
+		b_sub(b, delta);
+		__b_putblk(b, blk, len);
+		b_add(b, delta);
+	}
+	return len;
+}
 
 /* __b_put_varint(): encode 64-bit value <v> as a varint into buffer <b>. The
  * caller must have checked that the encoded value fits in the buffer so that