MEDIUM: buffer: always assign a dummy empty buffer to channels

Channels are now created with a valid pointer to a buffer before the
buffer is allocated. This buffer is a global one called "buf_empty" and
of size zero. Thus it prevents any activity from being performed on
the buffer and still ensures that chn->buf may always be dereferenced.
b_free() also resets the buffer to &buf_empty, and was split into
b_drop() which does not reset the buffer.

include/common/buffer.h

@@ -40,6 +40,7 @@ struct buffer {
 };
 
 extern struct pool_head *pool2_buffer;
+extern struct buffer buf_empty;
 
 int init_buffer();
 int buffer_replace2(struct buffer *b, char *pos, char *end, const char *str, int len);
@@ -409,13 +410,27 @@ static inline struct buffer *b_alloc(struct buffer **buf)
 	return *buf;
 }
 
-/* Releases buffer *buf.
+/* Releases buffer *buf (no check of emptiness) */
- */
+static inline void __b_drop(struct buffer **buf)
-static inline void b_free(struct buffer **buf)
 {
 	pool_free2(pool2_buffer, *buf);
 }
 
+/* Releases buffer *buf if allocated. */
+static inline void b_drop(struct buffer **buf)
+{
+	if (!(*buf)->size)
+		return;
+	__b_drop(buf);
+}
+
+/* Releases buffer *buf if allocated, and replaces it with &buf_empty. */
+static inline void b_free(struct buffer **buf)
+{
+	b_drop(buf);
+	*buf = &buf_empty;
+}
+
 #endif /* _COMMON_BUFFER_H */
 
 /*

include/proto/channel.h

@@ -51,6 +51,7 @@ int bo_getblk(struct channel *chn, char *blk, int len, int offset);
 /* Initialize all fields in the channel. */
 static inline void channel_init(struct channel *chn)
 {
+	chn->buf = &buf_empty;
 	chn->to_forward = 0;
 	chn->last_read = now_ms;
 	chn->xfer_small = chn->xfer_large = 0;

src/buffer.c

@@ -22,6 +22,10 @@
 
 struct pool_head *pool2_buffer;
 
+/* this buffer is used to have a valid pointer to an empty buffer in channels
+ * which convey no more data.
+ */
+struct buffer buf_empty  = { .p = buf_empty.data };
 
 /* perform minimal intializations, report 0 in case of error, 1 if OK. */
 int init_buffer()