MINOR: ring: add a function to compute max ring payload

Add a helper function to the ring API to compute the maximum payload
length that could fit into the ring based on ring size.

include/haproxy/ring.h

@@ -41,6 +41,8 @@ int ring_attach_cli(struct ring *ring, struct appctx *appctx, uint flags);
 int cli_io_handler_show_ring(struct appctx *appctx);
 void cli_io_release_show_ring(struct appctx *appctx);
 
+size_t ring_max_payload(const struct ring *ring);
+
 #endif /* _HAPROXY_RING_H */
 
 /*

src/ring.c

@@ -457,6 +457,22 @@ void cli_io_release_show_ring(struct appctx *appctx)
 	ring_detach_appctx(ring, appctx, ofs);
 }
 
+/* Returns the MAXIMUM payload len that could theoretically fit into the ring
+ * based on ring buffer size.
+ *
+ * Computation logic relies on implementation details from 'ring-t.h'.
+ */
+size_t ring_max_payload(const struct ring *ring)
+{
+	size_t max;
+
+	/* initial max = bufsize - 1 (initial RC) - 1 (payload RC) */
+	max = b_size(&ring->buf) - 1 - 1;
+
+	/* substract payload VI (varint-encoded size) */
+	max -= varint_bytes(max);
+	return max;
+}
 
 /*
  * Local variables: