From 5b295ff409d66b961db28955c844b958efcd0b8a Mon Sep 17 00:00:00 2001
From: Aurelien DARRAGON <adarragon@haproxy.com>
Date: Mon, 26 Jun 2023 19:22:38 +0200
Subject: [PATCH] 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 |  2 ++
 src/ring.c             | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/haproxy/ring.h b/include/haproxy/ring.h
index d6c527c5b..71217d523 100644
--- a/include/haproxy/ring.h
+++ b/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 */
 
 /*
diff --git a/src/ring.c b/src/ring.c
index 2b8e3c85e..9b8863afe 100644
--- a/src/ring.c
+++ b/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: