From: Andrey Rachitskiy <pl0h0yp1@gmail.com>
Date: Thu, 6 Aug 2026 18:48:00 +0500
Subject: [PATCH v2] Fix max_stack_depth error when ulimit -s is too small

When the platform stack rlimit is less than or equal to
STACK_DEPTH_SLOP, check_max_stack_depth() printed a zero or negative
allowed max_stack_depth in the DETAIL.  Keep rejecting that case, but
with an explicit message.  Unknown rlimits (-1) remain unchecked.

Author: Daria Shanina <vilensipkdm@gmail.com>
Reviewed-by: Andrey Rachitskiy <pl0h0yp1@gmail.com>
---
diff --git a/src/backend/utils/misc/stack_depth.c b/src/backend/utils/misc/stack_depth.c
index 914a4393bf3..13453db3163 100644
--- a/src/backend/utils/misc/stack_depth.c
+++ b/src/backend/utils/misc/stack_depth.c
@@ -159,11 +159,24 @@ check_max_stack_depth(int *newval, void **extra, GucSource source)
 {
 	ssize_t		newval_bytes = *newval * (ssize_t) 1024;
 	ssize_t		stack_rlimit = get_stack_depth_rlimit();
+	ssize_t		available;
 
-	if (stack_rlimit > 0 && newval_bytes > stack_rlimit - STACK_DEPTH_SLOP)
+	/* -1 means unknown; do not treat it as a too-small rlimit. */
+	if (stack_rlimit <= 0)
+		return true;
+
+	available = stack_rlimit - STACK_DEPTH_SLOP;
+	if (available <= 0)
+	{
+		GUC_check_errdetail("platform stack depth limit (%zdkB) is too small.",
+							stack_rlimit / 1024);
+		GUC_check_errhint("Increase the platform's stack depth limit via \"ulimit -s\" or local equivalent.");
+		return false;
+	}
+	if (newval_bytes > available)
 	{
 		GUC_check_errdetail("\"max_stack_depth\" must not exceed %zdkB.",
-							(stack_rlimit - STACK_DEPTH_SLOP) / 1024);
+							available / 1024);
 		GUC_check_errhint("Increase the platform's stack depth limit via \"ulimit -s\" or local equivalent.");
 		return false;
 	}