From 568c2e039bac388003068cd8debb2f93619dd462 Mon Sep 17 00:00:00 2001
From: Ozan Tezcan <ozantezcan@gmail.com>
Date: Thu, 6 Jan 2022 10:54:21 +0300
Subject: [PATCH] Set errno to EEXIST in redisFork() if child process exists
 (#10059)

Callers of redisFork() are logging `strerror(errno)` on failure.
`errno` is not set when there is already a child process, causing printing
current value of errno which was set before `redisFork()` call.

Setting errno to EEXIST on this failure to provide more meaningful error message.
---
 src/server.c                  | 4 +++-
 tests/unit/moduleapi/fork.tcl | 7 +++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/server.c b/src/server.c
index 42b6198d9..911441d56 100644
--- a/src/server.c
+++ b/src/server.c
@@ -5937,8 +5937,10 @@ void closeChildUnusedResourceAfterFork() {
 /* purpose is one of CHILD_TYPE_ types */
 int redisFork(int purpose) {
     if (isMutuallyExclusiveChildType(purpose)) {
-        if (hasActiveChildProcess())
+        if (hasActiveChildProcess()) {
+            errno = EEXIST;
             return -1;
+        }
 
         openChildInfoPipe();
     }
diff --git a/tests/unit/moduleapi/fork.tcl b/tests/unit/moduleapi/fork.tcl
index a6a9b8a1e..64184d01e 100644
--- a/tests/unit/moduleapi/fork.tcl
+++ b/tests/unit/moduleapi/fork.tcl
@@ -32,4 +32,11 @@ start_server {tags {"modules"}} {
         assert {[count_log_message "fork child exiting"] eq "1"}
     }
 
+    test {Module fork twice} {
+        r fork.create 0
+        after 250
+        catch {r fork.create 0}
+        assert {[count_log_message "Can't fork for module: File exists"] eq "1"}
+    }
+
 }