locale-util: do not call setlocale() when multi-threaded

Fixes #30141.

(cherry picked from commit ca13432d600593b8eda76721118763b63746eb33)

Related: RHEL-93425

Author: yuwata

src/basic/locale-util.c

@@ -20,6 +20,7 @@
 #include "fd-util.h"
 #include "hashmap.h"
 #include "locale-util.h"
+#include "missing_syscall.h"
 #include "path-util.h"
 #include "set.h"
 #include "string-table.h"
@@ -234,6 +235,12 @@ bool is_locale_utf8(void) {
         if (cached_answer >= 0)
                 goto out;
 
+        /* This function may be called from libsystemd, and setlocale() is not thread safe. Assuming yes. */
+        if (gettid() != raw_getpid()) {
+                cached_answer = true;
+                goto out;
+        }
+
         if (!setlocale(LC_ALL, "")) {
                 cached_answer = true;
                 goto out;