From 9a4d8f356782d5fcf0120fe0d98d55e8d9be39fd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tor=20Arne=20Vestb=C3=B8?= <torarnv@gmail.com>
Date: Tue, 11 Sep 2018 23:13:37 +0200
Subject: [PATCH] Make use of getgrnam() thread-safe

getgrnam is not thread-safe on Linux (but is on macOS). To ensure
we don't call it from two threads at the same time we move its use
inside a lambda that initializes a local static. This is fine, as
we don't expect the 'nogroup' gid to change.
---
 sparsebundlefs.cpp | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/sparsebundlefs.cpp b/sparsebundlefs.cpp
index ebe7948..ac22c66 100644
--- a/sparsebundlefs.cpp
+++ b/sparsebundlefs.cpp
@@ -114,16 +114,20 @@ static int sparsebundle_getattr(const char *path, struct stat *stbuf)
     } else
         return -ENOENT;
 
-    // Reflect user ID of the user who mounted the file system,
-    // but prefer 'nogroup' for the group, since the group has
-    // no effect on who can access the mount.
+    // Reflect user ID of the user who mounted the file system
     stbuf->st_uid = getuid();
-    if (group *nogroup = getgrnam("nogroup"))
-        stbuf->st_gid = nogroup->gr_gid;
-    else if (group *nobody = getgrnam("nobody"))
-        stbuf->st_gid = nobody->gr_gid;
-    else
-        stbuf->st_gid = getgid();
+
+    static gid_t gid = []() {
+        // But prefer 'nogroup' for the group, since the group
+        // has no actual effect on who can access the mount.
+        if (group *nogroup = getgrnam("nogroup"))
+            return nogroup->gr_gid;
+        if (group *nobody = getgrnam("nobody"))
+            return nobody->gr_gid;
+        // Fall back to the mounting user
+        return getgid();
+    }();
+    stbuf->st_gid = gid;
 
     // Once allow_other or allow_root is added into the mix we
     // want the permissions to also reflect the the situation.