Add "FUSE_NO_MTAB_UPDATE" env to disable mtab updates

This adds the env FUSE_NO_MTAB_UPDATE variable to be able
to disable mtab updates without recompiling libfuse.
The value needs to be set to 1.

doc/mount.fuse3.8

@@ -256,6 +256,9 @@ The user can only mount on a mountpoint, for which it has write permission
 The mountpoint is not a sticky directory which isn't owned by the user (like \fI/tmp\fP usually is)
 .IP 3.
 No other user (including root) can access the contents of the mounted filesystem.
+.SH Environment variables
+.TP
+FUSE_NO_MTAB_UPDATE can be set to 1 to disable mtab (nowadays actually /run/mount/utab) updates.
 .SH NOTE
 FUSE filesystems are unmounted using the \fBfusermount3\fP(1) command (\fBfusermount3 -u mountpoint\fP).
 .SH "AUTHORS"

lib/mount_util.c

@@ -48,6 +48,13 @@ static int mtab_needs_update(const char *mnt)
 	int res;
 	struct stat stbuf;
 
+	char *env = getenv("FUSE_NO_MTAB_UPDATE");
+	if (env != NULL) {
+		int value = atoi(env);
+		if (value == 1)
+			return 0;
+	}
+
 	/* If mtab is within new mount, don't touch it */
 	if (strncmp(mnt, _PATH_MOUNTED, strlen(mnt)) == 0 &&
 	    _PATH_MOUNTED[strlen(mnt)] == '/')