Fix flock cluster config may cause failure to restart after kill -9 (#7674)

After fork, the child process(redis-aof-rewrite) will get the fd opened
by the parent process(redis), when redis killed by kill -9, it will not
graceful exit(call prepareForShutdown()), so redis-aof-rewrite thread may still
alive, the fd(lock) will still be held by redis-aof-rewrite thread, and
redis restart will fail to get lock, means fail to start.

This issue was causing failures in the cluster tests in github actions.

Co-authored-by: Oran Agra <oran@redislabs.com>

src/cluster.c

@@ -418,7 +418,15 @@ int clusterLockConfig(char *filename) {
         return C_ERR;
     }
     /* Lock acquired: leak the 'fd' by not closing it, so that we'll retain the
-     * lock to the file as long as the process exists. */
+     * lock to the file as long as the process exists.
+     *
+     * After fork, the child process will get the fd opened by the parent process,
+     * we need save `fd` to `cluster_config_file_lock_fd`, so that in redisFork(),
+     * it will be closed in the child process.
+     * If it is not closed, when the main process is killed -9, but the child process
+     * (redis-aof-rewrite) is still alive, the fd(lock) will still be held by the
+     * child process, and the main process will fail to get lock, means fail to start. */
+    server.cluster_config_file_lock_fd = fd;
 #endif /* __sun */
 
     return C_OK;
@@ -468,6 +476,7 @@ void clusterInit(void) {
 
     /* Lock the cluster config file to make sure every node uses
      * its own nodes.conf. */
+    server.cluster_config_file_lock_fd = -1;
     if (clusterLockConfig(server.cluster_configfile) == C_ERR)
         exit(1);
 

src/server.c

@@ -4917,14 +4917,24 @@ void setupChildSignalHandlers(void) {
     return;
 }
 
+/* After fork, the child process will inherit the resources
+ * of the parent process, e.g. fd(socket or flock) etc.
+ * should close the resources not used by the child process, so that if the
+ * parent restarts it can bind/lock despite the child possibly still running. */
+void closeClildUnusedResourceAfterFork() {
+    closeListeningSockets(0);
+    if (server.cluster_enabled && server.cluster_config_file_lock_fd != -1)
+        close(server.cluster_config_file_lock_fd);  /* don't care if this fails */
+}
+
 int redisFork() {
     int childpid;
     long long start = ustime();
     if ((childpid = fork()) == 0) {
         /* Child */
         setOOMScoreAdj(CONFIG_OOM_BGCHILD);
-        closeListeningSockets(0);
         setupChildSignalHandlers();
+        closeClildUnusedResourceAfterFork();
     } else {
         /* Parent */
         server.stat_fork_time = ustime()-start;

src/server.h

@@ -1419,6 +1419,7 @@ struct redisServer {
                                       REDISMODULE_CLUSTER_FLAG_*. */
     int cluster_allow_reads_when_down; /* Are reads allowed when the cluster
                                         is down? */
+    int cluster_config_file_lock_fd;   /* cluster config fd, will be flock */
     /* Scripting */
     lua_State *lua; /* The Lua interpreter. We use just one for all clients */
     client *lua_client;   /* The "fake client" to query Redis from Lua */

tests/instances.tcl

@@ -116,7 +116,9 @@ proc spawn_instance {type base_port count {conf {}}} {
 
         # Check availability finally
         if {[server_is_up 127.0.0.1 $port 100] == 0} {
-            abort_sentinel_test "Problems starting $type #$j: ping timeout"
+            set logfile [file join $dirname log.txt]
+            puts [exec tail $logfile]
+            abort_sentinel_test "Problems starting $type #$j: ping timeout, maybe server start failed, check $logfile"
         }
 
         # Push the instance into the right list
@@ -493,12 +495,12 @@ proc kill_instance {type id} {
 
     # Wait for the port it was using to be available again, so that's not
     # an issue to start a new server ASAP with the same port.
-    set retry 10
+    set retry 100
     while {[incr retry -1]} {
-        set port_is_free [catch {set s [socket 127.0.01 $port]}]
+        set port_is_free [catch {set s [socket 127.0.0.1 $port]}]
         if {$port_is_free} break
         catch {close $s}
-        after 1000
+        after 100
     }
     if {$retry == 0} {
         error "Port $port does not return available after killing instance."
@@ -525,7 +527,9 @@ proc restart_instance {type id} {
 
     # Check that the instance is running
     if {[server_is_up 127.0.0.1 $port 100] == 0} {
-        abort_sentinel_test "Problems starting $type #$id: ping timeout"
+        set logfile [file join $dirname log.txt]
+        puts [exec tail $logfile]
+        abort_sentinel_test "Problems starting $type #$id: ping timeout, maybe server start failed, check $logfile"
     }
 
     # Connect with it with a fresh link