Disable rehash when redis has child process (#8007)

In redisFork(), we don't set child pid, so updateDictResizePolicy()
doesn't take effect, that isn't friendly for copy-on-write.

The bug was introduced this in redis 6.0: e70fbad

(cherry picked from commit 16e3af9d23bb4791a7571f889aad8f2c85546521)

src/aof.c

@@ -1652,6 +1652,7 @@ int rewriteAppendOnlyFileBackground(void) {
         server.aof_rewrite_scheduled = 0;
         server.aof_rewrite_time_start = time(NULL);
         server.aof_child_pid = childpid;
+        updateDictResizePolicy();
         /* We set appendseldb to -1 in order to force the next call to the
          * feedAppendOnlyFile() to issue a SELECT command, so the differences
          * accumulated by the parent into server.aof_rewrite_buf will start

src/module.c

@@ -6988,6 +6988,7 @@ int RM_Fork(RedisModuleForkDoneHandler cb, void *user_data) {
         server.module_child_pid = childpid;
         moduleForkInfo.done_handler = cb;
         moduleForkInfo.done_handler_user_data = user_data;
+        updateDictResizePolicy();
         serverLog(LL_VERBOSE, "Module fork started pid: %d ", childpid);
     }
     return childpid;

src/rdb.c

@@ -1412,6 +1412,7 @@ int rdbSaveBackground(char *filename, rdbSaveInfo *rsi) {
         server.rdb_save_time_start = time(NULL);
         server.rdb_child_pid = childpid;
         server.rdb_child_type = RDB_CHILD_TYPE_DISK;
+        updateDictResizePolicy();
         return C_OK;
     }
     return C_OK; /* unreached */
@@ -2590,6 +2591,7 @@ int rdbSaveToSlavesSockets(rdbSaveInfo *rsi) {
             server.rdb_save_time_start = time(NULL);
             server.rdb_child_pid = childpid;
             server.rdb_child_type = RDB_CHILD_TYPE_SOCKET;
+            updateDictResizePolicy();
             close(server.rdb_pipe_write); /* close write in parent so that it can detect the close on the child. */
             if (aeCreateFileEvent(server.el, server.rdb_pipe_read, AE_READABLE, rdbPipeReadHandler,NULL) == AE_ERR) {
                 serverPanic("Unrecoverable error creating server.rdb_pipe_read file event.");

src/server.c

@@ -2019,6 +2019,9 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
             }
         }
     }
+    /* Just for the sake of defensive programming, to avoid forgeting to
+     * call this function when need. */
+    updateDictResizePolicy();
 
 
     /* AOF postponed flush: Try at every cron cycle if the slow fsync
@@ -4969,7 +4972,6 @@ int redisFork(int purpose) {
         if (childpid == -1) {
             return -1;
         }
-        updateDictResizePolicy();
     }
     return childpid;
 }

tests/unit/other.tcl

@@ -250,3 +250,23 @@ start_server {tags {"other"}} {
         r save
     } {OK}
 }
+
+start_server {tags {"other"}} {
+    test {Don't rehash if redis has child proecess} {
+        r config set save ""
+        r config set rdb-key-save-delay 1000000
+
+        populate 4096 "" 1
+        r bgsave
+        r mset k1 v1 k2 v2
+        # Hash table should not rehash
+        assert_no_match "*table size: 8192*" [r debug HTSTATS 9]
+        exec kill -9 [get_child_pid 0]
+        after 200
+
+        # Hash table should rehash since there is no child process,
+        # size is power of two and over 4098, so it is 16384
+        r set k3 v3
+        assert_match "*table size: 16384*" [r debug HTSTATS 9]
+    }
+}