Make sure that fork child doesn't do incremental rehashing (#11692)

Turns out that a fork child calling getExpire while persisting keys (and
possibly also a result of some module fork tasks) could cause dictFind
to do incremental rehashing in the child process, which is both a waste
of time, and also causes COW harm.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/server.c b/src/server.c
index f01e7382d..ae83ec25c 100644
--- a/src/server.c
+++ b/src/server.c
@@ -622,13 +622,15 @@ int incrementallyRehash(int dbid) {
  * as we want to avoid resizing the hash tables when there is a child in order
  * to play well with copy-on-write (otherwise when a resize happens lots of
  * memory pages are copied). The goal of this function is to update the ability
- * for dict.c to resize the hash tables accordingly to the fact we have an
+ * for dict.c to resize or rehash the tables accordingly to the fact we have an
  * active fork child running. */
 void updateDictResizePolicy(void) {
-    if (!hasActiveChildProcess())
-        dictEnableResize();
+    if (server.in_fork_child != CHILD_TYPE_NONE)
+        dictSetResizeEnabled(DICT_RESIZE_FORBID);
+    else if (hasActiveChildProcess())
+        dictSetResizeEnabled(DICT_RESIZE_AVOID);
     else
-        dictDisableResize();
+        dictSetResizeEnabled(DICT_RESIZE_ENABLE);
 }
 
 const char *strChildType(int type) {
@@ -6496,6 +6498,7 @@ int redisFork(int purpose) {
         server.in_fork_child = purpose;
         setupChildSignalHandlers();
         setOOMScoreAdj(CONFIG_OOM_BGCHILD);
+        updateDictResizePolicy();
         dismissMemoryInChild();
         closeChildUnusedResourceAfterFork();
         /* Close the reading part, so that if the parent crashes, the child will