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.

(cherry picked from commit 2bec254d89)
(cherry picked from commit 3e82bdf738)
(cherry picked from commit 4803334cf6cb1eccdd33674a72a215ed6cd10069)

src/dict.c

@@ -51,15 +51,15 @@
 #include <assert.h>
 #endif
 
-/* Using dictEnableResize() / dictDisableResize() we make possible to
- * enable/disable resizing of the hash table as needed. This is very important
+/* Using dictEnableResize() / dictDisableResize() we make possible to disable
+ * resizing and rehashing of the hash table as needed. This is very important
  * for Redis, as we use copy-on-write and don't want to move too much memory
  * around when there is a child performing saving operations.
  *
  * Note that even when dict_can_resize is set to 0, not all resizes are
  * prevented: a hash table is still allowed to grow if the ratio between
  * the number of elements and the buckets > dict_force_resize_ratio. */
-static int dict_can_resize = 1;
+static dictResizeEnable dict_can_resize = DICT_RESIZE_ENABLE;
 static unsigned int dict_force_resize_ratio = 5;
 
 /* -------------------------- private prototypes ---------------------------- */
@@ -136,7 +136,7 @@ int dictResize(dict *d)
 {
     unsigned long minimal;
 
-    if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;
+    if (dict_can_resize != DICT_RESIZE_ENABLE || dictIsRehashing(d)) return DICT_ERR;
     minimal = d->ht[0].used;
     if (minimal < DICT_HT_INITIAL_SIZE)
         minimal = DICT_HT_INITIAL_SIZE;
@@ -187,7 +187,12 @@ int dictExpand(dict *d, unsigned long size)
  * work it does would be unbound and the function may block for a long time. */
 int dictRehash(dict *d, int n) {
     int empty_visits = n*10; /* Max number of empty buckets to visit. */
-    if (!dictIsRehashing(d)) return 0;
+    if (dict_can_resize == DICT_RESIZE_FORBID || !dictIsRehashing(d)) return 0;
+    if (dict_can_resize == DICT_RESIZE_AVOID && 
+        (d->ht[1].size / d->ht[0].size < dict_force_resize_ratio))
+    {
+        return 0;
+    }
 
     while(n-- && d->ht[0].used != 0) {
         dictEntry *de, *nextde;
@@ -962,9 +967,10 @@ static int _dictExpandIfNeeded(dict *d)
      * table (global setting) or we should avoid it but the ratio between
      * elements/buckets is over the "safe" threshold, we resize doubling
      * the number of buckets. */
-    if (d->ht[0].used >= d->ht[0].size &&
-        (dict_can_resize ||
-         d->ht[0].used/d->ht[0].size > dict_force_resize_ratio))
+    if ((dict_can_resize == DICT_RESIZE_ENABLE &&
+         d->ht[0].used >= d->ht[0].size) ||
+        (dict_can_resize != DICT_RESIZE_FORBID &&
+         d->ht[0].used / d->ht[0].size > dict_force_resize_ratio))
     {
         return dictExpand(d, d->ht[0].used*2);
     }
@@ -1023,12 +1029,8 @@ void dictEmpty(dict *d, void(callback)(void*)) {
     d->iterators = 0;
 }
 
-void dictEnableResize(void) {
-    dict_can_resize = 1;
-}
-
-void dictDisableResize(void) {
-    dict_can_resize = 0;
+void dictSetResizeEnabled(dictResizeEnable enable) {
+    dict_can_resize = enable;
 }
 
 uint64_t dictGetHash(dict *d, const void *key) {

src/dict.h

@@ -157,6 +157,12 @@ typedef void (dictScanBucketFunction)(void *privdata, dictEntry **bucketref);
 #define randomULong() random()
 #endif
 
+typedef enum {
+    DICT_RESIZE_ENABLE,
+    DICT_RESIZE_AVOID,
+    DICT_RESIZE_FORBID,
+} dictResizeEnable;
+
 /* API */
 dict *dictCreate(dictType *type, void *privDataPtr);
 int dictExpand(dict *d, unsigned long size);
@@ -182,8 +188,7 @@ void dictGetStats(char *buf, size_t bufsize, dict *d);
 uint64_t dictGenHashFunction(const void *key, int len);
 uint64_t dictGenCaseHashFunction(const unsigned char *buf, int len);
 void dictEmpty(dict *d, void(callback)(void*));
-void dictEnableResize(void);
-void dictDisableResize(void);
+void dictSetResizeEnabled(dictResizeEnable enable);
 int dictRehash(dict *d, int n);
 int dictRehashMilliseconds(dict *d, int ms);
 void dictSetHashFunctionSeed(uint8_t *seed);

src/server.c

@@ -1465,13 +1465,15 @@ 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);
 }
 
 /* Return true if there are no active children processes doing RDB saving,
@@ -5122,6 +5124,7 @@ int redisFork(int purpose) {
         server.in_fork_child = purpose;
         setOOMScoreAdj(CONFIG_OOM_BGCHILD);
         setupChildSignalHandlers();
+        updateDictResizePolicy();
         closeClildUnusedResourceAfterFork();
     } else {
         /* Parent */