Fix misuse of safe iterators (valkey-io#612)

Safe iterators must call resetIterators when they are done being used.
Fix one issue where a safe iterator was not correctly calling reset
during cluster slot caching and fixed a second issue where reset
iterator was being called twice. For the double release case,
kvstoreIteratorNextDict is responsible for patching up the iterator, but
we were calling it a second time in kvstoreIteratorNext.

In addition, I added some documentation around initializing iterators,
added an assert to prevent double initialization, and remove a function
from the public interface which isn't needed and might lead to incorrect
usage of the safe iterators.

Bumping srgsanky for finding it here:
valkey-io@c478206#r142867004.

---------

Signed-off-by: Madelyn Olson <[email protected]>

src/cluster_legacy.c

@@ -4888,7 +4888,7 @@ void bitmapClearBit(unsigned char *bitmap, int pos) {
  * MIGRATE_TO flag the when a primary gets the first slot. */
 int clusterPrimariesHaveReplicas(void) {
     dictIterator di;
-    dictInitSafeIterator(&di, server.cluster->nodes);
+    dictInitIterator(&di, server.cluster->nodes);
     dictEntry *de;
     int replicas = 0;
     while ((de = dictNext(&di)) != NULL) {
@@ -6509,7 +6509,7 @@ void clusterPromoteSelfToPrimary(void) {
 
 int detectAndUpdateCachedNodeHealth(void) {
     dictIterator di;
-    dictInitSafeIterator(&di, server.cluster->nodes);
+    dictInitIterator(&di, server.cluster->nodes);
     dictEntry *de;
     clusterNode *node;
     int overall_health_changed = 0;

src/dict.c

@@ -943,6 +943,8 @@ unsigned long long dictFingerprint(dict *d) {
     return hash;
 }
 
+/* Initiaize a normal iterator. This function should be called when initializing
+ * an iterator on the stack. */
 void dictInitIterator(dictIterator *iter, dict *d) {
     iter->d = d;
     iter->table = 0;
@@ -952,16 +954,19 @@ void dictInitIterator(dictIterator *iter, dict *d) {
     iter->nextEntry = NULL;
 }
 
+/* Initialize a safe iterator, which is allowed to modify the dictionary while iterating.
+ * You must call dictResetIterator when you are done with a safe iterator. */
 void dictInitSafeIterator(dictIterator *iter, dict *d) {
     dictInitIterator(iter, d);
     iter->safe = 1;
 }
 
 void dictResetIterator(dictIterator *iter) {
     if (!(iter->index == -1 && iter->table == 0)) {
-        if (iter->safe)
+        if (iter->safe) {
             dictResumeRehashing(iter->d);
-        else
+            assert(iter->d->pauserehash >= 0);
+        } else
             assert(iter->fingerprint == dictFingerprint(iter->d));
     }
 }

src/kvstore.c

@@ -48,6 +48,8 @@
 
 #define UNUSED(V) ((void)V)
 
+static dict *kvstoreIteratorNextDict(kvstoreIterator *kvs_it);
+
 struct _kvstore {
     int flags;
     dictType dtype;
@@ -572,7 +574,7 @@ void kvstoreIteratorRelease(kvstoreIterator *kvs_it) {
 }
 
 /* Returns next dictionary from the iterator, or NULL if iteration is complete. */
-dict *kvstoreIteratorNextDict(kvstoreIterator *kvs_it) {
+static dict *kvstoreIteratorNextDict(kvstoreIterator *kvs_it) {
     if (kvs_it->next_didx == -1) return NULL;
 
     /* The dict may be deleted during the iteration process, so here need to check for NULL. */
@@ -600,13 +602,6 @@ dictEntry *kvstoreIteratorNext(kvstoreIterator *kvs_it) {
     if (!de) { /* No current dict or reached the end of the dictionary. */
         dict *d = kvstoreIteratorNextDict(kvs_it);
         if (!d) return NULL;
-        if (kvs_it->di.d) {
-            /* Before we move to the next dict, reset the iter of the previous dict. */
-            dictIterator *iter = &kvs_it->di;
-            dictResetIterator(iter);
-            /* In the safe iterator context, we may delete entries. */
-            freeDictIfNeeded(kvs_it->kvs, kvs_it->didx);
-        }
         dictInitSafeIterator(&kvs_it->di, d);
         de = dictNext(&kvs_it->di);
     }

src/kvstore.h

@@ -40,7 +40,6 @@ uint64_t kvstoreGetHash(kvstore *kvs, const void *key);
 /* kvstore iterator specific functions */
 kvstoreIterator *kvstoreIteratorInit(kvstore *kvs);
 void kvstoreIteratorRelease(kvstoreIterator *kvs_it);
-dict *kvstoreIteratorNextDict(kvstoreIterator *kvs_it);
 int kvstoreIteratorGetCurrentDictIndex(kvstoreIterator *kvs_it);
 dictEntry *kvstoreIteratorNext(kvstoreIterator *kvs_it);