dict.c: put a bound to max work dictRehash() call can do.

Related to issue #2306.

src/dict.c

@@ -235,9 +235,15 @@ int dictExpand(dict *d, unsigned long size)
 
 /* Performs N steps of incremental rehashing. Returns 1 if there are still
  * keys to move from the old to the new hash table, otherwise 0 is returned.
+ *
  * Note that a rehashing step consists in moving a bucket (that may have more
- * than one key as we use chaining) from the old to the new hash table. */
+ * than one key as we use chaining) from the old to the new hash table, however
+ * since part of the hash table may be composed of empty spaces, it is not
+ * guaranteed that this function will rehash even a single bucket, since it
+ * will visit at max N*10 empty buckets in total, otherwise the amount of
+ * 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;
 
     while(n--) {
@@ -255,7 +261,10 @@ int dictRehash(dict *d, int n) {
         /* Note that rehashidx can't overflow as we are sure there are more
          * elements because ht[0].used != 0 */
         assert(d->ht[0].size > (unsigned long)d->rehashidx);
-        while(d->ht[0].table[d->rehashidx] == NULL) d->rehashidx++;
+        while(d->ht[0].table[d->rehashidx] == NULL) {
+            d->rehashidx++;
+            if (--empty_visits == 0) return 1;
+        }
         de = d->ht[0].table[d->rehashidx];
         /* Move all the keys in this bucket from the old to the new hash HT */
         while(de) {