dict.c: dictGetRandomKeys() visit pattern optimization.

We use the invariant that the original table ht[0] is never populated up
to the index before the current rehashing index.

Related to issue #2306.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/dict.c b/src/dict.c
index ad74d222a..fbcdd35f8 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -701,6 +701,10 @@ unsigned int dictGetRandomKeys(dict *d, dictEntry **des, unsigned int count) {
     unsigned int i = random() & maxsizemask;
     while(stored < count) {
         for (j = 0; j < tables; j++) {
+            /* Invariant of the dict.c rehashing: up to the indexes already
+             * visited in ht[0] during the rehashing, there are no populated
+             * buckets, so we can skip ht[0] for indexes between 0 and idx-1. */
+            if (j == 0 && i < d->rehashidx) continue;
             if (i >= d->ht[j].size) continue; /* Out of range for this table. */
             dictEntry *he = d->ht[j].table[i];
             while (he) {