Fix leak when shrinking a hashtable without entries

src/hashtable.c

@@ -652,7 +652,7 @@ static int resize(hashtable *ht, size_t min_capacity, int *malloc_failed) {
     if (ht->type->rehashingStarted) ht->type->rehashingStarted(ht);
 
     /* If the old table was empty, the rehashing is completed immediately. */
-    if (ht->tables[0] == NULL || ht->used[0] == 0) {
+    if (ht->tables[0] == NULL || (ht->used[0] == 0 && ht->child_buckets[0] == 0)) {
         rehashingCompleted(ht);
     } else if (ht->type->instant_rehashing) {
         while (hashtableIsRehashing(ht)) {
@@ -1761,7 +1761,9 @@ size_t hashtableScanDefrag(hashtable *ht, size_t cursor, hashtableScanFunction f
     if (!hashtableIsRehashing(ht)) {
         /* Emit entries at the cursor index. */
         size_t mask = expToMask(ht->bucket_exp[0]);
-        bucket *b = &ht->tables[0][cursor & mask];
+        size_t idx = cursor & mask;
+        size_t used_before = ht->used[0];
+        bucket *b = &ht->tables[0][idx];
         do {
             if (b->presence != 0) {
                 int pos;
@@ -1779,6 +1781,11 @@ size_t hashtableScanDefrag(hashtable *ht, size_t cursor, hashtableScanFunction f
             b = next;
         } while (b != NULL);
 
+        /* If any entries were deleted, fill the holes. */
+        if (ht->used[0] < used_before) {
+            compactBucketChain(ht, idx, 0);
+        }
+
         /* Advance cursor. */
         cursor = nextCursor(cursor, mask);
     } else {

tests/unit/expire.tcl

@@ -951,6 +951,43 @@ start_server {tags {"expire"}} {
     }
 }
 
+start_server {tags {expire} overrides {hz 100}} {
+    test {Active expiration triggers hashtable shrink} {
+        set persistent_keys 5
+        set volatile_keys 100
+        set total_keys [expr $persistent_keys + $volatile_keys]
+
+        for {set i 0} {$i < $persistent_keys} {incr i} {
+            r set "key_$i" "value_$i"
+        }
+        for {set i 0} {$i < $volatile_keys} {incr i} {
+            r psetex "expire_key_${i}" 100 "expire_value_${i}"
+        }
+        set table_size_before_expire [main_hash_table_size]
+
+        # Verify keys are set
+        assert_equal $total_keys [r dbsize]
+
+        # Wait for active expiration
+        wait_for_condition 100 50 {
+            [r dbsize] eq $persistent_keys
+        } else {
+            fail "Keys not expired"
+        }
+
+        # Wait for the table to shrink and active rehashing finish
+        wait_for_condition 100 50 {
+            [main_hash_table_size] < $table_size_before_expire
+        } else {
+            puts [r debug htstats 9]
+            fail "Table didn't shrink"
+        }
+
+        # Verify server is still responsive
+        assert_equal [r ping] {PONG}
+    } {} {needs:debug}
+}
+
 start_cluster 1 0 {tags {"expire external:skip cluster"}} {
     test "expire scan should skip dictionaries with lot's of empty buckets" {
         r debug set-active-expire 0