Reuse the immutable empty array in apcu_fetch

Instead of allocating brand new arrays or looking them up in the hash
map, return php's shared immutable empty array starting in php 7.3.

For #323

apc_persist.c

@@ -565,11 +565,15 @@ static zend_array *apc_unpersist_ht(
 	apc_unpersist_add_already_copied(ctxt, orig_ht, ht);
 	memcpy(ht, orig_ht, sizeof(HashTable));
 	GC_TYPE_INFO(ht) = GC_ARRAY;
-
+#if PHP_VERSION_ID >= 70300
+	/* Caller used ZVAL_EMPTY_ARRAY and set different zval flags instead */
+	ZEND_ASSERT(ht->nNumOfElements > 0 && ht->nNumUsed > 0);
+#else
 	if (ht->nNumUsed == 0) {
 		HT_SET_DATA_ADDR(ht, &uninitialized_bucket);
 		return ht;
 	}
+#endif
 
 	HT_SET_DATA_ADDR(ht, emalloc(HT_SIZE(ht)));
 	memcpy(HT_GET_DATA_ADDR(ht), HT_GET_DATA_ADDR(orig_ht), HT_HASH_SIZE(ht->nTableMask));
@@ -628,6 +632,12 @@ static void apc_unpersist_zval_impl(apc_unpersist_context_t *ctxt, zval *zv) {
 			Z_REF_P(zv) = apc_unpersist_ref(ctxt, Z_REF_P(zv));
 			return;
 		case IS_ARRAY:
+#if PHP_VERSION_ID >= 70300
+			if (Z_ARR_P(zv)->nNumOfElements == 0) {
+				ZVAL_EMPTY_ARRAY(zv); /* #323 */
+				return;
+			}
+#endif
 			Z_ARR_P(zv) = apc_unpersist_ht(ctxt, Z_ARR_P(zv));
 			return;
 		default:

package.xml

@@ -82,6 +82,7 @@
     <file name="apc_entry_002.phpt" role="test" />
     <file name="apc_entry_003.phpt" role="test" />
     <file name="apc_entry_recursion.phpt" role="test" />
+    <file name="apcu_fetch_empty_array_reference.phpt" role="test" />
     <file name="apc_inc_perf.phpt" role="test" />
     <file name="apc_store_array_int_keys.phpt" role="test" />
     <file name="apc_store_array_with_refs.phpt" role="test" />
@@ -718,7 +719,7 @@ and unpersisting values.
   - Bring back spinlocks, various issues (Joe)
   - Fixed symbol clashing, windows debug mode, again (Anatol)
   - apcu_key_info / apc_cache_stat functions (Joe)
-  
+
    </notes>
   </release>
   <release>

tests/apcu_fetch_empty_array_reference.phpt

@@ -0,0 +1,55 @@
+--TEST--
+apcu_fetch should work for multiple reference groups
+--SKIPIF--
+<?php require_once(dirname(__FILE__) . '/skipif.inc'); ?>
+--INI--
+apc.enabled=1
+apc.enable_cli=1
+--FILE--
+<?php
+
+$x = [];
+$y = [];
+$array = [&$x, &$x, &$y, &$y];
+apcu_store("ary", $array);
+$copy = apcu_fetch("ary");
+$copy[0][1] = new stdClass();
+var_dump($array);
+var_dump($copy);
+
+?>
+--EXPECT--
+array(4) {
+  [0]=>
+  &array(0) {
+  }
+  [1]=>
+  &array(0) {
+  }
+  [2]=>
+  &array(0) {
+  }
+  [3]=>
+  &array(0) {
+  }
+}
+array(4) {
+  [0]=>
+  &array(1) {
+    [1]=>
+    object(stdClass)#1 (0) {
+    }
+  }
+  [1]=>
+  &array(1) {
+    [1]=>
+    object(stdClass)#1 (0) {
+    }
+  }
+  [2]=>
+  &array(0) {
+  }
+  [3]=>
+  &array(0) {
+  }
+}