Fix issue #4476

src/t_hash.c

@@ -322,6 +322,7 @@ bool hashTypeHasStringRef(robj *o, sds field) {
     if (objectGetEncoding(o) == OBJ_ENCODING_LISTPACK) return false;
     hashtable *ht = objectGetVal(o);
     void **entry_ref = hashtableFindRef(ht, field);
+    if (!entry_ref) return false;
     return (entryHasStringRef(*entry_ref));
 }
 

tests/unit/moduleapi/hash_stringref.tcl

@@ -14,6 +14,17 @@ start_server {tags {"modules"}} {
         assert_equal "1" [r hash.has_stringref k f]
     }
 
+    test {Module hash has_stringref on non-existent field} {
+        r del k
+        r hset k f hello1
+        # Setting a stringref converts the hash to the hashtable encoding, which
+        # is the only encoding that looks the field up in the hashtable.
+        r hash.set_stringref k f hello1
+        assert_encoding hashtable k
+        # Missing field must return 0 rather than dereferencing a NULL entry.
+        assert_equal "0" [r hash.has_stringref k nonexistent]
+    }
+
     test "Unload the module - hash" {
         assert_equal {OK} [r module unload hash.stringref]
     }