Support empty set

Signed-off-by: Seungmin Lee <sungming@amazon.com>

src/config.c

@@ -3206,6 +3206,7 @@ standardConfig static_configs[] = {
     createBoolConfig("cluster-slot-stats-enabled", NULL, MODIFIABLE_CONFIG, server.cluster_slot_stats_enabled, 0, NULL, NULL),
     createBoolConfig("hide-user-data-from-log", NULL, MODIFIABLE_CONFIG, server.hide_user_data_from_log, 1, NULL, NULL),
     createBoolConfig("import-mode", NULL, DEBUG_CONFIG | MODIFIABLE_CONFIG, server.import_mode, 0, NULL, NULL),
+    createBoolConfig("allow-empty-set", NULL, MODIFIABLE_CONFIG, server.allow_empty_set, 0, NULL, NULL),
 
     /* String Configs */
     createStringConfig("aclfile", NULL, IMMUTABLE_CONFIG, ALLOW_EMPTY_STRING, server.acl_filename, "", NULL, NULL),

src/intset.c

@@ -320,9 +320,6 @@ int intsetValidateIntegrity(const unsigned char *p, size_t size, int deep) {
     uint32_t count = intrev32ifbe(is->length);
     if (sizeof(*is) + count * record_size != size) return 0;
 
-    /* check that the set is not empty. */
-    if (count == 0) return 0;
-
     if (!deep) return 1;
 
     /* check that there are no dup or out of order records. */

src/rdb.c

@@ -1900,7 +1900,7 @@ robj *rdbLoadObject(int rdbtype, rio *rdb, sds key, int dbid, int *error) {
     } else if (rdbtype == RDB_TYPE_SET) {
         /* Read Set value */
         if ((len = rdbLoadLen(rdb, NULL)) == RDB_LENERR) return NULL;
-        if (len == 0) goto emptykey;
+        if (len == 0 && !server.allow_empty_set) goto emptykey;
 
         /* Use a regular set when there are too many entries. */
         size_t max_entries = server.set_max_intset_entries;
@@ -2356,6 +2356,18 @@ robj *rdbLoadObject(int rdbtype, rio *rdb, sds key, int dbid, int *error) {
             }
             o->type = OBJ_SET;
             o->encoding = OBJ_ENCODING_INTSET;
+
+            if (intsetLen((intset *)encoded) == 0) {
+                if (!server.allow_empty_set) {
+                    zfree(encoded);
+                    o->ptr = NULL;
+                    decrRefCount(o);
+                    goto emptykey;
+                }
+
+                zfree(encoded);
+                o->ptr = intsetNew();
+            }
             if (intsetLen(o->ptr) > server.set_max_intset_entries) setTypeConvert(o, OBJ_ENCODING_HASHTABLE);
             break;
         case RDB_TYPE_SET_LISTPACK:
@@ -2371,10 +2383,15 @@ robj *rdbLoadObject(int rdbtype, rio *rdb, sds key, int dbid, int *error) {
             o->encoding = OBJ_ENCODING_LISTPACK;
 
             if (setTypeSize(o) == 0) {
+                if (!server.allow_empty_set) {
+                    zfree(encoded);
+                    o->ptr = NULL;
+                    decrRefCount(o);
+                    goto emptykey;
+                }
+
                 zfree(encoded);
-                o->ptr = NULL;
-                decrRefCount(o);
-                goto emptykey;
+                o->ptr = lpNew(0);
             }
             if (setTypeSize(o) > server.set_max_listpack_entries) setTypeConvert(o, OBJ_ENCODING_HASHTABLE);
             break;

src/server.h

@@ -1930,6 +1930,10 @@ struct valkeyServer {
                                                  invocation of the event loop. */
     unsigned int max_new_conns_per_cycle;     /* The maximum number of tcp connections that will be accepted during each
                                                  invocation of the event loop. */
+    int allow_empty_set;                      /* Flag to control whether empty set is allowed in the database
+                                                 1 empty sets are preserved in database even after all elements are removed
+                                                 0 by default, key for the set is deleted when it becomes empty */
+
     /* AOF persistence */
     int aof_enabled;                    /* AOF configuration */
     int aof_state;                      /* AOF_(ON|OFF|WAIT_REWRITE) */

src/t_set.c

@@ -624,8 +624,10 @@ void sremCommand(client *c) {
         if (setTypeRemove(set, c->argv[j]->ptr)) {
             deleted++;
             if (setTypeSize(set) == 0) {
-                dbDelete(c->db, c->argv[1]);
-                keyremoved = 1;
+                if (!server.allow_empty_set) {
+                    dbDelete(c->db, c->argv[1]);
+                    keyremoved = 1;
+                }
                 break;
             }
         }
@@ -668,8 +670,8 @@ void smoveCommand(client *c) {
     }
     notifyKeyspaceEvent(NOTIFY_SET, "srem", c->argv[1], c->db->id);
 
-    /* Remove the src set from the database when empty */
-    if (setTypeSize(srcset) == 0) {
+    /* Remove the src set from the database when empty and allow-empty-set is disabled */
+    if (!server.allow_empty_set && setTypeSize(srcset) == 0) {
         dbDelete(c->db, c->argv[1]);
         notifyKeyspaceEvent(NOTIFY_GENERIC, "del", c->argv[1], c->db->id);
     }
@@ -971,8 +973,8 @@ void spopCommand(client *c) {
     addReplyBulk(c, ele);
     decrRefCount(ele);
 
-    /* Delete the set if it's empty */
-    if (setTypeSize(set) == 0) {
+    /* Delete the set if it's empty and allow-empty-set is disabled */
+    if (!server.allow_empty_set && setTypeSize(set) == 0) {
         dbDelete(c->db, c->argv[1]);
         notifyKeyspaceEvent(NOTIFY_GENERIC, "del", c->argv[1], c->db->id);
     }

tests/support/server.tcl

@@ -769,7 +769,7 @@ proc start_multiple_servers {num options code} {
     uplevel 1 $code
 }
 
-proc restart_server {level wait_ready rotate_logs {reconnect 1} {shutdown sigterm}} {
+proc restart_server {level wait_ready rotate_logs {reconnect 1} {shutdown sigterm} {config_override {}}} {
     set srv [lindex $::servers end+$level]
     if {$shutdown ne {sigterm}} {
         catch {[dict get $srv "client"] shutdown $shutdown}
@@ -798,6 +798,15 @@ proc restart_server {level wait_ready rotate_logs {reconnect 1} {shutdown sigter
 
     set config_file [dict get $srv "config_file"]
 
+    # Apply config updates if provided
+    if {[llength $config_override] > 0} {
+        set fd [open $config_file "a"]
+        foreach {key value} $config_override {
+            puts $fd "$key $value"
+        }
+        close $fd
+    }
+
     set pid [spawn_server $config_file $stdout $stderr {}]
 
     # check that the server actually started

tests/unit/type/set.tcl

@@ -1168,6 +1168,111 @@ foreach type {single multiple single_multiple} {
     }
 }
 
+start_server {
+    tags {"set external:skip"}
+    overrides {
+        "set-max-intset-entries" 1
+        "allow-empty-set" "yes"
+    }
+} {
+    proc save_load_rdb {{config_override {}}} {
+        r save
+        restart_server 0 true false true now $config_override
+        wait_done_loading r
+    }
+    proc create_emptyset {key members} {
+        r del $key
+        foreach member $members { r sadd $key $member }
+        foreach member $members { r srem $key $member }
+    }
+    proc verify_emptyset {key} {
+        assert_equal 0 [r scard $key]
+        assert_equal 1 [r exists $key]
+    }
+
+    test {EMTPYSET save and reload empty listset from RDB} {
+        r flushdb
+        create_emptyset listset {a}
+        verify_emptyset listset
+        assert_equal listpack [r object encoding listset]
+        save_load_rdb
+        verify_emptyset listset
+    }
+
+    test {EMTPYSET save and reload empty intset from RDB} {
+        r flushdb
+        create_emptyset intset {1}
+        verify_emptyset intset
+        assert_equal intset [r object encoding intset]
+        save_load_rdb
+        verify_emptyset intset
+    }
+
+    test {EMTPYSET save and reload empty hashset from RDB} {
+        r flushdb
+        create_emptyset hashset {1 2}
+        verify_emptyset hashset
+        assert_equal hashtable [r object encoding hashset]
+        save_load_rdb
+        verify_emptyset hashset
+    }
+
+    test {EMTPYSET smove to make empty set} {
+        r flushdb
+        assert_equal 1 [r sadd myset a]
+        assert_equal 1 [r sadd myset2 b]
+        assert_equal 1 [r smove myset myset2 a]
+        assert_equal 2 [r scard myset2]
+        verify_emptyset myset
+        save_load_rdb
+        verify_emptyset myset
+    }
+
+    test {EMTPYSET spop to make empty set} {
+        r flushdb
+        assert_equal 1 [r sadd myset a]
+        assert_equal a [r spop myset]
+        verify_emptyset myset
+        save_load_rdb
+        verify_emptyset myset
+    }
+
+    test {EMTPYSET sunion with empty set} {
+        r flushdb
+        create_emptyset myset {a}
+        verify_emptyset myset
+        assert_equal 1 [r sadd myset2 b]
+        assert_equal b [r sunion myset myset2]
+        save_load_rdb
+        verify_emptyset myset
+    }
+
+    test {EMTPYSET skip loading empty sets when allow-empty-set is no} {
+        r flushdb
+        create_emptyset intset {1}
+        verify_emptyset intset
+        create_emptyset listset {a}
+        verify_emptyset listset
+        create_emptyset hashset {1 2}
+        verify_emptyset hashset
+        set config_override {
+            "allow-empty-set" "no"
+        }
+        save_load_rdb $config_override
+        wait_for_log_messages 0 {"*rdbLoadObject skipping empty key: intset*"} 0 1000 50
+        wait_for_log_messages 0 {"*rdbLoadObject skipping empty key: listset*"} 0 1000 50
+        wait_for_log_messages 0 {"*rdbLoadObject skipping empty key: hashset*"} 0 1000 50
+        wait_for_log_messages 0 {"*Done loading RDB, keys loaded: 0, keys expired: 0, empty keys skipped: 3.*"} 0 1000 50
+
+        assert_equal 0 [r exists intset]
+        assert_equal 0 [r scard intset]
+        assert_equal 0 [r exists listset]
+        assert_equal 0 [r scard listset]
+        assert_equal 0 [r exists hashset]
+        assert_equal 0 [r scard hashset]
+    }
+}
+
 run_solo {set-large-memory} {
 start_server [list overrides [list save ""] ] {
 

valkey.conf

@@ -496,6 +496,31 @@ locale-collate ""
 #
 # extended-redis-compatibility no
 
+# This configuration controls whether Valkey allows the representation of empty sets in the database.
+# By default, when the last element of a set is removed, the key associated with the set is deleted.
+# Enabling this option allows Valkey to retain the key and store an empty set instead.
+#
+# This can be useful for scenarios where an empty set has semantic meaning in the application,
+# such as indicating that a query was executed but returned no results.
+#
+# When this option is enabled:
+# - Empty sets are preserved in the database, and their key will not be automatically deleted.
+# - The `SCARD` command will return 0 for these keys.
+# - These keys will remain in the database until explicitly deleted using the `DEL` command.
+#
+# Note:
+# - This setting may result in additional memory usage due to the retention of keys for empty sets.
+# - Ensure client applications handle empty sets correctly, as their presence differs from the absence of a key.
+# - When loading RDB files, empty sets are skipped by default unless this option is enabled.
+#   This ensures backward compatibility with existing RDB files created in versions where empty
+#   sets were not explicitly supported.
+# - Ensure that this setting is consistently enabled if your application relies on the presence of
+#   empty sets during RDB restores.
+#
+# Default: no
+#
+# allow-empty-set no
+
 ################################ SNAPSHOTTING  ################################
 
 # Save the DB to disk.