RESP3 PUSH support for MONITOR command

src/replication.c

@@ -704,7 +704,16 @@ void replicationFeedMonitors(client *c, list *monitors, int dictid, robj **argv,
     listRewind(monitors, &li);
     while ((ln = listNext(&li))) {
         client *monitor = ln->value;
-        addReply(monitor, cmdobj);
+        if (monitor->resp > 2) {
+            struct ClientFlags old_flags = monitor->flag;
+            monitor->flag.pushing = 1;
+            addReplyPushLen(monitor, 2);
+            addReply(monitor, shared.monitorbulk);
+            addReply(monitor, cmdobj);
+            if (!old_flags.pushing) monitor->flag.pushing = 0;
+        } else {
+            addReply(monitor, cmdobj);
+        }
         updateClientMemUsageAndBucket(monitor);
     }
     decrRefCount(cmdobj);

src/server.c

@@ -2035,6 +2035,7 @@ void createSharedObjects(void) {
     shared.ssubscribebulk = createStringObject("$10\r\nssubscribe\r\n", 17);
     shared.sunsubscribebulk = createStringObject("$12\r\nsunsubscribe\r\n", 19);
     shared.smessagebulk = createStringObject("$8\r\nsmessage\r\n", 14);
+    shared.monitorbulk = createStringObject("$7\r\nmonitor\r\n", 13);
     shared.psubscribebulk = createStringObject("$10\r\npsubscribe\r\n", 17);
     shared.punsubscribebulk = createStringObject("$12\r\npunsubscribe\r\n", 19);
 
@@ -6181,8 +6182,11 @@ void monitorCommand(client *c) {
         return;
     }
 
-    /* ignore MONITOR if already replica or in monitor mode */
-    if (c->flag.replica) return;
+    /* Gently notify the client that the monitor command has already been issued. */
+    if (c->flag.replica) {
+        addReplyError(c, "The connection is already in monitoring mode.");
+        return;
+    }
 
     c->flag.replica = 1;
     c->flag.monitor = 1;

src/server.h

@@ -1437,7 +1437,7 @@ struct sharedObjectsStruct {
         *xgroup, *xclaim, *script, *replconf, *eval, *persist, *set, *pexpireat, *pexpire, *time, *pxat, *absttl,
         *retrycount, *force, *justid, *entriesread, *lastid, *ping, *setid, *keepttl, *load, *createconsumer, *getack,
         *special_asterick, *special_equals, *default_username, *redacted, *ssubscribebulk, *sunsubscribebulk,
-        *smessagebulk, *select[PROTO_SHARED_SELECT_CMDS], *integers[OBJ_SHARED_INTEGERS],
+        *smessagebulk, *monitorbulk, *select[PROTO_SHARED_SELECT_CMDS], *integers[OBJ_SHARED_INTEGERS],
         *mbulkhdr[OBJ_SHARED_BULKHDR_LEN], /* "*<value>\r\n" */
         *bulkhdr[OBJ_SHARED_BULKHDR_LEN],  /* "$<value>\r\n" */
         *maphdr[OBJ_SHARED_BULKHDR_LEN],   /* "%<value>\r\n" */

tests/unit/introspection.tcl

@@ -299,6 +299,60 @@ start_server {tags {"introspection"}} {
         set _ $res
     } {*"set" "foo"*"get" "foo"*}
 
+    test {MONITOR should support RESP3 protocol} {
+        set rd [valkey_deferring_client]
+        $rd HELLO 3
+        $rd read ; # Consume the HELLO reply
+
+        $rd monitor
+        $rd read ; # Consume the MONITOR reply
+        $rd readraw 1;
+
+        r set foo bar
+        assert_equal ">2" [$rd read]
+        assert_equal "\$7" [$rd read]
+        assert_equal "monitor" [$rd read]
+        assert_match {*"set"*"foo"*"bar"*} [$rd read]
+
+        $rd close
+    }
+
+    test {multiple MONITOR commands should result in ERR} {
+        set rd [valkey_deferring_client]
+        $rd HELLO 3
+        $rd read ; # Consume the HELLO reply
+
+        $rd readraw 1;
+
+        $rd monitor
+        assert_equal "+OK" [$rd read]
+
+        $rd monitor
+        assert_equal "-ERR The connection is already in monitoring mode." [$rd read]
+
+        $rd close
+    }
+
+    test {MONITOR should came after PONG reply} {
+        set rd [valkey_deferring_client]
+        $rd HELLO 3
+        $rd read ; # Consume the HELLO reply
+
+        $rd monitor
+        $rd read ; # Consume the MONITOR reply
+        $rd readraw 1;
+
+        $rd ping
+
+        assert_equal "+PONG" [$rd read]
+        assert_equal ">2" [$rd read]
+        assert_equal "\$7" [$rd read]
+        assert_equal "monitor" [$rd read]
+        assert_match {*"ping"*} [$rd read]
+
+        $rd close
+    }
+
     test {MONITOR can log commands issued by the scripting engine} {
         set rd [valkey_deferring_client]
         $rd monitor