Implement CLIENT REPLY ON|OFF|SKIP (#1626)

libs/resources/RespCommandsDocs.json

@@ -1216,6 +1216,43 @@
             ]
           }
         ]
+      },
+      {
+        "Command": "CLIENT_REPLY",
+        "Name": "CLIENT|REPLY",
+        "Summary": "Instructs the server whether to reply to commands.",
+        "Group": "Connection",
+        "Complexity": "O(1)",
+        "Arguments": [
+          {
+            "TypeDiscriminator": "RespCommandContainerArgument",
+            "Name": "ON-OFF-SKIP",
+            "Type": "OneOf",
+            "Arguments": [
+              {
+                "TypeDiscriminator": "RespCommandBasicArgument",
+                "Name": "ON",
+                "DisplayText": "on",
+                "Type": "PureToken",
+                "Token": "ON"
+              },
+              {
+                "TypeDiscriminator": "RespCommandBasicArgument",
+                "Name": "OFF",
+                "DisplayText": "off",
+                "Type": "PureToken",
+                "Token": "OFF"
+              },
+              {
+                "TypeDiscriminator": "RespCommandBasicArgument",
+                "Name": "SKIP",
+                "DisplayText": "skip",
+                "Type": "PureToken",
+                "Token": "SKIP"
+              }
+            ]
+          }
+        ]
       }
     ]
   },

libs/resources/RespCommandsInfo.json

@@ -590,6 +590,13 @@
         "Arity": -3,
         "Flags": "Admin, Loading, NoScript, Stale",
         "AclCategories": "Admin, Connection, Dangerous, Slow"
+      },
+      {
+        "Command": "CLIENT_REPLY",
+        "Name": "CLIENT|REPLY",
+        "Arity": 3,
+        "Flags": "Loading, NoScript, Stale",
+        "AclCategories": "Connection, Slow"
       }
     ]
   },

libs/server/Resp/ClientCommands.cs

@@ -11,6 +11,19 @@
 
 namespace Garnet.server
 {
+    /// <summary>
+    /// Reply suppression mode controlled by the <c>CLIENT REPLY</c> subcommand.
+    /// </summary>
+    internal enum ClientReplyMode : byte
+    {
+        /// <summary>Normal replies are sent (default).</summary>
+        On = 0,
+        /// <summary>All replies are suppressed until a <c>CLIENT REPLY ON</c> is received.</summary>
+        Off,
+        /// <summary>The reply for the next command is suppressed; mode returns to <see cref="On"/> after.</summary>
+        Skip,
+    }
+
     /// <summary>
     /// Server session for RESP protocol - client commands are in this file
     /// </summary>
@@ -640,5 +653,49 @@ private bool NetworkCLIENTUNBLOCK()
 
             return true;
         }
+
+        /// <summary>
+        /// CLIENT REPLY ON|OFF|SKIP — controls per-connection reply suppression.
+        /// OFF suppresses all replies until ON; SKIP suppresses only the next command's reply.
+        /// The OFF and SKIP commands themselves produce no reply; ON replies with +OK.
+        /// </summary>
+        private bool NetworkCLIENTREPLY()
+        {
+            if (parseState.Count != 1)
+            {
+                return AbortWithWrongNumberOfArguments("client|reply");
+            }
+
+            var modeSpan = parseState.GetArgSliceByRef(0).ReadOnlySpan;
+
+            if (modeSpan.EqualsUpperCaseSpanIgnoringCase(CmdStrings.ON))
+            {
+                clientReplyMode = ClientReplyMode.On;
+                // The ON command itself must reply +OK even if we just transitioned out of OFF/SKIP.
+                // Clear the suppression flag set at command-start so the +OK actually flushes.
+                suppressCurrentReply = false;
+                while (!RespWriteUtils.TryWriteDirect(CmdStrings.RESP_OK, ref dcurr, dend))
+                    SendAndReset();
+            }
+            else if (modeSpan.EqualsUpperCaseSpanIgnoringCase(CmdStrings.OFF))
+            {
+                clientReplyMode = ClientReplyMode.Off;
+                // No reply.
+            }
+            else if (modeSpan.EqualsUpperCaseSpanIgnoringCase(CmdStrings.SKIP))
+            {
+                // SKIP only arms when we're currently On. If already Off it stays Off; if already Skip it stays Skip
+                // (a second SKIP just re-arms — it does not stack).
+                if (clientReplyMode == ClientReplyMode.On)
+                    clientReplyMode = ClientReplyMode.Skip;
+                // No reply.
+            }
+            else
+            {
+                return AbortWithErrorMessage(CmdStrings.RESP_SYNTAX_ERROR);
+            }
+
+            return true;
+        }
     }
 }

libs/server/Resp/CmdStrings.cs

@@ -429,6 +429,8 @@ static partial class CmdStrings
         public static ReadOnlySpan<byte> GETNAME => "GETNAME"u8;
         public static ReadOnlySpan<byte> SETINFO => "SETINFO"u8;
         public static ReadOnlySpan<byte> UNBLOCK => "UNBLOCK"u8;
+        public static ReadOnlySpan<byte> REPLY => "REPLY"u8;
+        public static ReadOnlySpan<byte> SKIP => "SKIP"u8;
         public static ReadOnlySpan<byte> USER => "USER"u8;
         public static ReadOnlySpan<byte> ADDR => "ADDR"u8;
         public static ReadOnlySpan<byte> LADDR => "LADDR"u8;

libs/server/Resp/Parser/RespCommand.cs

@@ -282,6 +282,7 @@ public enum RespCommand : ushort
         CLIENT_SETNAME,
         CLIENT_SETINFO,
         CLIENT_UNBLOCK,
+        CLIENT_REPLY,
 
         MONITOR,
         MODULE,
@@ -465,6 +466,7 @@ public static class RespCommandExtensions
             RespCommand.CLIENT_SETNAME,
             RespCommand.CLIENT_SETINFO,
             RespCommand.CLIENT_UNBLOCK,
+            RespCommand.CLIENT_REPLY,
             // Command
             RespCommand.COMMAND,
             RespCommand.COMMAND_COUNT,
@@ -2125,6 +2127,10 @@ private RespCommand SlowParseCommand(ReadOnlySpan<byte> command, ref int count,
                 {
                     return RespCommand.CLIENT_UNBLOCK;
                 }
+                else if (subCommand.SequenceEqual(CmdStrings.REPLY))
+                {
+                    return RespCommand.CLIENT_REPLY;
+                }
 
                 string errMsg = string.Format(CmdStrings.GenericErrUnknownSubCommandNoHelp,
                                               Encoding.UTF8.GetString(subCommand),

libs/server/Resp/RespServerSession.cs

@@ -207,6 +207,18 @@ public IGarnetServer Server
         /// </summary>
         string clientLibVersion = null;
 
+        /// <summary>
+        /// Current CLIENT REPLY mode for this connection (controls reply suppression).
+        /// </summary>
+        internal ClientReplyMode clientReplyMode = ClientReplyMode.On;
+
+        /// <summary>
+        /// True while the current command's output should be discarded (i.e. CLIENT REPLY OFF/SKIP is active for this command).
+        /// Set at the start of each command in ProcessMessages and may be cleared by NetworkCLIENTREPLY when it processes ON
+        /// (so the +OK reply for `CLIENT REPLY ON` is allowed through).
+        /// </summary>
+        bool suppressCurrentReply = false;
+
         /// <summary>
         /// Flag indicating whether any of the commands in one message
         /// requires us to block on AOF before sending response over the network
@@ -623,6 +635,18 @@ private void ProcessMessages<TBasicApi, TTxnApi>(ref TBasicApi basicApi, ref TTx
 
             while (bytesRead - readHead >= 4)
             {
+                // CLIENT REPLY: snapshot reply-suppression state for this command BEFORE parsing.
+                // - modeAtStart drives whether the buffer position we capture now (cmdStartPtr) gets
+                //   rewound after the command runs (discarding the reply bytes).
+                // - suppressCurrentReply also gates SendAndReset() so any mid-command flush is
+                //   discarded instead of being sent to the network. We set it before ParseCommand so
+                //   parse-time error replies (e.g. "unknown command") are also gated.
+                // CLIENT REPLY ON clears suppressCurrentReply inside its handler so the +OK reply
+                // for the ON command itself is allowed through.
+                var modeAtStart = clientReplyMode;
+                var cmdStartPtr = dcurr;
+                suppressCurrentReply = modeAtStart != ClientReplyMode.On;
+
                 // First, parse the command, making sure we have the entire command available
                 // We use endReadHead to track the end of the current command
                 // On success, readHead is left at the start of the command payload for legacy operators
@@ -632,6 +656,7 @@ private void ProcessMessages<TBasicApi, TTxnApi>(ref TBasicApi basicApi, ref TTx
                 if (!commandReceived)
                 {
                     endReadHead = readHead = _origReadHead;
+                    suppressCurrentReply = false;
                     break;
                 }
 
@@ -695,6 +720,27 @@ private void ProcessMessages<TBasicApi, TTxnApi>(ref TBasicApi basicApi, ref TTx
                     containsSlowCommand = true;
                 }
 
+                // CLIENT REPLY: end-of-command suppression handling. Applies to INVALID/parse-error
+                // commands as well so things like "GET" with no args don't leak "-ERR unknown command".
+                // suppressCurrentReply may have been cleared by NetworkCLIENTREPLY when it
+                // executed `CLIENT REPLY ON` (so the +OK reply is preserved).
+                if (suppressCurrentReply)
+                {
+                    // Discard anything this command wrote (mid-command SendAndReset has already
+                    // been gated, so the bytes still live within the current buffer between
+                    // cmdStartPtr and dcurr).
+                    dcurr = cmdStartPtr;
+                }
+                suppressCurrentReply = false;
+
+                // Burn off a one-shot SKIP. CLIENT REPLY commands themselves never burn the skip —
+                // a SKIP issued while already in Skip mode just re-arms (does not stack), and a
+                // following SKIP/ON/OFF transitions the mode directly inside the handler.
+                if (modeAtStart == ClientReplyMode.Skip && cmd != RespCommand.CLIENT_REPLY && cmd != RespCommand.INVALID)
+                {
+                    clientReplyMode = ClientReplyMode.On;
+                }
+
                 // Advance read head variables to process the next command
                 _origReadHead = readHead = endReadHead;
 
@@ -1050,6 +1096,7 @@ private bool ProcessOtherCommands<TGarnetApi>(RespCommand command, ref TGarnetAp
                 RespCommand.CLIENT_SETNAME => NetworkCLIENTSETNAME(),
                 RespCommand.CLIENT_SETINFO => NetworkCLIENTSETINFO(),
                 RespCommand.CLIENT_UNBLOCK => NetworkCLIENTUNBLOCK(),
+                RespCommand.CLIENT_REPLY => NetworkCLIENTREPLY(),
                 RespCommand.COMMAND => NetworkCOMMAND(),
                 RespCommand.COMMAND_COUNT => NetworkCOMMAND_COUNT(),
                 RespCommand.COMMAND_DOCS => NetworkCOMMAND_DOCS(),
@@ -1318,6 +1365,15 @@ private unsafe bool Write(int seqNo, ref byte* dst, int length)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal void SendAndReset()
         {
+            // CLIENT REPLY OFF/SKIP: discard whatever was written into the current buffer
+            // without sending it. We deliberately do NOT rotate to a fresh buffer or call Send —
+            // we just rewind dcurr to the head so the suppressed bytes are dropped.
+            if (suppressCurrentReply)
+            {
+                dcurr = networkSender.GetResponseObjectHead();
+                return;
+            }
+
             byte* d = networkSender.GetResponseObjectHead();
             if ((int)(dcurr - d) > 0)
             {

playground/CommandInfoUpdater/SupportedCommand.cs

@@ -54,6 +54,7 @@ public class SupportedCommand
                 new("CLIENT|SETNAME", RespCommand.CLIENT_SETNAME),
                 new("CLIENT|SETINFO", RespCommand.CLIENT_SETINFO),
                 new("CLIENT|UNBLOCK", RespCommand.CLIENT_UNBLOCK),
+                new("CLIENT|REPLY", RespCommand.CLIENT_REPLY),
             ]),
             new("CLUSTER", RespCommand.CLUSTER, StoreType.None,
             [