Reduce scope of HELLO implementation (#129)

* Reduce scope of HELLO implementation

Only accept protover=2 as a valid argument as that is
the only thing that is supported at the moment. For all
other arguments degrade to 'unknown command'

The previous implementation creates issues for clients
expecting the presence of the HELLO command to also signal
the presence of RESP3 (as technically the command appeared in
redis at the same time as support for RESP3).

It also did not raise any errors when unsupported (or invalid)
arguments were passed to the command (such as AUTH, SETNAME)

* Fix error in test

* Remove unnecessary context in assertion

* Fix construction of unknown command error string

* Fix casing of error string

src/facade/facade_types.h

@@ -18,6 +18,11 @@ using MutableSlice = absl::Span<char>;
 using CmdArgList = absl::Span<MutableSlice>;
 using CmdArgVec = std::vector<MutableSlice>;
 
+struct CmdArgListFormatter {
+  void operator()(std::string* out, MutableSlice arg) const {
+    out->append(absl::StrCat("`", std::string_view(arg.data(), arg.size()), "`"));
+  }
+};
 
 struct ConnectionStats {
   absl::flat_hash_map<std::string, uint64_t> err_count_map;

src/server/dragonfly_test.cc

@@ -295,17 +295,22 @@ TEST_F(DflyEngineTest, EvalResp) {
 }
 
 TEST_F(DflyEngineTest, Hello) {
-  auto resp_no_param = Run({"hello"});
-  ASSERT_THAT(resp_no_param, ArrLen(12));
-
-  auto resp = Run({"hello", "2"});
+  auto resp = Run({"hello"});
+  ASSERT_THAT(resp, ArrLen(12));
+  resp = Run({"hello", "2"});
   ASSERT_THAT(resp, ArrLen(12));
-  EXPECT_THAT(resp.GetVec(),
-              ElementsAre("server", "redis", "version", ArgType(RespExpr::STRING), "proto",
-                          IntArg(2), "id", ArgType(RespExpr::INT64), "mode",
-                          "standalone", "role", "master"));
 
-  EXPECT_THAT(Run({"hello", "3"}), ErrArg("ERR NOPROTO unsupported protocol"));
+  EXPECT_THAT(resp.GetVec(), ElementsAre("server", "redis", "version", ArgType(RespExpr::STRING),
+                                         "proto", IntArg(2), "id", ArgType(RespExpr::INT64), "mode",
+                                         "standalone", "role", "master"));
+
+  // These are valid arguments to HELLO, however as they are not yet supported the implementation
+  // is degraded to 'unknown command'.
+  EXPECT_THAT(Run({"hello", "3"}),
+              ErrArg("ERR unknown command 'HELLO' with args beginning with: `3`"));
+  EXPECT_THAT(
+      Run({"hello", "2", "AUTH", "uname", "pwd"}),
+      ErrArg("ERR unknown command 'HELLO' with args beginning with: `2`, `AUTH`, `uname`, `pwd`"));
 }
 
 TEST_F(DflyEngineTest, EvalSha) {

src/server/server_family.cc

@@ -92,6 +92,11 @@ string UnknownSubCmd(string_view subcmd, string cmd) {
                       cmd, " HELP.");
 }
 
+string UnknownCmd(string cmd, CmdArgList args) {
+  return absl::StrCat("unknown command '", cmd, "' with args beginning with: ",
+                      StrJoin(args.begin(), args.end(), ", ", CmdArgListFormatter()));
+}
+
 string InferLoadFile(fs::path data_dir) {
   const auto& dbname = GetFlag(FLAGS_dbfilename);
 
@@ -953,11 +958,16 @@ void ServerFamily::Info(CmdArgList args, ConnectionContext* cntx) {
 }
 
 void ServerFamily::Hello(CmdArgList args, ConnectionContext* cntx) {
+  // Allow calling this commands with no arguments or protover=2
+  // technically that is all that is supported at the moment.
+  // For all other cases degrade to 'unknown command' so that clients
+  // checking for the existence of the command to detect if RESP3 is
+  // supported or whether authentication can be performed using HELLO
+  // will gracefully fallback to RESP2 and using the AUTH command explicitly.
   if (args.size() > 1) {
     string_view proto_version = ArgS(args, 1);
-
-    if (proto_version != "2") {
-      (*cntx)->SendError("NOPROTO unsupported protocol version");
+    if (proto_version != "2" || args.size() > 2) {
+      (*cntx)->SendError(UnknownCmd("HELLO", args.subspan(1)));
       return;
     }
   }

src/server/stream_family_test.cc

@@ -24,7 +24,7 @@ TEST_F(StreamFamilyTest, Add) {
   auto resp = Run({"xadd", "key", "*", "field", "value"});
   ASSERT_THAT(resp, ArgType(RespExpr::STRING));
   string id = string(ToSV(resp.GetBuf()));
-  EXPECT_TRUE(id.ends_with("-0")) << id;
+  EXPECT_THAT(id, EndsWith("-0"));
 
   resp = Run({"xrange", "null", "-", "+"});
   EXPECT_THAT(resp, ArrLen(0));