Allow some basic client commands before AOF data loads

This commit accepts incoming connections even before the AOF
dataset has been loaded into memory. Though only a very limited
command set is allowed.

Allowed commands:
  PING, ECHO, OUTPUT, QUIT

All other commands will return:
  LOADING Tile38 is loading the dataset in memory

This is useful for establishing connections for the purpose of
checking process and network state.

internal/server/server.go

@@ -94,6 +94,7 @@ type Server struct {
 	lastShrinkDuration aint
 	stopServer         abool
 	outOfMemory        abool
+	loadedAndReady     abool // server is loaded and ready for commands
 
 	connsmu sync.RWMutex
 	conns   map[int]*Client
@@ -245,6 +246,12 @@ func Serve(opts Options) error {
 	}
 	log.Debugf("Multi indexing: RTree (%d points)", server.geomParseOpts.IndexChildren)
 
+	nerr := make(chan error)
+	go func() {
+		// Start the server in the background
+		nerr <- server.netServe()
+	}()
+
 	// Load the queue before the aof
 	qdb, err := buntdb.Open(core.QueueFileName)
 	if err != nil {
@@ -288,7 +295,6 @@ func Serve(opts Options) error {
 			server.aof.Sync()
 		}()
 	}
-	// server.fillExpiresList()
 
 	// Start background routines
 	if server.config.followHost() != "" {
@@ -322,8 +328,9 @@ func Serve(opts Options) error {
 		server.lcond.L.Lock()
 	}()
 
-	// Start the network server
-	return server.netServe()
+	// Server is now loaded and ready. Wait for network error messages.
+	server.loadedAndReady.set(true)
+	return <-nerr
 }
 
 func (server *Server) isProtected() bool {
@@ -426,7 +433,6 @@ func (server *Server) netServe() error {
 				rdbuf := bytes.NewBuffer(packet)
 				pr.rd = rdbuf
 				pr.wr = client
-
 				msgs, err := pr.ReadMessages()
 				for _, msg := range msgs {
 					// Just closing connection if we have deprecated HTTP or WS connection,
@@ -827,12 +833,34 @@ func (server *Server) handleInputCommand(client *Client, msg *Message) error {
 			if errMsg == errInvalidNumberOfArguments.Error() {
 				return writeOutput("-ERR wrong number of arguments for '" + cmd + "' command\r\n")
 			}
-			v, _ := resp.ErrorValue(errors.New("ERR " + errMsg)).MarshalRESP()
+			var ucprefix bool
+			word := strings.Split(errMsg, " ")[0]
+			if len(word) > 0 {
+				ucprefix = true
+				for i := 0; i < len(word); i++ {
+					if word[i] < 'A' || word[i] > 'Z' {
+						ucprefix = false
+						break
+					}
+				}
+			}
+			if !ucprefix {
+				errMsg = "ERR " + errMsg
+			}
+			v, _ := resp.ErrorValue(errors.New(errMsg)).MarshalRESP()
 			return writeOutput(string(v))
 		}
 		return nil
 	}
 
+	if !server.loadedAndReady.on() {
+		switch msg.Command() {
+		case "output", "ping", "echo":
+		default:
+			return writeErr("LOADING Tile38 is loading the dataset in memory")
+		}
+	}
+
 	if cmd == "timeout" {
 		if err := rewriteTimeoutMsg(msg); err != nil {
 			return writeErr(err.Error())