cmd/dlv: add --client-addr flag to run dap with a predefined client

Documentation/usage/dlv_dap.md

@@ -20,14 +20,19 @@ The server does not yet accept multiple client connections (--accept-multiclient
 While --continue is not supported, stopOnEntry launch/attach attribute can be used to control if
 execution is resumed at the start of the debug session.
 
+The --client-addr flag is a special flag that makes the server initiate a debug session
+by dialing in to the host:port where a DAP client is waiting. This server process
+will exit when the debug session ends.
+
 ```
 dlv dap [flags]
 ```
 
 ### Options
 
 ```
-  -h, --help   help for dap
+      --client-addr string   host:port where the DAP client is waiting for the DAP server to dial in
+  -h, --help                 help for dap
 ```
 
 ### Options inherited from parent commands

cmd/dlv/cmds/commands.go

@@ -62,6 +62,11 @@ var (
 	// disableASLR is used to disable ASLR
 	disableASLR bool
 
+	// dapClientAddr is dap subcommand's flag that specifies the address of a DAP client.
+	// If it is specified, the dap server starts a debug session by dialing to the client.
+	// The dap server will serve only for the debug session.
+	dapClientAddr string
+
 	// backend selection
 	backend string
 
@@ -191,9 +196,15 @@ Program and output binary paths will be interpreted relative to dlv's working di
 
 The server does not yet accept multiple client connections (--accept-multiclient).
 While --continue is not supported, stopOnEntry launch/attach attribute can be used to control if
-execution is resumed at the start of the debug session.`,
+execution is resumed at the start of the debug session.
+
+The --client-addr flag is a special flag that makes the server initiate a debug session
+by dialing in to the host:port where a DAP client is waiting. This server process
+will exit when the debug session ends.`,
 		Run: dapCmd,
 	}
+	dapCommand.Flags().StringVar(&dapClientAddr, "client-addr", "", "host:port where the DAP client is waiting for the DAP server to dial in")
+
 	// TODO(polina): support --tty when dlv dap allows to launch a program from command-line
 	rootCommand.AddCommand(dapCommand)
 
@@ -451,14 +462,8 @@ func dapCmd(cmd *cobra.Command, args []string) {
 			fmt.Fprintf(os.Stderr, "Warning: program flags ignored with dap; specify via launch/attach request instead\n")
 		}
 
-		listener, err := net.Listen("tcp", addr)
-		if err != nil {
-			fmt.Printf("couldn't start listener: %s\n", err)
-			return 1
-		}
 		disconnectChan := make(chan struct{})
-		server := dap.NewServer(&service.Config{
-			Listener:       listener,
+		config := &service.Config{
 			DisconnectChan: disconnectChan,
 			Debugger: debugger.Config{
 				Backend:              backend,
@@ -467,10 +472,31 @@ func dapCmd(cmd *cobra.Command, args []string) {
 				CheckGoVersion:       checkGoVersion,
 			},
 			CheckLocalConnUser: checkLocalConnUser,
-		})
-		defer server.Stop()
+		}
+		var conn net.Conn
+		if dapClientAddr == "" {
+			listener, err := net.Listen("tcp", addr)
+			if err != nil {
+				fmt.Printf("couldn't start listener: %s\n", err)
+				return 1
+			}
+			config.Listener = listener
+		} else { // with a predetermined client.
+			var err error
+			conn, err = net.Dial("tcp", dapClientAddr)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "Failed to connect to the DAP client: %v\n", err)
+				return 1
+			}
+		}
 
-		server.Run()
+		server := dap.NewServer(config)
+		defer server.Stop()
+		if conn == nil {
+			server.Run()
+		} else { // work with a predetermined client.
+			server.RunWithClient(conn)
+		}
 		waitForDisconnectSignal(disconnectChan)
 		return 0
 	}()

cmd/dlv/dlv_test.go

@@ -10,6 +10,7 @@ import (
 	"go/types"
 	"io"
 	"io/ioutil"
+	"net"
 	"os"
 	"os/exec"
 	"os/user"
@@ -690,6 +691,48 @@ func TestDap(t *testing.T) {
 	cmd.Wait()
 }
 
+// TestDapWithClient tests dlv dap --client-addr can be started and shut down.
+func TestDapWithClient(t *testing.T) {
+	listener, err := net.Listen("tcp", ":0")
+	if err != nil {
+		t.Fatalf("cannot setup listener required for testing: %v", err)
+	}
+	defer listener.Close()
+
+	dlvbin, tmpdir := getDlvBin(t)
+	defer os.RemoveAll(tmpdir)
+
+	cmd := exec.Command(dlvbin, "dap", "--log-output=dap", "--log", "--client-addr", listener.Addr().String())
+	buf := &bytes.Buffer{}
+	cmd.Stdin = buf
+	cmd.Stdout = buf
+	assertNoError(cmd.Start(), t, "start dlv dap process with --client-addr flag")
+
+	// Wait for the connection.
+	conn, err := listener.Accept()
+	if err != nil {
+		cmd.Process.Kill() // release the port
+		t.Fatalf("Failed to get connection: %v", err)
+	}
+	t.Log("dlv dap process dialed in successfully")
+
+	client := daptest.NewClientFromConn(conn)
+	client.InitializeRequest()
+	client.ExpectInitializeResponse(t)
+
+	// Close the connection.
+	if err := conn.Close(); err != nil {
+		cmd.Process.Kill()
+		t.Fatalf("Failed to get connection: %v", err)
+	}
+
+	// Connection close should trigger dlv-reverse command's normal exit.
+	if err := cmd.Wait(); err != nil {
+		cmd.Process.Kill()
+		t.Fatalf("command failed: %v\n%s\n%v", err, buf.Bytes(), cmd.Process.Pid)
+	}
+}
+
 func TestTrace(t *testing.T) {
 	dlvbin, tmpdir := getDlvBin(t)
 	defer os.RemoveAll(tmpdir)

service/dap/daptest/client.go

@@ -37,6 +37,12 @@ func NewClient(addr string) *Client {
 	if err != nil {
 		log.Fatal("dialing:", err)
 	}
+	return NewClientFromConn(conn)
+}
+
+// NewClientFromConn creates a new Client with the given TCP connection.
+// Call Close to close the connection.
+func NewClientFromConn(conn net.Conn) *Client {
 	c := &Client{conn: conn, reader: bufio.NewReader(conn)}
 	c.seq = 1 // match VS Code numbering
 	return c

service/dap/server.go

@@ -97,6 +97,7 @@ type Server struct {
 	// config is all the information necessary to start the debugger and server.
 	config *Config
 	// listener is used to accept the client connection.
+	// When working with a predetermined client, this is nil.
 	listener net.Listener
 	// session is the debug session that comes with an client connection.
 	session   *Session
@@ -242,9 +243,17 @@ var (
 // it will be closed by the server when the client fails to connect,
 // disconnects or requests shutdown. Once config.DisconnectChan is closed,
 // Server.Stop() must be called to shutdown this single-user server.
+//
+// NewServer can be used to create a special DAP Server that works
+// only with a predetermined client. In that case, config.Listener is
+// nil and its RunWithClient must be used instead of Run.
 func NewServer(config *service.Config) *Server {
 	logger := logflags.DAPLogger()
-	logflags.WriteDAPListeningMessage(config.Listener.Addr())
+	if config.Listener != nil {
+		logflags.WriteDAPListeningMessage(config.Listener.Addr())
+	} else {
+		logger.Debug("DAP server for a predetermined client")
+	}
 	logger.Debug("DAP server pid = ", os.Getpid())
 	if config.AcceptMulti {
 		logger.Warn("DAP server does not support accept-multiclient mode")
@@ -308,8 +317,11 @@ func (s *Server) Stop() {
 	s.config.log.Debug("DAP server stopping...")
 	defer s.config.log.Debug("DAP server stopped")
 	close(s.config.stopTriggered)
-	// If run goroutine is blocked on accept, this will unblock it.
-	_ = s.listener.Close()
+
+	if s.listener != nil {
+		// If run goroutine is blocked on accept, this will unblock it.
+		_ = s.listener.Close()
+	}
 
 	s.sessionMu.Lock()
 	defer s.sessionMu.Unlock()
@@ -381,6 +393,11 @@ func (c *Config) triggerServerStop() {
 // So if we want to reuse this server for multiple independent debugging sessions
 // we need to take that into consideration.
 func (s *Server) Run() {
+	if s.listener == nil {
+		s.config.log.Fatal("Misconfigured server: no Listener is configured.")
+		return
+	}
+
 	go func() {
 		conn, err := s.listener.Accept() // listener is closed in Stop()
 		if err != nil {
@@ -399,13 +416,30 @@ func (s *Server) Run() {
 				return
 			}
 		}
-		s.sessionMu.Lock()
-		s.session = NewSession(conn, s.config) // closed in Stop()
-		s.sessionMu.Unlock()
-		s.session.serveDAPCodec()
+		s.runSession(conn)
 	}()
 }
 
+func (s *Server) runSession(conn io.ReadWriteCloser) {
+	s.sessionMu.Lock()
+	s.session = NewSession(conn, s.config) // closed in Stop()
+	s.sessionMu.Unlock()
+	s.session.serveDAPCodec()
+}
+
+// RunWithClient is similar to Run but works only with an already established
+// connection instead of waiting on the listener to accept a new client.
+// RunWithClient takes ownership of conn. Debugger won't be started
+// until a launch/attach request is received over the connection.
+func (s *Server) RunWithClient(conn net.Conn) {
+	if s.listener != nil {
+		s.config.log.Fatal("RunWithClient must not be used when the Server is configured with a Listener")
+		return
+	}
+	s.config.log.Debugf("Connected to the client at %s", conn.RemoteAddr())
+	go s.runSession(conn)
+}
+
 // serveDAPCodec reads and decodes requests from the client
 // until it encounters an error or EOF, when it sends
 // a disconnect signal and returns.