proc/gdbserial: fix redirect bugs with debugserver

1. Forward stdin/stdout/stderr to the target process when in foreground
   mode instead of always forwarding the current tty (issue #1964)
2. When redirecting a file descriptor make sure to also specify
   something for all three otherwise debugserver will misbehave (either
   exit on launch or run but giving the target process a closed file
   descriptor).

Fixes #1964

_fixtures/cat.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	s := bufio.NewScanner(os.Stdin)
+	for s.Scan() {
+		fmt.Printf("read %q\n", s.Text())
+	}
+	os.Stdout.Close()
+}

cmd/dlv/dlv_test.go

@@ -296,6 +296,38 @@ func TestChildProcessExitWhenNoDebugInfo(t *testing.T) {
 	}
 }
 
+// TestRedirect verifies that redirecting stdin works
+func TestRedirect(t *testing.T) {
+	const listenAddr = "127.0.0.1:40573"
+
+	dlvbin, tmpdir := getDlvBin(t)
+	defer os.RemoveAll(tmpdir)
+
+	catfixture := filepath.Join(protest.FindFixturesDir(), "cat.go")
+	cmd := exec.Command(dlvbin, "debug", "--headless", "--continue", "--accept-multiclient", "--listen", listenAddr, "-r", catfixture, catfixture)
+	stdout, err := cmd.StdoutPipe()
+	assertNoError(err, t, "stderr pipe")
+	defer stdout.Close()
+
+	assertNoError(cmd.Start(), t, "start headless instance")
+
+	scan := bufio.NewScanner(stdout)
+	// wait for the debugger to start
+	for scan.Scan() {
+		t.Log(scan.Text())
+		if scan.Text() == "read \"}\"" {
+			break
+		}
+	}
+
+	// and detach from and kill the headless instance
+	client := rpc2.NewClient(listenAddr)
+	if err := client.Detach(true); err != nil {
+		t.Fatalf("error detaching from headless instance: %v", err)
+	}
+	cmd.Wait()
+}
+
 func checkAutogenDoc(t *testing.T, filename, gencommand string, generated []byte) {
 	saved := slurpFile(t, filepath.Join(projectRoot(), filename))
 

pkg/proc/gdbserial/gdbserver.go

@@ -349,20 +349,13 @@ func LLDBLaunch(cmd []string, wd string, flags proc.LaunchFlags, debugInfoDirs [
 
 	foreground := flags&proc.LaunchForeground != 0
 
-	if foreground {
-		// Disable foregrounding if we can't open /dev/tty or debugserver will
-		// crash. See issue #1215.
-		if !isatty.IsTerminal(os.Stdin.Fd()) {
-			foreground = false
-		}
-	}
-
 	var (
 		isDebugserver bool
 		listener      net.Listener
 		port          string
 		process       *exec.Cmd
 		err           error
+		hasRedirects  bool
 	)
 
 	if debugserverExecutable := getDebugServerAbsolutePath(); debugserverExecutable != "" {
@@ -382,18 +375,15 @@ func LLDBLaunch(cmd []string, wd string, flags proc.LaunchFlags, debugInfoDirs [
 			for i := range redirects {
 				if redirects[i] != "" {
 					found[i] = true
+					hasRedirects = true
 					args = append(args, fmt.Sprintf("--%s-path", names[i]), redirects[i])
 				}
 			}
 
-			if foreground {
-				if !found[0] && !found[1] && !found[2] {
-					args = append(args, "--stdio-path", "/dev/tty")
-				} else {
-					for i := range found {
-						if !found[i] {
-							args = append(args, fmt.Sprintf("--%s-path", names[i]), "/dev/tty")
-						}
+			if foreground || hasRedirects {
+				for i := range found {
+					if !found[i] {
+						args = append(args, fmt.Sprintf("--%s-path", names[i]), "/dev/"+names[i])
 					}
 				}
 			}
@@ -423,19 +413,23 @@ func LLDBLaunch(cmd []string, wd string, flags proc.LaunchFlags, debugInfoDirs [
 		process = commandLogger("lldb-server", args...)
 	}
 
-	if logflags.LLDBServerOutput() || logflags.GdbWire() || foreground {
+	if logflags.LLDBServerOutput() || logflags.GdbWire() || foreground || hasRedirects {
 		process.Stdout = os.Stdout
 		process.Stderr = os.Stderr
 	}
-	if foreground {
-		foregroundSignalsIgnore()
+	if foreground || hasRedirects {
+		if isatty.IsTerminal(os.Stdin.Fd()) {
+			foregroundSignalsIgnore()
+		}
 		process.Stdin = os.Stdin
 	}
 	if wd != "" {
 		process.Dir = wd
 	}
 
-	process.SysProcAttr = sysProcAttr(foreground)
+	if isatty.IsTerminal(os.Stdin.Fd()) {
+		process.SysProcAttr = sysProcAttr(foreground)
+	}
 
 	if runtime.GOOS == "darwin" {
 		process.Env = proc.DisableAsyncPreemptEnv()