exp/ssh: simplify Stdin/out/errPipe methods

If a Pipe method is called, return the underlying
reader/writer from session.clientChan, bypassing the
io.Copy and io.Pipe harness.

StdoutPipe and StderrPipe now return an io.Reader not
an io.ReadCloser as SSH cannot signal the close of the
local reader to the remote process.

R=rsc, agl, gustav.paul, cw
CC=golang-dev
https://golang.org/cl/5493047

src/pkg/exp/ssh/session.go

@@ -68,10 +68,12 @@ type Session struct {
 
 	*clientChan // the channel backing this session
 
-	started        bool // true once Start, Run or Shell is invoked.
-	closeAfterWait []io.Closer
-	copyFuncs      []func() error
-	errch          chan error // one send per copyFunc
+	started   bool // true once Start, Run or Shell is invoked.
+	copyFuncs []func() error
+	errch     chan error // one send per copyFunc
+
+	// true if pipe method is active
+	stdinpipe, stdoutpipe, stderrpipe bool
 }
 
 // RFC 4254 Section 6.4.
@@ -237,11 +239,9 @@ func (s *Session) waitForResponse() error {
 func (s *Session) start() error {
 	s.started = true
 
-	type F func(*Session) error
+	type F func(*Session)
 	for _, setupFd := range []F{(*Session).stdin, (*Session).stdout, (*Session).stderr} {
-		if err := setupFd(s); err != nil {
-			return err
-		}
+		setupFd(s)
 	}
 
 	s.errch = make(chan error, len(s.copyFuncs))
@@ -274,9 +274,6 @@ func (s *Session) Wait() error {
 			copyError = err
 		}
 	}
-	for _, fd := range s.closeAfterWait {
-		fd.Close()
-	}
 	if waitErr != nil {
 		return waitErr
 	}
@@ -341,7 +338,10 @@ func (s *Session) wait() error {
 	return &ExitError{wm}
 }
 
-func (s *Session) stdin() error {
+func (s *Session) stdin() {
+	if s.stdinpipe {
+		return
+	}
 	if s.Stdin == nil {
 		s.Stdin = new(bytes.Buffer)
 	}
@@ -352,10 +352,12 @@ func (s *Session) stdin() error {
 		}
 		return err
 	})
-	return nil
 }
 
-func (s *Session) stdout() error {
+func (s *Session) stdout() {
+	if s.stdoutpipe {
+		return
+	}
 	if s.Stdout == nil {
 		s.Stdout = ioutil.Discard
 	}
@@ -363,10 +365,12 @@ func (s *Session) stdout() error {
 		_, err := io.Copy(s.Stdout, s.clientChan.stdout)
 		return err
 	})
-	return nil
 }
 
-func (s *Session) stderr() error {
+func (s *Session) stderr() {
+	if s.stderrpipe {
+		return
+	}
 	if s.Stderr == nil {
 		s.Stderr = ioutil.Discard
 	}
@@ -374,7 +378,6 @@ func (s *Session) stderr() error {
 		_, err := io.Copy(s.Stderr, s.clientChan.stderr)
 		return err
 	})
-	return nil
 }
 
 // StdinPipe returns a pipe that will be connected to the
@@ -386,10 +389,8 @@ func (s *Session) StdinPipe() (io.WriteCloser, error) {
 	if s.started {
 		return nil, errors.New("ssh: StdinPipe after process started")
 	}
-	pr, pw := io.Pipe()
-	s.Stdin = pr
-	s.closeAfterWait = append(s.closeAfterWait, pr)
-	return pw, nil
+	s.stdinpipe = true
+	return s.clientChan.stdin, nil
 }
 
 // StdoutPipe returns a pipe that will be connected to the
@@ -398,17 +399,15 @@ func (s *Session) StdinPipe() (io.WriteCloser, error) {
 // stdout and stderr streams. If the StdoutPipe reader is
 // not serviced fast enought it may eventually cause the
 // remote command to block.
-func (s *Session) StdoutPipe() (io.ReadCloser, error) {
+func (s *Session) StdoutPipe() (io.Reader, error) {
 	if s.Stdout != nil {
 		return nil, errors.New("ssh: Stdout already set")
 	}
 	if s.started {
 		return nil, errors.New("ssh: StdoutPipe after process started")
 	}
-	pr, pw := io.Pipe()
-	s.Stdout = pw
-	s.closeAfterWait = append(s.closeAfterWait, pw)
-	return pr, nil
+	s.stdoutpipe = true
+	return s.clientChan.stdout, nil
 }
 
 // StderrPipe returns a pipe that will be connected to the
@@ -417,17 +416,15 @@ func (s *Session) StdoutPipe() (io.ReadCloser, error) {
 // stdout and stderr streams. If the StderrPipe reader is
 // not serviced fast enought it may eventually cause the
 // remote command to block.
-func (s *Session) StderrPipe() (io.ReadCloser, error) {
+func (s *Session) StderrPipe() (io.Reader, error) {
 	if s.Stderr != nil {
 		return nil, errors.New("ssh: Stderr already set")
 	}
 	if s.started {
 		return nil, errors.New("ssh: StderrPipe after process started")
 	}
-	pr, pw := io.Pipe()
-	s.Stderr = pw
-	s.closeAfterWait = append(s.closeAfterWait, pw)
-	return pr, nil
+	s.stderrpipe = true
+	return s.clientChan.stderr, nil
 }
 
 // TODO(dfc) add Output and CombinedOutput helpers