sshbytestream & IOCommand: fix handling of dead child process

SSH catches SIGTERM, tears down its connection, then exits with
platform-specific exit code.

sshbytestream/ssh.go

@@ -2,9 +2,12 @@ package sshbytestream
 
 import (
 	"fmt"
-	"github.com/zrepl/zrepl/util"
 	"io"
 	"os"
+	"runtime"
+	"syscall"
+
+	"github.com/zrepl/zrepl/util"
 )
 
 type Error struct {
@@ -53,7 +56,11 @@ func (f IncomingReadWriteCloser) Close() (err error) {
 	return
 }
 
-func Outgoing(remote SSHTransport) (c *util.IOCommand, err error) {
+type OutgoingSSHByteStream struct {
+	c *util.IOCommand
+}
+
+func Outgoing(remote SSHTransport) (s OutgoingSSHByteStream, err error) {
 
 	sshArgs := make([]string, 0, 2*len(remote.Options)+4)
 	sshArgs = append(sshArgs,
@@ -73,13 +80,44 @@ func Outgoing(remote SSHTransport) (c *util.IOCommand, err error) {
 		sshCommand = SSHCommand
 	}
 
-	if c, err = util.NewIOCommand(sshCommand, sshArgs, util.IOCommandStderrBufSize); err != nil {
+	if s.c, err = util.NewIOCommand(sshCommand, sshArgs, util.IOCommandStderrBufSize); err != nil {
 		return
 	}
 
 	// Clear environment of cmd, ssh shall not rely on SSH_AUTH_SOCK, etc.
-	c.Cmd.Env = []string{}
+	s.c.Cmd.Env = []string{}
+
+	err = s.c.Start()
+	return
+}
+
+func (s OutgoingSSHByteStream) Read(p []byte) (n int, err error) {
+	return s.c.Read(p)
+}
+
+func (s OutgoingSSHByteStream) Write(p []byte) (n int, err error) {
+	return s.c.Write(p)
+}
+
+func (s OutgoingSSHByteStream) Close() (err error) {
+	err = s.c.Close()
+	if err == nil || s.c.ExitResult == nil {
+		return
+	}
+
+	// SSH catches SIGTERM and has different exit codes on different platforms
+	ws := s.c.ExitResult.WaitStatus
+	switch runtime.GOOS {
+	case "linux":
+		if ws.ExitStatus() == 128+int(syscall.SIGTERM) { // OpenSSH_7.5p1, OpenSSL 1.1.0f  25 May 2017 Arch Linux
+			err = nil
+		}
+	case "freebsd": // OpenSSH_7.2p2, OpenSSL 1.0.2k-freebsd  26 Jan 2017
+		if ws.ExitStatus() == 255 {
+			err = nil
+		}
+	default: // TODO
+	}
 
-	err = c.Start()
 	return
 }