cmd/buildlet/stage0: close serial port before running buildlet

Jeff thinks the stage0 binary is hogging the COM1 port and preventing
the buildlet from using it.

Updates golang/go#21153

Change-Id: I73e39eeed90269c179818d06864ab1c35ce9fa79
Reviewed-on: https://go-review.googlesource.com/51190
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

cmd/buildlet/stage0/stage0.go

@@ -41,9 +41,13 @@ var (
 	untarDestDir = flag.String("untar-dest-dir", "", "destination directory to untar --untar-file to")
 )
 
-// configureSerialLogOutput is set non-nil on some platforms to configure
-// log output to go to the serial console.
-var configureSerialLogOutput func()
+// configureSerialLogOutput and closeSerialLogOutput are set non-nil
+// on some platforms to configure log output to go to the serial
+// console and to close the serial port, respectively.
+var (
+	configureSerialLogOutput func()
+	closeSerialLogOutput     func()
+)
 
 func main() {
 	if configureSerialLogOutput != nil {
@@ -148,7 +152,16 @@ func main() {
 	case "solaris/amd64":
 		cmd.Args = append(cmd.Args, legacyReverseBuildletArgs("solaris-amd64-smartosbuildlet")...)
 	}
+	// Release the serial port (if we opened it) so the buildlet
+	// process can open & write to it. At least on Windows, only
+	// one process can have it open.
+	if closeSerialLogOutput != nil {
+		closeSerialLogOutput()
+	}
 	if err := cmd.Run(); err != nil {
+		if configureSerialLogOutput != nil {
+			configureSerialLogOutput()
+		}
 		sleepFatalf("Error running buildlet: %v", err)
 	}
 }

cmd/buildlet/stage0/stage0_windows.go

@@ -6,17 +6,22 @@ package main
 
 import (
 	"log"
+	"os"
 
 	"github.com/tarm/serial"
 )
 
 func init() {
 	configureSerialLogOutput = configureSerialLogOutputWindows
+	closeSerialLogOutput = closeSerialLogOutputWindows
 }
 
+var com1 *serial.Port
+
 func configureSerialLogOutputWindows() {
 	c := &serial.Config{Name: "COM1", Baud: 9600}
-	s, err := serial.OpenPort(c)
+	var err error
+	com1, err = serial.OpenPort(c)
 	if err != nil {
 		// Oh well, we tried. This empirically works
 		// on Windows on GCE.
@@ -25,5 +30,13 @@ func configureSerialLogOutputWindows() {
 		log.Printf("serial.OpenPort: %v", err)
 		return
 	}
-	log.SetOutput(s)
+	log.SetOutput(com1)
+}
+
+func closeSerialLogOutputWindows() {
+	if com1 != nil {
+		com1.Close()
+		com1 = nil
+		log.SetOutput(os.Stderr)
+	}
 }