main: redirectStdFds: keep logger from holding stdout open

We passed our stdout and stderr to the new logger instance,
which makes sense to see any error message, but also means that
the fd is kept open even when we close it.

Fixes the new TestMountBackground test and
https://github.com/rfjakob/gocryptfs/issues/130 .

daemonize.go

@@ -56,7 +56,9 @@ func forkChild() int {
 
 // redirectStdFds redirects stderr and stdout to syslog; stdin to /dev/null
 func redirectStdFds() {
-	// stderr and stdout
+	// Create a pipe pair "pw" -> "pr" and start logger reading from "pr".
+	// We do it ourselves instead of using StdinPipe() because we need access
+	// to the fd numbers.
 	pr, pw, err := os.Pipe()
 	if err != nil {
 		tlog.Warn.Printf("redirectStdFds: could not create pipe: %v\n", err)
@@ -64,14 +66,15 @@ func redirectStdFds() {
 	}
 	tag := fmt.Sprintf("gocryptfs-%d-logger", os.Getpid())
 	cmd := exec.Command("logger", "-t", tag)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
 	cmd.Stdin = pr
 	err = cmd.Start()
 	if err != nil {
 		tlog.Warn.Printf("redirectStdFds: could not start logger: %v\n", err)
+		return
 	}
+	// The logger now reads on "pr". We can close it.
 	pr.Close()
+	// Redirect stout and stderr to "pw".
 	err = syscallcompat.Dup3(int(pw.Fd()), 1, 0)
 	if err != nil {
 		tlog.Warn.Printf("redirectStdFds: stdout dup error: %v\n", err)
@@ -80,9 +83,9 @@ func redirectStdFds() {
 	if err != nil {
 		tlog.Warn.Printf("redirectStdFds: stderr dup error: %v\n", err)
 	}
+	// Our stout and stderr point to "pw". We can close the extra copy.
 	pw.Close()
-
-	// stdin
+	// Redirect stdin to /dev/null
 	nullFd, err := os.Open("/dev/null")
 	if err != nil {
 		tlog.Warn.Printf("redirectStdFds: could not open /dev/null: %v\n", err)