From e9529e8d361480fd064b92c58b64752fc228e3d8 Mon Sep 17 00:00:00 2001
From: Aaron Jacobs <jacobsa@google.com>
Date: Fri, 24 Jul 2015 16:18:07 +1000
Subject: [PATCH] Gave mount a makeover.

---
 mount_linux.go | 94 +++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 70 insertions(+), 24 deletions(-)

diff --git a/mount_linux.go b/mount_linux.go
index de7a060..94858bd 100644
--- a/mount_linux.go
+++ b/mount_linux.go
@@ -32,81 +32,127 @@ func lineLogger(wg *sync.WaitGroup, prefix string, r io.ReadCloser) {
 	}
 }
 
-func mount(dir string, conf *mountConfig, ready chan<- struct{}, errp *error) (fusefd *os.File, err error) {
-	// linux mount is never delayed
-	close(ready)
+// Begin the process of mounting at the given directory, returning a connection
+// to the kernel. Mounting continues in the background, and is complete when an
+// error is written to the supplied channel. The file system may need to
+// service the connection in order for mounting to complete.
+func mount(
+	dir string,
+	cfg *MountConfig,
+	ready chan<- error) (dev *os.File, err error) {
+	// On linux, mounting is never delayed.
+	ready <- nil
 
+	// Create a socket pair.
 	fds, err := syscall.Socketpair(syscall.AF_FILE, syscall.SOCK_STREAM, 0)
 	if err != nil {
-		return nil, fmt.Errorf("socketpair error: %v", err)
+		err = fmt.Errorf("Socketpair: %v", err)
+		return
 	}
 
+	// Wrap the sockets into os.File objects that we will pass off to fusermount.
 	writeFile := os.NewFile(uintptr(fds[0]), "fusermount-child-writes")
 	defer writeFile.Close()
 
 	readFile := os.NewFile(uintptr(fds[1]), "fusermount-parent-reads")
 	defer readFile.Close()
 
+	// Start fusermount, passing it pipes for stdout and stderr.
 	cmd := exec.Command(
 		"fusermount",
-		"-o", conf.getOptions(),
+		"-o", cfg.toOptionsString(),
 		"--",
 		dir,
 	)
-	cmd.Env = append(os.Environ(), "_FUSE_COMMFD=3")
 
+	cmd.Env = append(os.Environ(), "_FUSE_COMMFD=3")
 	cmd.ExtraFiles = []*os.File{writeFile}
 
-	var wg sync.WaitGroup
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
-		return nil, fmt.Errorf("setting up fusermount stderr: %v", err)
-	}
-	stderr, err := cmd.StderrPipe()
-	if err != nil {
-		return nil, fmt.Errorf("setting up fusermount stderr: %v", err)
+		err = fmt.Errorf("StdoutPipe: %v", err)
+		return
 	}
 
-	if err := cmd.Start(); err != nil {
-		return nil, fmt.Errorf("fusermount: %v", err)
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		err = fmt.Errorf("StderrPipe: %v", err)
+		return
 	}
+
+	err = cmd.Start()
+	if err != nil {
+		err = fmt.Errorf("Starting fusermount: %v", err)
+		return
+	}
+
+	// Log fusermount output until it closes stdout and stderr.
+	var wg sync.WaitGroup
 	wg.Add(2)
 	go lineLogger(&wg, "mount helper output", stdout)
 	go lineLogger(&wg, "mount helper error", stderr)
 	wg.Wait()
-	if err := cmd.Wait(); err != nil {
-		return nil, fmt.Errorf("fusermount: %v", err)
+
+	// Wait for the command.
+	err = cmd.Wait()
+	if err != nil {
+		err = fmt.Errorf("fusermount: %v", err)
+		return
 	}
 
+	// Wrap the socket file in a connection.
 	c, err := net.FileConn(readFile)
 	if err != nil {
-		return nil, fmt.Errorf("FileConn from fusermount socket: %v", err)
+		err = fmt.Errorf("FileConn: %v", err)
+		return
 	}
 	defer c.Close()
 
+	// We expect to have a Unix domain socket.
 	uc, ok := c.(*net.UnixConn)
 	if !ok {
-		return nil, fmt.Errorf("unexpected FileConn type; expected UnixConn, got %T", c)
+		err = fmt.Errorf("Expected UnixConn, got %T", c)
+		return
 	}
 
+	// Read a message.
 	buf := make([]byte, 32) // expect 1 byte
 	oob := make([]byte, 32) // expect 24 bytes
 	_, oobn, _, _, err := uc.ReadMsgUnix(buf, oob)
+	if err != nil {
+		err = fmt.Errorf("ReadMsgUnix: %v", err)
+		return
+	}
+
+	// Parse the message.
 	scms, err := syscall.ParseSocketControlMessage(oob[:oobn])
 	if err != nil {
-		return nil, fmt.Errorf("ParseSocketControlMessage: %v", err)
+		err = fmt.Errorf("ParseSocketControlMessage: %v", err)
+		return
 	}
+
+	// We expect one message.
 	if len(scms) != 1 {
-		return nil, fmt.Errorf("expected 1 SocketControlMessage; got scms = %#v", scms)
+		err = fmt.Errorf("expected 1 SocketControlMessage; got scms = %#v", scms)
+		return
 	}
+
 	scm := scms[0]
+
+	// Pull out the FD returned by fusermount
 	gotFds, err := syscall.ParseUnixRights(&scm)
 	if err != nil {
-		return nil, fmt.Errorf("syscall.ParseUnixRights: %v", err)
+		err = fmt.Errorf("syscall.ParseUnixRights: %v", err)
+		return
 	}
+
 	if len(gotFds) != 1 {
-		return nil, fmt.Errorf("wanted 1 fd; got %#v", gotFds)
+		err = fmt.Errorf("wanted 1 fd; got %#v", gotFds)
+		return
 	}
-	f := os.NewFile(uintptr(gotFds[0]), "/dev/fuse")
-	return f, nil
+
+	// Turn the FD into an os.File.
+	dev = os.NewFile(uintptr(gotFds[0]), "/dev/fuse")
+
+	return
 }