feat: use squashfuse ready notifier if available

When using squasfuse, check whether it supports -o notifypipe.
If it does, then use that instead of our manual checking the
mountpoint inode, which is less reliable.

Co-Developed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Ariel Miculas <amiculas@cisco.com>

pkg/squashfs/squashfs.go

@@ -19,6 +19,7 @@ import (
 	"golang.org/x/sys/unix"
 	"stackerbuild.io/stacker/pkg/log"
 	"stackerbuild.io/stacker/pkg/mount"
+	"github.com/Masterminds/semver/v3"
 )
 
 var checkZstdSupported sync.Once
@@ -205,6 +206,32 @@ func findSquashfusePath() string {
 	return which("squashfuse")
 }
 
+// sqfuseSupportsMountNotification - returns true if squashfuse supports mount
+// notification, false otherwise
+// sqfuse is the path to the squashfuse binary
+func sqfuseSupportsMountNotification(sqfuse string) (bool) {
+    cmd := exec.Command(sqfuse)
+
+    // `squashfuse` always returns an error...  so we ignore it.
+    out, _ := cmd.CombinedOutput()
+
+    first_line := strings.Split(string(out[:]), "\n")[0];
+    version := strings.Split(first_line, " ")[1];
+    v, err := semver.NewVersion(version)
+    if err != nil {
+        return false
+    }
+    // squashfuse notify mechanism was merged in 0.5.0
+    constraint, err := semver.NewConstraint(">= 0.5.0");
+    if err != nil {
+        return false
+    }
+    if constraint.Check(v) {
+        return true
+    }
+    return false
+}
+
 var squashNotFound = errors.Errorf("squashfuse program not found")
 
 // squashFuse - mount squashFile to extractDir
@@ -217,6 +244,22 @@ func squashFuse(squashFile, extractDir string) (*exec.Cmd, error) {
 		return cmd, squashNotFound
 	}
 
+	sqNotify := sqfuseSupportsMountNotification(sqfuse)
+
+	notifyOpts := ""
+	notifyPath := ""
+	if sqNotify {
+		sockdir, err := os.MkdirTemp("", "sock")
+		if err != nil {
+			return cmd, err
+		}
+		notifyPath = filepath.Join(sockdir, "notifypipe")
+		if err := syscall.Mkfifo(notifyPath, 0640); err != nil {
+			return cmd, err
+		}
+		notifyOpts = "notify_pipe=" + notifyPath
+	}
+
 	// given extractDir of path/to/some/dir[/], log to path/to/some/.dir-squashfs.log
 	extractDir = strings.TrimSuffix(extractDir, "/")
 
@@ -240,7 +283,11 @@ func squashFuse(squashFile, extractDir string) (*exec.Cmd, error) {
 	// It would be nice to only enable debug (or maybe to only log to file at all)
 	// if 'stacker --debug', but we do not have access to that info here.
 	// to debug squashfuse, use "allow_other,debug"
-	cmd = exec.Command(sqfuse, "-f", "-o", "allow_other,debug", squashFile, extractDir)
+	optionArgs := "allow_other,debug"
+	if notifyOpts != "" {
+		optionArgs += "," + notifyOpts
+	}
+	cmd = exec.Command(sqfuse, "-f", "-o", optionArgs, squashFile, extractDir)
 	cmd.Stdin = nil
 	cmd.Stdout = cmdOut
 	cmd.Stderr = cmdOut
@@ -260,9 +307,49 @@ func squashFuse(squashFile, extractDir string) (*exec.Cmd, error) {
 	// c. a timeout (timeLimit) was hit
 	startTime := time.Now()
 	timeLimit := 30 * time.Second
+	alarmCh := make(chan struct{})
 	go func() {
 		cmd.Wait()
+		close(alarmCh)
 	}()
+	if sqNotify {
+		notifyCh := make(chan byte)
+		log.Infof("%s supports notify pipe, watching %q", sqfuse, notifyPath)
+		go func() {
+			f, err := os.Open(notifyPath)
+			if err != nil {
+				return
+			}
+			defer f.Close()
+			b1 := make([]byte, 1)
+			for {
+				n1, err := f.Read(b1)
+				if err != nil {
+					return
+				}
+				if err == nil && n1 >= 1 {
+					break
+				}
+			}
+			notifyCh <- b1[0]
+		}()
+		if err != nil {
+			return cmd, errors.Wrapf(err, "Failed reading %q", notifyPath)
+		}
+
+		select {
+		case <-alarmCh:
+			cmd.Process.Kill()
+			return cmd, errors.Wrapf(err, "Gave up on squashFuse mount of %s with %s after %s", squashFile, sqfuse, timeLimit)
+		case ret := <-notifyCh:
+			if ret == 's' {
+				return cmd, nil
+			} else {
+				return cmd, fmt.Errorf("squashfuse returned an error, check %s", logf)
+			}
+		}
+	}
+	log.Infof("%s does not support notify pipe", sqfuse)
 	for count := 0; !fileChanged(fiPre, extractDir); count++ {
 		if cmd.ProcessState != nil {
 			// process exited, the Wait() call in the goroutine above