mountinfo.Mounted: optimize by adding fast paths

Add two fast paths to avoid (slow) mountinfo parsing:

1. In case path does not exist, it is definitely NOT a mount point.

2. In case path's st.Dev differs from that of its parent, it is
   definitely a mount point.

In all the other cases (including bind mounts, case of path="/", other
errors from unix.Lstat) the code falls back to the old slow method
of parsing mountinfo.

Signed-off-by: Kir Kolyshkin <[email protected]>

mountinfo/go.mod

@@ -1,3 +1,5 @@
 module github.com/moby/sys/mountinfo
 
 go 1.14
+
+require golang.org/x/sys v0.0.0-20200625212154-ddb9806d33ae

mountinfo/go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sys v0.0.0-20200625212154-ddb9806d33ae h1:Ih9Yo4hSPImZOpfGuA4bR/ORKTAbhZo2AbWNRCnevdo=
+golang.org/x/sys v0.0.0-20200625212154-ddb9806d33ae/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=

mountinfo/mountinfo.go

@@ -16,19 +16,13 @@ func GetMountsFromReader(reader io.Reader, f FilterFunc) ([]*Info, error) {
 	return parseInfoFile(reader, f)
 }
 
-// Mounted determines if a specified mountpoint has been mounted.
-// On Linux it looks at /proc/self/mountinfo.
+// Mounted determines if a specified path is a mount point.
 //
 // The argument must be an absolute path, with all symlinks resolved, and clean.
-// One way to ensure it is to process the path using filepath.EvalSymlinks before
-// calling this function.
-func Mounted(mountpoint string) (bool, error) {
-	entries, err := GetMounts(SingleEntryFilter(mountpoint))
-	if err != nil {
-		return false, err
-	}
-
-	return len(entries) > 0, nil
+// One way to ensure it is to process the path using filepath.Abs followed by
+// filepath.EvalSymlinks before calling this function.
+func Mounted(path string) (bool, error) {
+	return mounted(path)
 }
 
 // Info reveals information about a particular mounted filesystem. This

mountinfo/mountinfo_unix.go

@@ -0,0 +1,35 @@
+// +build !windows
+
+package mountinfo
+
+import (
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+func mounted(path string) (bool, error) {
+	var st unix.Stat_t
+
+	err := unix.Lstat(path, &st)
+	switch err {
+	case unix.ENOENT:
+		// Nonexistent path, so not a mount point.
+		return false, nil
+	case nil:
+		dev := st.Dev
+		err = unix.Lstat(filepath.Dir(path), &st)
+		if err == nil && dev != st.Dev {
+			// Device number differs from that of parent,
+			// so definitely a mount point.
+			return true, nil
+		}
+	}
+
+	entries, err := GetMounts(SingleEntryFilter(path))
+	if err != nil {
+		return false, err
+	}
+
+	return len(entries) > 0, nil
+}

mountinfo/mountinfo_unsupported.go

@@ -8,10 +8,16 @@ import (
 	"runtime"
 )
 
+var errNotImplemented = fmt.Errorf("not implemented on %s/%s", runtime.GOOS, runtime.GOARCH)
+
 func parseMountTable(_ FilterFunc) ([]*Info, error) {
-	return nil, fmt.Errorf("mount.parseMountTable is not implemented on %s/%s", runtime.GOOS, runtime.GOARCH)
+	return nil, errNotImplemented
 }
 
 func parseInfoFile(_ io.Reader, f FilterFunc) ([]*Info, error) {
 	return parseMountTable(f)
 }
+
+func mounted(path string) (bool, error) {
+	return false, errNotImplemented
+}

mountinfo/mountinfo_windows.go

@@ -10,3 +10,7 @@ func parseMountTable(_ FilterFunc) ([]*Info, error) {
 func parseInfoFile(_ io.Reader, f FilterFunc) ([]*Info, error) {
 	return parseMountTable(f)
 }
+
+func mounted(_ string) (bool, error) {
+	return false, nil
+}