snapshotter: use syncfs system call

`sync` system call triggers a full page cache sync which may not always
work, especially in kubernetes environment where it is easy to be
interfered by others. I have seen several cases where a broken nfs mount
is blocking kaniko from doing its job.

With `syncfs`, it only writes cache back to disk for the current
filesystem that is used by kaniko which is supposed to be more reliable.

pkg/snapshot/snapshot.go

@@ -22,6 +22,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"syscall"
 
@@ -155,7 +156,21 @@ func (s *Snapshotter) scanFullFilesystem() ([]string, []string, error) {
 	// for example the hashing function that determines if files are equal uses the mtime of the files,
 	// which can lag if sync is not called. Unfortunately there can still be lag if too much data needs
 	// to be flushed or the disk does its own caching/buffering.
-	syscall.Sync()
+	if runtime.GOOS == "linux" {
+		dir, err := os.Open(s.directory)
+		if err != nil {
+			return nil, nil, err
+		}
+		defer dir.Close()
+		// 306 (linux specific) is the number for sys_syncfs
+		_, _, errno := syscall.Syscall(SYS_SYNCFS, dir.Fd(), 0, 0)
+		if errno != 0 {
+			return nil, nil, errno
+		}
+	} else {
+		// fallback to full page cache sync
+		syscall.Sync()
+	}
 
 	s.l.Snapshot()
 

pkg/snapshot/syncfs.go

@@ -0,0 +1,9 @@
+//go:build !linux
+// +build !linux
+
+package snapshot
+
+const (
+	// don't use it at all
+	SYS_SYNCFS = uintptr(0)
+)

pkg/snapshot/syncfs_linux.go

@@ -0,0 +1,12 @@
+//go:build linux
+// +build linux
+
+package snapshot
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+const (
+	SYS_SYNCFS = unix.SYS_SYNCFS
+)