Copy errors & fs packages from the restic repo and fix import paths

Tedious, but I see no better way...

fs/file_linux.go

@@ -0,0 +1,58 @@
+// +build linux,go1.4
+
+package fs
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/zcalusic/restic-server/errors"
+	"golang.org/x/sys/unix"
+)
+
+// Open opens a file for reading, without updating the atime and without caching data on read.
+func Open(name string) (File, error) {
+	file, err := os.OpenFile(name, os.O_RDONLY|syscall.O_NOATIME, 0)
+	if os.IsPermission(errors.Cause(err)) {
+		file, err = os.OpenFile(name, os.O_RDONLY, 0)
+	}
+	return &nonCachingFile{File: file}, err
+}
+
+// nonCachingFile wraps an *os.File and calls fadvise() to instantly forget
+// data that has been read or written.
+type nonCachingFile struct {
+	*os.File
+	readOffset int64
+}
+
+func (f *nonCachingFile) Read(p []byte) (int, error) {
+	n, err := f.File.Read(p)
+
+	if n > 0 {
+		ferr := unix.Fadvise(int(f.File.Fd()), f.readOffset, int64(n), unix.FADV_DONTNEED)
+
+		f.readOffset += int64(n)
+
+		if err == nil {
+			err = ferr
+		}
+	}
+
+	return n, err
+}
+
+// ClearCache syncs and then removes the file's content from the OS cache.
+func ClearCache(file File) error {
+	f, ok := file.(*os.File)
+	if !ok {
+		panic("ClearCache called for file not *os.File")
+	}
+
+	err := f.Sync()
+	if err != nil {
+		return err
+	}
+
+	return unix.Fadvise(int(f.Fd()), 0, 0, unix.FADV_DONTNEED)
+}