fs: add Fingerprint to detect changes in an object

fs/fingerprint.go

@@ -0,0 +1,54 @@
+package fs
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/rclone/rclone/fs/hash"
+)
+
+// Fingerprint produces a unique-ish string for an object.
+//
+// This is for detecting whether an object has changed since we last
+// saw it, not for checking object identity between two different
+// remotes - operations.Equal should be used for that.
+//
+// If fast is set then Fingerprint will only include attributes where
+// usually another operation is not required to fetch them. For
+// example if fast is set then this won't include hashes on the local
+// backend.
+func Fingerprint(ctx context.Context, o ObjectInfo, fast bool) string {
+	var (
+		out      strings.Builder
+		f        = o.Fs()
+		features = f.Features()
+	)
+	fmt.Fprintf(&out, "%d", o.Size())
+	// Whether we want to do a slow operation or not
+	//
+	//  fast     true  false true  false
+	//  opIsSlow true  true  false false
+	//  do Op    false true  true  true
+	//
+	// If !fast (slow) do the operation or if !OpIsSlow ==
+	// OpIsFast do the operation.
+	//
+	// Eg don't do this for S3 where modtimes are expensive
+	if !fast || !features.SlowModTime {
+		if f.Precision() != ModTimeNotSupported {
+			fmt.Fprintf(&out, ",%v", o.ModTime(ctx).UTC())
+		}
+	}
+	// Eg don't do this for SFTP/local where hashes are expensive?
+	if !fast || !features.SlowHash {
+		hashType := f.Hashes().GetOne()
+		if hashType != hash.None {
+			hash, err := o.Hash(ctx, hashType)
+			if err == nil {
+				fmt.Fprintf(&out, ",%v", hash)
+			}
+		}
+	}
+	return out.String()
+}