backend: Improve Save()

As mentioned in issue [#1560](https://github.com/restic/restic/pull/1560#issuecomment-364689346)
this changes the signature for `backend.Save()`. It now takes a
parameter of interface type `RewindReader`, so that the backend
implementations or our `RetryBackend` middleware can reset the reader to
the beginning and then retry an upload operation.

The `RewindReader` interface also provides a `Length()` method, which is
used in the backend to get the size of the data to be saved. This
removes several ugly hacks we had to do to pull the size back out of the
`io.Reader` passed to `Save()` before. In the `s3` and `rest` backend
this is actively used.

internal/backend/test/benchmarks.go

@@ -14,7 +14,8 @@ func saveRandomFile(t testing.TB, be restic.Backend, length int) ([]byte, restic
 	data := test.Random(23, length)
 	id := restic.Hash(data)
 	handle := restic.Handle{Type: restic.DataFile, Name: id.String()}
-	if err := be.Save(context.TODO(), handle, bytes.NewReader(data)); err != nil {
+	err := be.Save(context.TODO(), handle, restic.NewByteReader(data))
+	if err != nil {
 		t.Fatalf("Save() error: %+v", err)
 	}
 	return data, handle
@@ -148,16 +149,11 @@ func (s *Suite) BenchmarkSave(t *testing.B) {
 	id := restic.Hash(data)
 	handle := restic.Handle{Type: restic.DataFile, Name: id.String()}
 
-	rd := bytes.NewReader(data)
-
+	rd := restic.NewByteReader(data)
 	t.SetBytes(int64(length))
 	t.ResetTimer()
 
 	for i := 0; i < t.N; i++ {
-		if _, err := rd.Seek(0, 0); err != nil {
-			t.Fatal(err)
-		}
-
 		if err := be.Save(context.TODO(), handle, rd); err != nil {
 			t.Fatal(err)
 		}

internal/backend/test/tests.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"reflect"
 	"sort"
-	"strings"
 	"testing"
 	"time"
 
@@ -85,7 +84,7 @@ func (s *Suite) TestConfig(t *testing.T) {
 		t.Fatalf("did not get expected error for non-existing config")
 	}
 
-	err = b.Save(context.TODO(), restic.Handle{Type: restic.ConfigFile}, strings.NewReader(testString))
+	err = b.Save(context.TODO(), restic.Handle{Type: restic.ConfigFile}, restic.NewByteReader([]byte(testString)))
 	if err != nil {
 		t.Fatalf("Save() error: %+v", err)
 	}
@@ -135,7 +134,7 @@ func (s *Suite) TestLoad(t *testing.T) {
 	id := restic.Hash(data)
 
 	handle := restic.Handle{Type: restic.DataFile, Name: id.String()}
-	err = b.Save(context.TODO(), handle, bytes.NewReader(data))
+	err = b.Save(context.TODO(), handle, restic.NewByteReader(data))
 	if err != nil {
 		t.Fatalf("Save() error: %+v", err)
 	}
@@ -250,7 +249,7 @@ func (s *Suite) TestList(t *testing.T) {
 		data := test.Random(rand.Int(), rand.Intn(100)+55)
 		id := restic.Hash(data)
 		h := restic.Handle{Type: restic.DataFile, Name: id.String()}
-		err := b.Save(context.TODO(), h, bytes.NewReader(data))
+		err := b.Save(context.TODO(), h, restic.NewByteReader(data))
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -340,7 +339,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 		data := []byte(fmt.Sprintf("random test blob %v", i))
 		id := restic.Hash(data)
 		h := restic.Handle{Type: restic.DataFile, Name: id.String()}
-		err := b.Save(context.TODO(), h, bytes.NewReader(data))
+		err := b.Save(context.TODO(), h, restic.NewByteReader(data))
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -443,7 +442,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 }
 
 type errorCloser struct {
-	io.Reader
+	io.ReadSeeker
 	l int
 	t testing.TB
 }
@@ -453,10 +452,15 @@ func (ec errorCloser) Close() error {
 	return errors.New("forbidden method close was called")
 }
 
-func (ec errorCloser) Len() int {
+func (ec errorCloser) Length() int {
 	return ec.l
 }
 
+func (ec errorCloser) Rewind() error {
+	_, err := ec.ReadSeeker.Seek(0, io.SeekStart)
+	return err
+}
+
 // TestSave tests saving data in the backend.
 func (s *Suite) TestSave(t *testing.T) {
 	seedRand(t)
@@ -480,7 +484,7 @@ func (s *Suite) TestSave(t *testing.T) {
 			Type: restic.DataFile,
 			Name: fmt.Sprintf("%s-%d", id, i),
 		}
-		err := b.Save(context.TODO(), h, bytes.NewReader(data))
+		err := b.Save(context.TODO(), h, restic.NewByteReader(data))
 		test.OK(t, err)
 
 		buf, err := backend.LoadAll(context.TODO(), b, h)
@@ -532,7 +536,7 @@ func (s *Suite) TestSave(t *testing.T) {
 
 	// wrap the tempfile in an errorCloser, so we can detect if the backend
 	// closes the reader
-	err = b.Save(context.TODO(), h, errorCloser{t: t, l: length, Reader: tmpfile})
+	err = b.Save(context.TODO(), h, errorCloser{t: t, l: length, ReadSeeker: tmpfile})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -542,25 +546,10 @@ func (s *Suite) TestSave(t *testing.T) {
 		t.Fatalf("error removing item: %+v", err)
 	}
 
-	// try again directly with the temp file
-	if _, err = tmpfile.Seek(588, io.SeekStart); err != nil {
-		t.Fatal(err)
-	}
-
-	err = b.Save(context.TODO(), h, tmpfile)
-	if err != nil {
-		t.Fatal(err)
-	}
-
 	if err = tmpfile.Close(); err != nil {
 		t.Fatal(err)
 	}
 
-	err = b.Remove(context.TODO(), h)
-	if err != nil {
-		t.Fatalf("error removing item: %+v", err)
-	}
-
 	if err = os.Remove(tmpfile.Name()); err != nil {
 		t.Fatal(err)
 	}
@@ -585,7 +574,7 @@ func (s *Suite) TestSaveFilenames(t *testing.T) {
 
 	for i, test := range filenameTests {
 		h := restic.Handle{Name: test.name, Type: restic.DataFile}
-		err := b.Save(context.TODO(), h, strings.NewReader(test.data))
+		err := b.Save(context.TODO(), h, restic.NewByteReader([]byte(test.data)))
 		if err != nil {
 			t.Errorf("test %d failed: Save() returned %+v", i, err)
 			continue
@@ -622,7 +611,7 @@ var testStrings = []struct {
 func store(t testing.TB, b restic.Backend, tpe restic.FileType, data []byte) restic.Handle {
 	id := restic.Hash(data)
 	h := restic.Handle{Name: id.String(), Type: tpe}
-	err := b.Save(context.TODO(), h, bytes.NewReader(data))
+	err := b.Save(context.TODO(), h, restic.NewByteReader([]byte(data)))
 	test.OK(t, err)
 	return h
 }
@@ -776,7 +765,7 @@ func (s *Suite) TestBackend(t *testing.T) {
 		test.Assert(t, !ok, "removed blob still present")
 
 		// create blob
-		err = b.Save(context.TODO(), h, strings.NewReader(ts.data))
+		err = b.Save(context.TODO(), h, restic.NewByteReader([]byte(ts.data)))
 		test.OK(t, err)
 
 		// list items