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/repository/key.go

@@ -1,7 +1,6 @@
 package repository
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -250,7 +249,7 @@ func AddKey(ctx context.Context, s *Repository, password string, template *crypt
 		Name: restic.Hash(buf).String(),
 	}
 
-	err = s.be.Save(ctx, h, bytes.NewReader(buf))
+	err = s.be.Save(ctx, h, restic.NewByteReader(buf))
 	if err != nil {
 		return nil, err
 	}

internal/repository/packer_manager.go

@@ -3,7 +3,6 @@ package repository
 import (
 	"context"
 	"crypto/sha256"
-	"io"
 	"os"
 	"sync"
 
@@ -19,7 +18,7 @@ import (
 
 // Saver implements saving data in a backend.
 type Saver interface {
-	Save(context.Context, restic.Handle, io.Reader) error
+	Save(context.Context, restic.Handle, restic.RewindReader) error
 }
 
 // Packer holds a pack.Packer together with a hash writer.
@@ -96,15 +95,15 @@ func (r *Repository) savePacker(ctx context.Context, t restic.BlobType, p *Packe
 		return err
 	}
 
-	_, err = p.tmpfile.Seek(0, 0)
-	if err != nil {
-		return errors.Wrap(err, "Seek")
-	}
-
 	id := restic.IDFromHash(p.hw.Sum(nil))
 	h := restic.Handle{Type: restic.DataFile, Name: id.String()}
 
-	err = r.be.Save(ctx, h, p.tmpfile)
+	rd, err := restic.NewFileReader(p.tmpfile)
+	if err != nil {
+		return err
+	}
+
+	err = r.be.Save(ctx, h, rd)
 	if err != nil {
 		debug.Log("Save(%v) error: %v", h, err)
 		return err

internal/repository/packer_manager_test.go

@@ -50,11 +50,17 @@ func randomID(rd io.Reader) restic.ID {
 
 const maxBlobSize = 1 << 20
 
-func saveFile(t testing.TB, be Saver, f *os.File, id restic.ID) {
+func saveFile(t testing.TB, be Saver, length int, f *os.File, id restic.ID) {
 	h := restic.Handle{Type: restic.DataFile, Name: id.String()}
 	t.Logf("save file %v", h)
 
-	if err := be.Save(context.TODO(), h, f); err != nil {
+	rd, err := restic.NewFileReader(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = be.Save(context.TODO(), h, rd)
+	if err != nil {
 		t.Fatal(err)
 	}
 
@@ -101,12 +107,8 @@ func fillPacks(t testing.TB, rnd *randReader, be Saver, pm *packerManager, buf [
 			t.Fatal(err)
 		}
 
-		if _, err = packer.tmpfile.Seek(0, 0); err != nil {
-			t.Fatal(err)
-		}
-
 		packID := restic.IDFromHash(packer.hw.Sum(nil))
-		saveFile(t, be, packer.tmpfile, packID)
+		saveFile(t, be, int(packer.Size()), packer.tmpfile, packID)
 	}
 
 	return bytes
@@ -122,7 +124,7 @@ func flushRemainingPacks(t testing.TB, rnd *randReader, be Saver, pm *packerMana
 			bytes += int(n)
 
 			packID := restic.IDFromHash(packer.hw.Sum(nil))
-			saveFile(t, be, packer.tmpfile, packID)
+			saveFile(t, be, int(packer.Size()), packer.tmpfile, packID)
 		}
 	}
 
@@ -147,7 +149,7 @@ func BenchmarkPackerManager(t *testing.B) {
 	rnd := newRandReader(rand.NewSource(23))
 
 	be := &mock.Backend{
-		SaveFn: func(context.Context, restic.Handle, io.Reader) error { return nil },
+		SaveFn: func(context.Context, restic.Handle, restic.RewindReader) error { return nil },
 	}
 	blobBuf := make([]byte, maxBlobSize)
 

internal/repository/repository.go

@@ -282,7 +282,7 @@ func (r *Repository) SaveUnpacked(ctx context.Context, t restic.FileType, p []by
 	id = restic.Hash(ciphertext)
 	h := restic.Handle{Type: t, Name: id.String()}
 
-	err = r.be.Save(ctx, h, bytes.NewReader(ciphertext))
+	err = r.be.Save(ctx, h, restic.NewByteReader(ciphertext))
 	if err != nil {
 		debug.Log("error saving blob %v: %v", h, err)
 		return restic.ID{}, err
@@ -456,11 +456,7 @@ func (r *Repository) LoadIndex(ctx context.Context) error {
 		}
 	}
 
-	if err := <-errCh; err != nil {
-		return err
-	}
-
-	return nil
+	return <-errCh
 }
 
 // LoadIndex loads the index id from backend and returns it.