backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/defaults_test.go

@@ -0,0 +1,64 @@
+package util_test
+
+import (
+	"context"
+	"io"
+	"testing"
+
+	"github.com/restic/restic/internal/backend/util"
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+type mockReader struct {
+	closed bool
+}
+
+func (rd *mockReader) Read(_ []byte) (n int, err error) {
+	return 0, nil
+}
+func (rd *mockReader) Close() error {
+	rd.closed = true
+	return nil
+}
+
+func TestDefaultLoad(t *testing.T) {
+
+	h := restic.Handle{Name: "id", Type: restic.PackFile}
+	rd := &mockReader{}
+
+	// happy case, assert correct parameters are passed around and content stream is closed
+	err := util.DefaultLoad(context.TODO(), h, 10, 11, func(ctx context.Context, ih restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+		rtest.Equals(t, h, ih)
+		rtest.Equals(t, int(10), length)
+		rtest.Equals(t, int64(11), offset)
+
+		return rd, nil
+	}, func(ird io.Reader) error {
+		rtest.Equals(t, rd, ird)
+		return nil
+	})
+	rtest.OK(t, err)
+	rtest.Equals(t, true, rd.closed)
+
+	// unhappy case, assert producer errors are handled correctly
+	err = util.DefaultLoad(context.TODO(), h, 10, 11, func(ctx context.Context, ih restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+		return nil, errors.Errorf("producer error")
+	}, func(ird io.Reader) error {
+		t.Fatalf("unexpected consumer invocation")
+		return nil
+	})
+	rtest.Equals(t, "producer error", err.Error())
+
+	// unhappy case, assert consumer errors are handled correctly
+	rd = &mockReader{}
+	err = util.DefaultLoad(context.TODO(), h, 10, 11, func(ctx context.Context, ih restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+		return rd, nil
+	}, func(ird io.Reader) error {
+		return errors.Errorf("consumer error")
+	})
+	rtest.Equals(t, true, rd.closed)
+	rtest.Equals(t, "consumer error", err.Error())
+}