[#63] Simplify multiObjectReader and add tests

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/layer/multi_object_reader_test.go

@@ -0,0 +1,127 @@
+package layer
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"testing"
+
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+	oidtest "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id/test"
+	"github.com/stretchr/testify/require"
+)
+
+type readerInitiatorMock struct {
+	parts map[oid.ID][]byte
+}
+
+func (r *readerInitiatorMock) initFrostFSObjectPayloadReader(_ context.Context, p getFrostFSParams) (io.Reader, error) {
+	partPayload, ok := r.parts[p.oid]
+	if !ok {
+		return nil, errors.New("part not found")
+	}
+
+	if p.off+p.ln == 0 {
+		return bytes.NewReader(partPayload), nil
+	}
+
+	if p.off > uint64(len(partPayload)-1) {
+		return nil, fmt.Errorf("invalid offset: %d/%d", p.off, len(partPayload))
+	}
+
+	if p.off+p.ln > uint64(len(partPayload)) {
+		return nil, fmt.Errorf("invalid range: %d-%d/%d", p.off, p.off+p.ln, len(partPayload))
+	}
+
+	return bytes.NewReader(partPayload[p.off : p.off+p.ln]), nil
+}
+
+func prepareDataReader() ([]byte, []partObj, *readerInitiatorMock) {
+	mockInitReader := &readerInitiatorMock{
+		parts: map[oid.ID][]byte{
+			oidtest.ID(): []byte("first part 1"),
+			oidtest.ID(): []byte("second part 2"),
+			oidtest.ID(): []byte("third part 3"),
+		},
+	}
+
+	var fullPayload []byte
+	parts := make([]partObj, 0, len(mockInitReader.parts))
+	for id, payload := range mockInitReader.parts {
+		parts = append(parts, partObj{OID: id, Size: uint64(len(payload))})
+		fullPayload = append(fullPayload, payload...)
+	}
+
+	return fullPayload, parts, mockInitReader
+}
+
+func TestMultiReader(t *testing.T) {
+	ctx := context.Background()
+
+	fullPayload, parts, mockInitReader := prepareDataReader()
+
+	for _, tc := range []struct {
+		name string
+		off  uint64
+		ln   uint64
+		err  error
+	}{
+		{
+			name: "simple read all",
+		},
+		{
+			name: "simple read with length",
+			ln:   uint64(len(fullPayload)),
+		},
+		{
+			name: "middle of parts",
+			off:  parts[0].Size + 2,
+			ln:   4,
+		},
+		{
+			name: "first and second",
+			off:  parts[0].Size - 4,
+			ln:   8,
+		},
+		{
+			name: "first and third",
+			off:  parts[0].Size - 4,
+			ln:   parts[1].Size + 8,
+		},
+		{
+			name: "offset out of range",
+			off:  uint64(len(fullPayload) + 1),
+			ln:   1,
+			err:  errOffsetIsOutOfRange,
+		},
+		{
+			name: "zero length",
+			off:  parts[1].Size + 1,
+			ln:   0,
+			err:  errorZeroRangeLength,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			multiReader, err := newMultiObjectReader(ctx, multiObjectReaderConfig{
+				layer: mockInitReader,
+				parts: parts,
+				off:   tc.off,
+				ln:    tc.ln,
+			})
+			require.ErrorIs(t, err, tc.err)
+
+			if tc.err == nil {
+				off := tc.off
+				ln := tc.ln
+				if off+ln == 0 {
+					ln = uint64(len(fullPayload))
+				}
+				data, err := io.ReadAll(multiReader)
+				require.NoError(t, err)
+				require.Equal(t, fullPayload[off:off+ln], data)
+			}
+		})
+	}
+}