lib/readers: add FakeSeeker to adapt io.Reader to io.ReadSeeker #5422

lib/readers/fakeseeker.go

@@ -0,0 +1,72 @@
+package readers
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+// FakeSeeker adapts an io.Seeker into an io.ReadSeeker
+type FakeSeeker struct {
+	in      io.Reader
+	readErr error
+	length  int64
+	offset  int64
+	read    bool
+}
+
+// NewFakeSeeker creates a fake io.ReadSeeker from an io.Reader
+//
+// This can be seeked before reading to discover the length passed in.
+func NewFakeSeeker(in io.Reader, length int64) io.ReadSeeker {
+	if rs, ok := in.(io.ReadSeeker); ok {
+		return rs
+	}
+	return &FakeSeeker{
+		in:     in,
+		length: length,
+	}
+}
+
+// Seek the stream - possible only before reading
+func (r *FakeSeeker) Seek(offset int64, whence int) (abs int64, err error) {
+	if r.readErr != nil {
+		return 0, r.readErr
+	}
+	if r.read {
+		return 0, fmt.Errorf("FakeSeeker: can't Seek(%d, %d) after reading", offset, whence)
+	}
+	switch whence {
+	case io.SeekStart:
+		abs = offset
+	case io.SeekCurrent:
+		abs = r.offset + offset
+	case io.SeekEnd:
+		abs = r.length + offset
+	default:
+		return 0, errors.New("FakeSeeker: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("FakeSeeker: negative position")
+	}
+	r.offset = abs
+	return abs, nil
+}
+
+// Read data from the stream. Will give an error if seeked.
+func (r *FakeSeeker) Read(p []byte) (n int, err error) {
+	if r.readErr != nil {
+		return 0, r.readErr
+	}
+	if !r.read && r.offset != 0 {
+		return 0, errors.New("FakeSeeker: not at start: can't read")
+	}
+	n, err = r.in.Read(p)
+	if n != 0 {
+		r.read = true
+	}
+	if err != nil {
+		r.readErr = err
+	}
+	return n, err
+}

lib/readers/fakeseeker_test.go

@@ -0,0 +1,87 @@
+package readers
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Check interface
+var _ io.ReadSeeker = &FakeSeeker{}
+
+func TestFakeSeeker(t *testing.T) {
+	// Test that passing in an io.ReadSeeker just passes it through
+	bufReader := bytes.NewReader([]byte{1})
+	r := NewFakeSeeker(bufReader, 5)
+	assert.Equal(t, r, bufReader)
+
+	in := bytes.NewBufferString("hello")
+	buf := make([]byte, 16)
+	r = NewFakeSeeker(in, 5)
+	assert.NotEqual(t, r, in)
+
+	// check the seek offset is as passed in
+	checkPos := func(pos int64) {
+		abs, err := r.Seek(0, io.SeekCurrent)
+		require.NoError(t, err)
+		assert.Equal(t, pos, abs)
+	}
+
+	// Test some seeking
+	checkPos(0)
+
+	abs, err := r.Seek(2, io.SeekStart)
+	require.NoError(t, err)
+	assert.Equal(t, int64(2), abs)
+	checkPos(2)
+
+	abs, err = r.Seek(-1, io.SeekEnd)
+	require.NoError(t, err)
+	assert.Equal(t, int64(4), abs)
+	checkPos(4)
+
+	// Check can't read if not at start
+	_, err = r.Read(buf)
+	require.ErrorContains(t, err, "not at start")
+
+	// Seek back to start
+	abs, err = r.Seek(-4, io.SeekCurrent)
+	require.NoError(t, err)
+	assert.Equal(t, int64(0), abs)
+	checkPos(0)
+
+	_, err = r.Seek(42, 17)
+	require.ErrorContains(t, err, "invalid whence")
+
+	_, err = r.Seek(-1, io.SeekStart)
+	require.ErrorContains(t, err, "negative position")
+
+	// Test reading now seeked back to the start
+	n, err := r.Read(buf)
+	require.NoError(t, err)
+	assert.Equal(t, 5, n)
+	assert.Equal(t, []byte("hello"), buf[:5])
+
+	// Seeking should give an error now
+	_, err = r.Seek(-1, io.SeekEnd)
+	require.ErrorContains(t, err, "after reading")
+}
+
+func TestFakeSeekerError(t *testing.T) {
+	in := bytes.NewBufferString("hello")
+	r := NewFakeSeeker(in, 5)
+	assert.NotEqual(t, r, in)
+
+	buf, err := io.ReadAll(r)
+	require.NoError(t, err)
+	assert.Equal(t, []byte("hello"), buf)
+
+	_, err = r.Read(buf)
+	assert.Equal(t, io.EOF, err)
+
+	_, err = r.Seek(0, io.SeekStart)
+	assert.Equal(t, io.EOF, err)
+}