async reader + test (#1464)

2025-10-11 04:32:29 +02:00 · 2025-07-25 11:29:53 +02:00
parent a95c0bcd66
commit 0e0714b5a1
2 changed files with 788 additions and 0 deletions
--- a/asyncbuffer/buffer.go
+++ b/asyncbuffer/buffer.go
@@ -0,0 +1,440 @@
+// Package asyncbuffer provides an asynchronous buffer that reads data from an
+// io.Reader in the background.
+//
+// When created, AsyncBuffer starts reading from the upstream reader in the
+// background. If a read error occurs, it is stored and can be checked with
+// AsyncBuffer.Error().
+//
+// When reading through AsyncBuffer.Reader().Read(), the error is only returned
+// once the reader reaches the point where the error occurred. In other words,
+// errors are delayed until encountered by the reader.
+//
+// However, AsyncBuffer.Close() and AsyncBuffer.Error() will immediately return
+// any stored error, even if the reader has not yet reached the error point.
+package asyncbuffer
+
+import (
+	"errors"
+	"io"
+	"sync"
+	"sync/atomic"
+)
+
+// ChunkSize is the size of each chunk in bytes
+const ChunkSize = 4096
+
+// byteChunk is a struct that holds a buffer and the data read from the upstream reader
+// data slice is required since the chunk read may be smaller than ChunkSize
+type byteChunk struct {
+	buf  []byte
+	data []byte
+}
+
+// chunkPool is a global sync.Pool that holds byteChunk objects for
+// all readers
+var chunkPool = sync.Pool{
+	New: func() any {
+		buf := make([]byte, ChunkSize)
+
+		return &byteChunk{
+			buf:  buf,
+			data: buf[:0],
+		}
+	},
+}
+
+// AsyncBuffer is a wrapper around io.Reader that reads data in chunks
+// in background and allows reading from synchronously.
+type AsyncBuffer struct {
+	r io.Reader // Upstream reader
+
+	chunks []*byteChunk // References to the chunks read from the upstream reader
+
+	err      atomic.Value // Error that occurred during reading
+	finished atomic.Bool  // Indicates that the reader has finished reading
+	len      atomic.Int64 // Total length of the data read
+	closed   atomic.Bool  // Indicates that the reader was closed
+
+	mu             sync.RWMutex  // Mutex on chunks slice
+	newChunkSignal chan struct{} // Tick-tock channel that indicates that a new chunk is ready
+}
+
+// Underlying Reader that provides io.ReadSeeker interface for the actual data reading
+// What is the purpose of this Reader?
+type Reader struct {
+	ab  *AsyncBuffer
+	pos int64
+}
+
+// FromReadCloser creates a new AsyncBuffer that reads from the given io.Reader in background
+func FromReader(r io.Reader) *AsyncBuffer {
+	ab := &AsyncBuffer{
+		r:              r,
+		newChunkSignal: make(chan struct{}),
+	}
+
+	go ab.readChunks()
+
+	return ab
+}
+
+// getNewChunkSignal returns the channel that signals when a new chunk is ready
+// Lock is required to read the channel, so it is not closed while reading
+func (ab *AsyncBuffer) getNewChunkSignal() chan struct{} {
+	ab.mu.RLock()
+	defer ab.mu.RUnlock()
+
+	return ab.newChunkSignal
+}
+
+// addChunk adds a new chunk to the AsyncBuffer, increments len and signals that a chunk is ready
+func (ab *AsyncBuffer) addChunk(chunk *byteChunk) {
+	ab.mu.Lock()
+	defer ab.mu.Unlock()
+
+	// Store the chunk, increase chunk size, increase length of the data read
+	ab.chunks = append(ab.chunks, chunk)
+	ab.len.Add(int64(len(chunk.data)))
+
+	// Signal that a chunk is ready
+	currSignal := ab.newChunkSignal
+	ab.newChunkSignal = make(chan struct{})
+	close(currSignal)
+}
+
+// finish marks the reader as finished
+func (ab *AsyncBuffer) finish() {
+	// Indicate that the reader has finished reading
+	ab.finished.Store(true)
+
+	// This indicates that Close() was called before all the chunks were read, we do not need to close the channel
+	// since it was closed already.
+	if !ab.closed.Load() {
+		close(ab.newChunkSignal)
+	}
+}
+
+// readChunks reads data from the upstream reader in background and stores them in the pool
+func (ab *AsyncBuffer) readChunks() {
+	defer ab.finish()
+
+	// Stop reading if the reader is finished
+	for !ab.finished.Load() {
+		// Get a chunk from the pool
+		// If the pool is empty, it will create a new byteChunk with ChunkSize
+		chunk, ok := chunkPool.Get().(*byteChunk)
+		if !ok {
+			ab.err.Store(errors.New("asyncbuffer.AsyncBuffer.readChunks: failed to get chunk from pool"))
+			return
+		}
+
+		// Read data into the chunk's buffer
+		n, err := io.ReadFull(ab.r, chunk.buf)
+
+		// If it's not the EOF, we need to store the error
+		if err != nil && err != io.EOF && err != io.ErrUnexpectedEOF {
+			ab.err.Store(err)
+			return
+		}
+
+		// No bytes were read (n == 0), we can return the chunk to the pool
+		if err == io.EOF || n == 0 {
+			chunkPool.Put(chunk)
+			return
+		}
+
+		// Resize the chunk's data slice to the number of bytes read
+		chunk.data = chunk.buf[:n]
+
+		// Store the reference to the chunk in the AsyncBuffer
+		ab.addChunk(chunk)
+
+		// We got ErrUnexpectedEOF meaning that some bytes were read, but this is the
+		// end of the stream, so we can stop reading
+		if err == io.ErrUnexpectedEOF {
+			return
+		}
+	}
+}
+
+// closedError returns an error if the attempt to read on a closed reader was made.
+// If the reader had an error, it returns that error instead.
+func (ab *AsyncBuffer) closedError() error {
+	// If the reader is closed, we return the error or nil
+	if ab.closed.Load() {
+		err := ab.Error()
+		if err == nil {
+			err = errors.New("asyncbuffer.AsyncBuffer.ReadAt: attempt to read on closed reader")
+		}
+
+		return err
+	}
+
+	return nil
+}
+
+// offsetAvailable checks if the data at the given offset is available for reading.
+// It may return io.EOF if the reader is finished reading and the offset is beyond the end of the stream.
+func (ab *AsyncBuffer) offsetAvailable(off int64) (bool, error) {
+	// We can not read data from the closed reader, none
+	if ab.closed.Load() {
+		return false, ab.closedError()
+	}
+
+	// In case the offset falls within the already read chunks, we can return immediately,
+	// even if error has occurred in the future
+	if off < ab.len.Load() {
+		return true, nil
+	}
+
+	// In case the reader is finished reading, and we have not read enough
+	// data yet, return either error or EOF
+	if ab.finished.Load() {
+		// In case, error has occurred, we need to return it
+		err := ab.Error()
+		if err != nil {
+			return false, err
+		}
+
+		// Otherwise, it's EOF if the offset is beyond the end of the stream
+		return false, io.EOF
+	}
+
+	// No available data
+	return false, nil
+}
+
+// WaitFor waits for the data to be ready at the given offset. nil means ok.
+// It guarantees that the chunk at the given offset is ready to be read.
+func (ab *AsyncBuffer) WaitFor(off int64) error {
+	for {
+		ok, err := ab.offsetAvailable(off)
+		if ok || err != nil {
+			return err
+		}
+
+		<-ab.getNewChunkSignal()
+	}
+}
+
+// Wait waits for the reader to finish reading all data and returns
+// the total length of the data read.
+func (ab *AsyncBuffer) Wait() (int64, error) {
+	for {
+		// We can not read data from the closed reader even if there were no errors
+		if ab.closed.Load() {
+			return 0, ab.closedError()
+		}
+
+		// In case the reader is finished reading, we can return immediately
+		if ab.finished.Load() {
+			size := ab.len.Load()
+
+			// If there was an error during reading, we need to return it no matter what position
+			// had the error happened
+			err := ab.err.Load()
+			if err != nil {
+				err, ok := err.(error)
+				if !ok {
+					return size, errors.New("asyncbuffer.AsyncBuffer.Wait: failed to get error")
+				}
+
+				return size, err
+			}
+
+			return size, nil
+		}
+
+		// Lock until the next chunk is ready
+		<-ab.getNewChunkSignal()
+	}
+}
+
+// Error returns the error that occurred during reading data in background.
+func (ab *AsyncBuffer) Error() error {
+	err := ab.err.Load()
+	if err == nil {
+		return nil
+	}
+
+	errCast, ok := err.(error)
+	if !ok {
+		return errors.New("asyncbuffer.AsyncBuffer.Error: failed to get error")
+	}
+
+	return errCast
+}
+
+// readChunkAt copies data from the chunk at the given absolute offset to the provided slice.
+// Chunk must be available when this method is called.
+// Returns the number of bytes copied to the slice or 0 if chunk has no data
+// (eg. offset is beyond the end of the stream).
+func (ab *AsyncBuffer) readChunkAt(p []byte, off, rem int64) int {
+	// If the chunk is not available, we return 0
+	if off >= ab.len.Load() {
+		return 0
+	}
+
+	ind := off / ChunkSize // chunk index
+	chunk := ab.chunks[ind]
+
+	startOffset := off % ChunkSize // starting offset in the chunk
+
+	// If the offset in current chunk is greater than the data
+	// it has, we return 0
+	if startOffset >= int64(len(chunk.data)) {
+		return 0
+	}
+
+	// How many bytes we could read from the chunk. No more than:
+	// - left to read totally
+	// - chunk size minus the start offset
+	// - chunk has
+	size := min(rem, ChunkSize-startOffset, int64(len(chunk.data)))
+
+	if size == 0 {
+		return 0
+	}
+
+	return copy(p, chunk.data[startOffset:startOffset+size])
+}
+
+// readAt reads data from the AsyncBuffer at the given offset.
+//
+// If full is true:
+//
+// The behaviour is similar to io.ReaderAt.ReadAt. It blocks until the maxumum amount of data possible
+// is read from the buffer. It may return io.UnexpectedEOF in case we tried to read more data than was
+// available in the buffer.
+//
+// If full is false:
+//
+// It behaves like a regular non-blocking Read.
+func (ab *AsyncBuffer) readAt(p []byte, off int64) (int, error) {
+	size := int64(len(p)) // total size of the data to read
+
+	if off < 0 {
+		return 0, errors.New("asyncbuffer.AsyncBuffer.readAt: negative offset")
+	}
+
+	// Wait for the offset to be available.
+	// It may return io.EOF if the offset is beyond the end of the stream.
+	err := ab.WaitFor(off)
+	if err != nil {
+		return 0, err
+	}
+
+	ab.mu.RLock()
+	defer ab.mu.RUnlock()
+
+	// If the reader is closed, we return an error
+	if ab.closed.Load() {
+		return 0, ab.closedError()
+	}
+
+	// Read data from the first chunk
+	n := ab.readChunkAt(p, off, size)
+	if n == 0 {
+		return 0, io.EOF // Failed to read any data: means we tried to read beyond the end of the stream
+	}
+
+	size -= int64(n)
+	off += int64(n) // Here and beyond off always points to the last read byte + 1
+
+	// Now, let's try to read the rest of the data from next chunks while they are available
+	for size > 0 {
+		// If data is not available at the given offset, we can return data read so far.
+		ok, err := ab.offsetAvailable(off)
+		if !ok || err != nil {
+			return n, err
+		}
+
+		// Read data from the next chunk
+		nX := ab.readChunkAt(p[n:], off, size)
+		n += nX
+		size -= int64(nX)
+		off += int64(nX)
+
+		// If we read data shorter than ChunkSize or, in case that was the last chunk, less than
+		// the size of the tail, return kind of EOF
+		if int64(nX) < min(size, int64(ChunkSize)) {
+			return n, io.EOF
+		}
+	}
+
+	return n, nil
+}
+
+// Close closes the AsyncBuffer and releases all resources.
+// It returns an error if the reader was already closed or if there was
+// an error during reading data in background even if none of the subsequent
+// readers have reached the position where the error occurred.
+func (ab *AsyncBuffer) Close() error {
+	ab.mu.Lock()
+	defer ab.mu.Unlock()
+
+	// If the reader is already closed, we return immediately error or nil
+	if ab.closed.Load() {
+		return ab.Error()
+	}
+
+	ab.closed.Store(true)
+
+	// If the reader is still running, we need to signal that it should stop and close the channel
+	if !ab.finished.Load() {
+		ab.finished.Store(true)
+		close(ab.newChunkSignal)
+	}
+
+	// Return all chunks to the pool
+	for _, chunk := range ab.chunks {
+		chunkPool.Put(chunk)
+	}
+
+	return nil
+}
+
+// Reader returns an io.ReadSeeker+io.ReaderAt that can be used to read actual data from the AsyncBuffer
+func (ab *AsyncBuffer) Reader() *Reader {
+	return &Reader{ab: ab, pos: 0}
+}
+
+// Read reads data from the AsyncBuffer.
+func (r *Reader) Read(p []byte) (int, error) {
+	n, err := r.ab.readAt(p, r.pos)
+	if err != nil {
+		return n, err
+	}
+
+	r.pos += int64(n)
+
+	return n, nil
+}
+
+// Seek sets the position of the reader to the given offset and returns the new position
+func (r *Reader) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		r.pos = offset
+
+	case io.SeekCurrent:
+		r.pos += offset
+
+	case io.SeekEnd:
+		size, err := r.ab.Wait()
+		if err != nil {
+			return 0, err
+		}
+
+		r.pos = size + offset
+
+	default:
+		return 0, errors.New("asyncbuffer.AsyncBuffer.ReadAt: invalid whence")
+	}
+
+	if r.pos < 0 {
+		return 0, errors.New("asyncbuffer.AsyncBuffer.ReadAt: negative position")
+	}
+
+	return r.pos, nil
+}
--- a/asyncbuffer/buffer_test.go
+++ b/asyncbuffer/buffer_test.go
@@ -0,0 +1,348 @@
+package asyncbuffer
+
+import (
+	"bytes"
+	"crypto/rand"
+	"errors"
+	"io"
+	"sync"
+	"sync/atomic"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	halfChunkSize   = ChunkSize / 2
+	quaterChunkSize = ChunkSize / 4
+)
+
+// erraticReader is a test reader that simulates a slow read and can fail after reading a certain number of bytes
+type erraticReader struct {
+	reader bytes.Reader
+	failAt int64 // if set, will return an error after reading this many bytes
+}
+
+// Read reads data from the testReader, simulating a slow read and a potential failure
+func (r *erraticReader) Read(p []byte) (n int, err error) {
+	cur, _ := r.reader.Seek(0, io.SeekCurrent)
+	if r.failAt > 0 && r.failAt < cur+int64(len(p)) {
+		return 0, errors.New("simulated read failure")
+	}
+	return r.reader.Read(p)
+}
+
+// blockingReader is a test reader which flushes data in chunks
+type blockingReader struct {
+	reader    bytes.Reader
+	mu        sync.Mutex  // locked reader does not return anything
+	unlocking atomic.Bool // if true, will proceed without locking each chunk
+}
+
+// newBlockingReader creates a new partialReader in locked state
+func newBlockingReader(reader bytes.Reader) *blockingReader {
+	r := &blockingReader{
+		reader: reader,
+	}
+	r.mu.Lock()
+	return r
+}
+
+// flushNextChunk unlocks the reader, allowing it to return the next chunk of data
+func (r *blockingReader) flushNextChunk() {
+	r.mu.Unlock()
+}
+
+// flush unlocks the reader, allowing it to return all data as usual
+func (r *blockingReader) flush() {
+	r.unlocking.Store(true) // allow reading data without blocking
+	r.mu.Unlock()           // and continue
+}
+
+// Read reads data from the testReader, simulating a slow read and a potential failure
+func (r *blockingReader) Read(p []byte) (n int, err error) {
+	if !r.unlocking.Load() {
+		r.mu.Lock()
+	}
+
+	n, err = r.reader.Read(p)
+	return n, err
+}
+
+// generateSourceData generates a byte slice with 4.5 chunks of data
+func generateSourceData(t *testing.T, size int64) ([]byte, *bytes.Reader) {
+	// We use small chunks for tests, let's check the ChunkSize just in case
+	assert.GreaterOrEqual(t, ChunkSize, 20, "ChunkSize required for tests must be greater than 10 bytes")
+
+	// Create a byte slice with 4 chunks of ChunkSize
+	source := make([]byte, size)
+
+	// Fill the source with random data
+	_, err := rand.Read(source)
+	require.NoError(t, err)
+	return source, bytes.NewReader(source)
+}
+
+// TestAsyncBufferRead tests reading from AsyncBuffer using readAt method which is base for all other methods
+func TestAsyncBufferReadAt(t *testing.T) {
+	// Let's use source buffer which is 4.5 chunks long
+	source, bytesReader := generateSourceData(t, int64(ChunkSize*4)+halfChunkSize)
+	asyncBuffer := FromReader(bytesReader)
+	defer asyncBuffer.Close()
+
+	asyncBuffer.Wait() // Wait for all chunks to be read since we're going to read all data
+
+	// Let's read all the data
+	target := make([]byte, len(source))
+
+	n, err := asyncBuffer.readAt(target, 0)
+	require.NoError(t, err)
+	assert.Equal(t, len(source), n)
+	assert.Equal(t, target, source)
+
+	// Let's read all the data + a bit more
+	target = make([]byte, len(source)+1)
+
+	n, err = asyncBuffer.readAt(target, 0)
+	require.ErrorIs(t, err, io.EOF) // We read all the data, and reached end
+	assert.Equal(t, len(source), n)
+	assert.Equal(t, target[:n], source)
+
+	// Let's read > 1 chunk, but with offset from the beginning and the end
+	target = make([]byte, len(source)-halfChunkSize)
+	n, err = asyncBuffer.readAt(target, quaterChunkSize)
+	require.NoError(t, err)
+	assert.Equal(t, len(target), n)
+	assert.Equal(t, target, source[quaterChunkSize:len(source)-quaterChunkSize])
+
+	// Let's read some data from the middle of the stream < chunk size
+	target = make([]byte, ChunkSize/4)
+	n, err = asyncBuffer.readAt(target, ChunkSize+ChunkSize/4)
+	require.NoError(t, err)
+	assert.Equal(t, quaterChunkSize, n)
+	assert.Equal(t, target, source[ChunkSize+quaterChunkSize:ChunkSize+quaterChunkSize*2])
+
+	// Let's read some data from the latest half chunk
+	target = make([]byte, quaterChunkSize)
+	n, err = asyncBuffer.readAt(target, ChunkSize*4+quaterChunkSize)
+	require.NoError(t, err)
+	assert.Equal(t, quaterChunkSize, n)
+	assert.Equal(t, target, source[ChunkSize*4+quaterChunkSize:ChunkSize*4+halfChunkSize])
+
+	// Let's try to read more data then available in the stream
+	target = make([]byte, ChunkSize*2)
+	n, err = asyncBuffer.readAt(target, ChunkSize*4)
+	require.Error(t, err)
+	assert.Equal(t, err, io.EOF)
+	assert.Equal(t, ChunkSize/2, n)
+	assert.Equal(t, target[:ChunkSize/2], source[ChunkSize*4:]) // We read only last half chunk
+
+	// Let's try to read data beyond the end of the stream
+	target = make([]byte, ChunkSize*2)
+	n, err = asyncBuffer.readAt(target, ChunkSize*5)
+	require.Error(t, err)
+	assert.Equal(t, err, io.EOF)
+	assert.Equal(t, 0, n)
+}
+
+// TestAsyncBufferRead tests reading from AsyncBuffer using ReadAt method
+func TestAsyncBufferReadAtSmallBuffer(t *testing.T) {
+	source, bytesReader := generateSourceData(t, 20)
+	asyncBuffer := FromReader(bytesReader)
+	defer asyncBuffer.Close()
+
+	// First, let's read all the data
+	target := make([]byte, len(source))
+
+	n, err := asyncBuffer.readAt(target, 0)
+	require.NoError(t, err)
+	assert.Equal(t, len(source), n)
+	assert.Equal(t, target, source)
+
+	// Let's read some data
+	target = make([]byte, 2)
+	n, err = asyncBuffer.readAt(target, 1)
+	require.NoError(t, err)
+	assert.Equal(t, len(target), n)
+	assert.Equal(t, target, source[1:3])
+
+	// Let's read some data beyond the end of the stream
+	target = make([]byte, 2)
+	n, err = asyncBuffer.readAt(target, 50)
+	require.Error(t, err)
+	assert.Equal(t, err, io.EOF)
+	assert.Equal(t, 0, n)
+}
+
+func TestAsyncBufferReader(t *testing.T) {
+	source, bytesReader := generateSourceData(t, int64(ChunkSize*4)+halfChunkSize)
+
+	// Create an AsyncBuffer with the byte slice
+	asyncBuffer := FromReader(bytesReader)
+	defer asyncBuffer.Close()
+
+	// Let's wait for all chunks to be read
+	size, err := asyncBuffer.Wait()
+	require.NoError(t, err, "AsyncBuffer failed to wait for all chunks")
+	assert.Equal(t, int64(ChunkSize*4+halfChunkSize), size)
+
+	reader := asyncBuffer.Reader()
+
+	// Ensure the total length of the data is ChunkSize*4
+	require.NoError(t, err)
+
+	// Read the first two chunks
+	twoChunks := make([]byte, ChunkSize*2)
+	n, err := reader.Read(twoChunks)
+	require.NoError(t, err)
+	assert.Equal(t, ChunkSize*2, n)
+	assert.Equal(t, source[:ChunkSize*2], twoChunks)
+
+	// Seek to the last chunk + 10 bytes
+	pos, err := reader.Seek(ChunkSize*3+5, io.SeekStart)
+	require.NoError(t, err)
+	assert.Equal(t, int64(ChunkSize*3+5), pos)
+
+	// Read the next 10 bytes
+	smallSlice := make([]byte, 10)
+	n, err = reader.Read(smallSlice)
+	require.NoError(t, err)
+	assert.Equal(t, 10, n)
+	assert.Equal(t, source[ChunkSize*3+5:ChunkSize*3+5+10], smallSlice)
+
+	// Seek -10 bytes from the current position
+	pos, err = reader.Seek(-10, io.SeekCurrent)
+	require.NoError(t, err)
+	assert.Equal(t, int64(ChunkSize*3+5), pos)
+
+	// Read data again
+	n, err = reader.Read(smallSlice)
+	require.NoError(t, err)
+	assert.Equal(t, 10, n)
+	assert.Equal(t, source[ChunkSize*3+5:ChunkSize*3+5+10], smallSlice)
+
+	// Seek -10 bytes from end of the stream
+	pos, err = reader.Seek(-10, io.SeekEnd)
+	require.NoError(t, err)
+	assert.Equal(t, size-10, pos)
+
+	// Read last 10 bytes
+	n, err = reader.Read(smallSlice)
+	require.NoError(t, err)
+	assert.Equal(t, 10, n)
+	assert.Equal(t, source[size-10:], smallSlice)
+
+	// Seek beyond the end of the stream and try to read
+	pos, err = reader.Seek(1024, io.SeekCurrent)
+	require.NoError(t, err)
+	assert.Equal(t, size+1024, pos)
+
+	_, err = reader.Read(smallSlice)
+	require.ErrorIs(t, err, io.EOF)
+}
+
+// TestAsyncBufferClose tests closing the AsyncBuffer
+func TestAsyncBufferClose(t *testing.T) {
+	_, bytesReader := generateSourceData(t, int64(ChunkSize*4)+halfChunkSize)
+
+	// Create an AsyncBuffer with the byte slice
+	asyncBuffer := FromReader(bytesReader)
+
+	reader1 := asyncBuffer.Reader()
+	reader2 := asyncBuffer.Reader()
+
+	asyncBuffer.Close()
+
+	b := make([]byte, 10)
+	_, err := reader1.Read(b)
+	require.Error(t, err, "asyncbuffer.AsyncBuffer.ReadAt: attempt to read on closed reader")
+
+	_, err = reader2.Read(b)
+	require.Error(t, err, "asyncbuffer.AsyncBuffer.ReadAt: attempt to read on closed reader")
+
+	// After closing the closed reader, it should not panic
+	asyncBuffer.Close()
+
+	_, err = reader2.Read(b)
+	require.Error(t, err, "asyncbuffer.AsyncBuffer.ReadAt: attempt to read on closed reader")
+}
+
+// TestAsyncBufferReadAtErrAtSomePoint tests reading from AsyncBuffer using readAt method
+// which would fail somewhere
+func TestAsyncBufferReadAtErrAtSomePoint(t *testing.T) {
+	// Let's use source buffer which is 4.5 chunks long
+	source, bytesReader := generateSourceData(t, int64(ChunkSize*4)+halfChunkSize)
+	slowReader := &erraticReader{reader: *bytesReader, failAt: ChunkSize*3 + 5} // fails at last chunk
+	asyncBuffer := FromReader(slowReader)
+	defer asyncBuffer.Close()
+
+	// Let's wait for all chunks to be read
+	_, err := asyncBuffer.Wait()
+	require.Error(t, err, "simulated read failure")
+
+	// Let's read something, but before error occurs
+	target := make([]byte, halfChunkSize)
+	n, err := asyncBuffer.readAt(target, 0)
+	require.NoError(t, err)
+	assert.Equal(t, len(target), n)
+	assert.Equal(t, target, source[:halfChunkSize])
+
+	// And again
+	target = make([]byte, halfChunkSize)
+	n, err = asyncBuffer.readAt(target, halfChunkSize)
+	require.NoError(t, err)
+	assert.Equal(t, len(target), n)
+	assert.Equal(t, target, source[halfChunkSize:halfChunkSize*2])
+
+	// Let's read something, but when error occurs
+	target = make([]byte, halfChunkSize)
+	_, err = asyncBuffer.readAt(target, ChunkSize*3)
+	require.Error(t, err, "simulated read failure")
+}
+
+// TestAsyncBufferReadAsync tests reading from AsyncBuffer using readAt method
+// with full = false
+func TestAsyncBufferReadAsync(t *testing.T) {
+	// Let's use source buffer which is 4.5 chunks long
+	source, bytesReader := generateSourceData(t, int64(ChunkSize)*3)
+	blockingReader := newBlockingReader(*bytesReader)
+	asyncBuffer := FromReader(blockingReader)
+	defer asyncBuffer.Close()
+
+	// flush the first chunk to allow reading
+	blockingReader.flushNextChunk()
+
+	// Let's try to read first two chunks, however,
+	// we know that only the first chunk is available
+	target := make([]byte, ChunkSize*2)
+	n, err := asyncBuffer.readAt(target, 0)
+	require.NoError(t, err)
+	assert.Equal(t, ChunkSize, n)
+	assert.Equal(t, target[:ChunkSize], source[:ChunkSize])
+
+	blockingReader.flushNextChunk()    // unlock reader to allow read second chunk
+	asyncBuffer.WaitFor(ChunkSize + 1) // wait for the second chunk to be available
+
+	target = make([]byte, ChunkSize*2)
+	n, err = asyncBuffer.readAt(target, 0)
+	require.NoError(t, err)
+	assert.Equal(t, ChunkSize*2, n)
+	assert.Equal(t, target, source[:ChunkSize*2])
+
+	blockingReader.flush() // Flush the rest of the data
+	asyncBuffer.Wait()
+
+	// Try to read near end of the stream, EOF
+	target = make([]byte, ChunkSize)
+	n, err = asyncBuffer.readAt(target, ChunkSize*3-1)
+	require.ErrorIs(t, err, io.EOF)
+	assert.Equal(t, 1, n)
+	assert.Equal(t, target[0], source[ChunkSize*3-1])
+
+	// Try to read beyond the end of the stream == eof
+	target = make([]byte, ChunkSize)
+	n, err = asyncBuffer.readAt(target, ChunkSize*3)
+	require.ErrorIs(t, io.EOF, err)
+	assert.Equal(t, 0, n)
+}