Add Reader.ReadContext

Allows cancelling reads etc. Torrents that get stuck can result in Reads that won't return until the torrent is dropped.

reader.go

@@ -1,6 +1,7 @@
 package torrent
 
 import (
+	"context"
 	"errors"
 	"io"
 	"os"
@@ -91,11 +92,31 @@ func (r *Reader) waitReadable(off int64) {
 }
 
 func (r *Reader) Read(b []byte) (n int, err error) {
+	return r.ReadContext(b, context.Background())
+}
+
+func (r *Reader) ReadContext(b []byte, ctx context.Context) (n int, err error) {
+	// This is set under the Client lock if the Context is canceled.
+	var ctxErr error
+	if ctx.Done() != nil {
+		ctx, cancel := context.WithCancel(ctx)
+		// Abort the goroutine when the function returns.
+		defer cancel()
+		go func() {
+			<-ctx.Done()
+			r.t.cl.mu.Lock()
+			ctxErr = ctx.Err()
+			r.t.cl.event.Broadcast()
+			r.t.cl.mu.Unlock()
+		}()
+	}
+	// Hmmm, if a Read gets stuck, this means you can't change position for
+	// other purposes. That seems reasonable, but unusual.
 	r.opMu.Lock()
 	defer r.opMu.Unlock()
 	for len(b) != 0 {
 		var n1 int
-		n1, err = r.readOnceAt(b, r.pos)
+		n1, err = r.readOnceAt(b, r.pos, &ctxErr)
 		if n1 == 0 {
 			if err == nil {
 				panic("expected error")
@@ -123,28 +144,32 @@ func (r *Reader) torrentClosed() bool {
 
 // Wait until some data should be available to read. Tickles the client if it
 // isn't. Returns how much should be readable without blocking.
-func (r *Reader) waitAvailable(pos, wanted int64) (avail int64) {
+func (r *Reader) waitAvailable(pos, wanted int64, ctxErr *error) (avail int64) {
 	r.t.cl.mu.Lock()
 	defer r.t.cl.mu.Unlock()
-	for !r.readable(pos) {
+	for !r.readable(pos) && *ctxErr == nil {
 		r.waitReadable(pos)
 	}
 	return r.available(pos, wanted)
 }
 
 // Performs at most one successful read to torrent storage.
-func (r *Reader) readOnceAt(b []byte, pos int64) (n int, err error) {
+func (r *Reader) readOnceAt(b []byte, pos int64, ctxErr *error) (n int, err error) {
 	if pos >= r.t.length {
 		err = io.EOF
 		return
 	}
 	for {
-		avail := r.waitAvailable(pos, int64(len(b)))
+		avail := r.waitAvailable(pos, int64(len(b)), ctxErr)
 		if avail == 0 {
 			if r.torrentClosed() {
 				err = errors.New("torrent closed")
 				return
 			}
+			if *ctxErr != nil {
+				err = *ctxErr
+				return
+			}
 		}
 		b1 := b[:avail]
 		pi := int(pos / r.t.Info().PieceLength)

reader_test.go

@@ -0,0 +1,25 @@
+package torrent
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/anacrolix/torrent/internal/testutil"
+)
+
+func TestReaderReadContext(t *testing.T) {
+	cl, err := NewClient(&TestingConfig)
+	require.NoError(t, err)
+	defer cl.Close()
+	tt, err := cl.AddTorrent(testutil.GreetingMetaInfo())
+	require.NoError(t, err)
+	defer tt.Drop()
+	ctx, _ := context.WithDeadline(context.Background(), time.Now().Add(time.Millisecond))
+	r := tt.NewReader()
+	defer r.Close()
+	_, err = r.ReadContext(make([]byte, 1), ctx)
+	require.EqualValues(t, context.DeadlineExceeded, err)
+}