Close piped processes on cancellation

internal/git/blob_reader.go

@@ -51,6 +51,9 @@ func (reader *BlobReader) Read(ctx context.Context, objectID string) ([]byte, er
 
 	reader.mu.Lock()
 	defer reader.mu.Unlock()
+	if err := ctx.Err(); err != nil {
+		return nil, fmt.Errorf("read blob %s: %w", objectID, err)
+	}
 	if reader.terminal != nil {
 		return nil, reader.terminal
 	}

internal/git/blob_reader_test.go

@@ -10,6 +10,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -156,6 +157,46 @@ exec sleep 60`)
 	}
 }
 
+func TestBlobReaderCanceledQueuedReadKeepsSessionUsable(t *testing.T) {
+	fixture := newRemoteFixture(t)
+	object := gitText(t, fixture.work, "rev-parse", "HEAD:source.txt")
+	reader := NewBlobReader(t.Context(), fixture.work)
+	defer reader.Close()
+	if body, err := reader.Read(t.Context(), object); err != nil || string(body) != "candidate" {
+		t.Fatalf("first read = %q, %v", body, err)
+	}
+	reader.mu.Lock()
+	ctx, cancel := context.WithCancel(t.Context())
+	observed := make(chan struct{})
+	queued := &observedContext{Context: ctx, observed: observed}
+	result := make(chan error, 1)
+	go func() {
+		_, err := reader.Read(queued, object)
+		result <- err
+	}()
+	<-observed
+	cancel()
+	reader.mu.Unlock()
+	if err := <-result; !errors.Is(err, context.Canceled) {
+		t.Fatalf("queued read error = %v", err)
+	}
+	if body, err := reader.Read(t.Context(), object); err != nil || string(body) != "candidate" {
+		t.Fatalf("later read = %q, %v", body, err)
+	}
+}
+
+type observedContext struct {
+	context.Context
+	once     sync.Once
+	observed chan struct{}
+}
+
+func (ctx *observedContext) Err() error {
+	err := ctx.Context.Err()
+	ctx.once.Do(func() { close(ctx.observed) })
+	return err
+}
+
 func TestBlobReaderPreservesGitFailureAndStderr(t *testing.T) {
 	installFakeGit(t, `printf 'fixture Git failure\n' >&2
 exit 23`)