fix(playback): finish segment acknowledgements

internal/transcodeproxy/completion.go

@@ -85,7 +85,11 @@ func Acknowledge(ctx context.Context, client *http.Client, targetURL, jwtSecret,
 	if client == nil {
 		client = http.DefaultClient
 	}
-	ackCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+	// The downstream request is commonly canceled as soon as its response body
+	// reaches the client, while the handler is still issuing this acknowledgement.
+	// Preserve request-scoped values, but give the post-response hop its own
+	// bounded lifetime so a successful delivery is not mistaken for a disconnect.
+	ackCtx, cancel := context.WithTimeout(context.WithoutCancel(ctx), 5*time.Second)
 	defer cancel()
 	req, err := http.NewRequestWithContext(ackCtx, http.MethodPost, targetURL+"/downloaded", nil)
 	if err != nil {

internal/transcodeproxy/completion_test.go

@@ -0,0 +1,42 @@
+package transcodeproxy
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestAcknowledgeSurvivesCompletedDownstreamRequest(t *testing.T) {
+	const (
+		jwtSecret  = "test-secret"
+		generation = "incarnation:generation"
+	)
+
+	acknowledged := make(chan struct{}, 1)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			t.Errorf("method = %q, want POST", r.Method)
+		}
+		if got := r.Header.Get("Authorization"); got != "Bearer "+jwtSecret {
+			t.Errorf("Authorization = %q", got)
+		}
+		if got := r.Header.Get(GenerationHeader); got != generation {
+			t.Errorf("generation = %q, want %q", got, generation)
+		}
+		acknowledged <- struct{}{}
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	t.Cleanup(server.Close)
+
+	requestCtx, cancelRequest := context.WithCancel(context.Background())
+	cancelRequest()
+	if err := Acknowledge(requestCtx, server.Client(), server.URL+"/segment", jwtSecret, generation); err != nil {
+		t.Fatalf("Acknowledge() error = %v", err)
+	}
+	select {
+	case <-acknowledged:
+	default:
+		t.Fatal("acknowledgement request was not delivered")
+	}
+}