chore: maybe improved integration test for additional headers? (#1773)

## Which problem is this PR solving?

I was playing with the tests to get to know the changes in #1771. The
result is mostly the same, but I propose we maintain this version.

## Short description of the changes

The multiple commits show my fiddling, but the major changes are:

* a single table-driven test
* the lock for synchronizing the captured headers is hidden by using an
atomic type

Author: robbkidd

transmit/direct_transmit_test.go

@@ -1508,158 +1508,104 @@ func TestBuildRequestURL(t *testing.T) {
 	}
 }
 
-func TestDirectTransmitAdditionalHeaders(t *testing.T) {
-	// Create test server that captures headers
-	var capturedHeaders http.Header
-	var headerMutex sync.Mutex
-
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		headerMutex.Lock()
-		capturedHeaders = r.Header.Clone()
-		headerMutex.Unlock()
-
-		// Read and discard body
-		io.ReadAll(r.Body)
-		r.Body.Close()
-
-		// Send proper response
-		w.Header().Set("Content-Type", "application/json")
-		w.WriteHeader(http.StatusOK)
-		w.Write([]byte(`[{"status": 202}]`))
-	}))
-	defer server.Close()
-
-	// Create DirectTransmission with additional headers
-	additionalHeaders := map[string]string{
-		"FORWARD_TO_URL":  "https://proxy.example.com",
-		"X-Custom-Header": "custom-value",
-	}
-
-	mockMetrics := &metrics.MockMetrics{}
-	mockMetrics.Start()
-	mockLogger := &logger.MockLogger{}
-
-	dt := NewDirectTransmission(
-		types.TransmitTypeUpstream,
-		http.DefaultTransport.(*http.Transport),
-		10,
-		100*time.Millisecond,
-		10*time.Second,
-		false, // no compression to simplify test
-		additionalHeaders,
-	)
-	dt.Logger = mockLogger
-	dt.Version = "test-version"
-	dt.Metrics = mockMetrics
-
-	err := dt.Start()
-	require.NoError(t, err)
-	defer dt.Stop()
-
-	// Send a test event
-	mockCfg := &config.MockConfig{}
-	eventData := types.NewPayload(mockCfg, map[string]any{"test": "value"})
-	eventData.ExtractMetadata()
-
-	event := &types.Event{
-		Context:    context.Background(),
-		APIHost:    server.URL,
-		APIKey:     "test-api-key",
-		Dataset:    "test-dataset",
-		SampleRate: 1,
-		Timestamp:  time.Now().UTC(),
-		Data:       eventData,
+// TestDirectTransmission_AdditionalHeaders validates that additional headers configured
+// in Network.AdditionalHeaders are properly sent with HTTP requests.
+// Config-level validation (reserved headers, etc.) is tested in config/config_test.go
+func TestDirectTransmission_AdditionalHeaders(t *testing.T) {
+	testCases := []struct {
+		desc              string
+		additionalHeaders map[string]string
+	}{
+		{
+			desc: "with-additional-headers",
+			additionalHeaders: map[string]string{
+				"FORWARD_TO_URL":  "https://proxy.example.com",
+				"X-Custom-Header": "custom-value",
+			},
+		},
+		{
+			desc:              "with-empty-additional-headers",
+			additionalHeaders: map[string]string{},
+		},
+		{
+			desc:              "with-nil-additional-headers",
+			additionalHeaders: nil,
+		},
 	}
-	dt.EnqueueEvent(event)
-
-	// Wait for the event to be sent
-	require.Eventually(t, func() bool {
-		headerMutex.Lock()
-		defer headerMutex.Unlock()
-		return capturedHeaders != nil
-	}, 5*time.Second, 50*time.Millisecond, "headers should be captured")
-
-	// Verify custom headers were sent
-	headerMutex.Lock()
-	defer headerMutex.Unlock()
-	assert.Equal(t, "https://proxy.example.com", capturedHeaders.Get("FORWARD_TO_URL"))
-	assert.Equal(t, "custom-value", capturedHeaders.Get("X-Custom-Header"))
-
-	// Verify standard Honeycomb headers are also present
-	assert.Equal(t, "test-api-key", capturedHeaders.Get("X-Honeycomb-Team"))
-	assert.Equal(t, "application/msgpack", capturedHeaders.Get("Content-Type"))
-}
-
-func TestDirectTransmitNoAdditionalHeaders(t *testing.T) {
-	// Create test server that captures headers
-	var capturedHeaders http.Header
-	var headerMutex sync.Mutex
-
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		headerMutex.Lock()
-		capturedHeaders = r.Header.Clone()
-		headerMutex.Unlock()
-
-		io.ReadAll(r.Body)
-		r.Body.Close()
-
-		w.Header().Set("Content-Type", "application/json")
-		w.WriteHeader(http.StatusOK)
-		w.Write([]byte(`[{"status": 202}]`))
-	}))
-	defer server.Close()
-
-	// Create DirectTransmission without additional headers (nil)
-	mockMetrics := &metrics.MockMetrics{}
-	mockMetrics.Start()
-	mockLogger := &logger.MockLogger{}
-
-	dt := NewDirectTransmission(
-		types.TransmitTypeUpstream,
-		http.DefaultTransport.(*http.Transport),
-		10,
-		100*time.Millisecond,
-		10*time.Second,
-		false,
-		nil, // No additional headers
-	)
-	dt.Logger = mockLogger
-	dt.Version = "test-version"
-	dt.Metrics = mockMetrics
-
-	err := dt.Start()
-	require.NoError(t, err)
-	defer dt.Stop()
-
-	// Send a test event
-	mockCfg := &config.MockConfig{}
-	eventData := types.NewPayload(mockCfg, map[string]any{"test": "value"})
-	eventData.ExtractMetadata()
+	for _, tC := range testCases {
+		t.Run(tC.desc, func(t *testing.T) {
+			t.Parallel()
+
+			mockMetrics := &metrics.MockMetrics{}
+			mockMetrics.Start()
+			t.Cleanup(mockMetrics.Stop)
+
+			dt := NewDirectTransmission(
+				types.TransmitTypeUpstream,
+				http.DefaultTransport.(*http.Transport),
+				10,
+				100*time.Millisecond,
+				10*time.Second,
+				false, // no compression to simplify test
+				tC.additionalHeaders,
+			)
+			dt.Logger = &logger.MockLogger{}
+			dt.Metrics = mockMetrics
+			dt.Version = tC.desc + "-test"
+
+			// Store http.Header(s) received for validation outside handler
+			var receivedHeaders atomic.Value
+
+			capturingHeadersServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				receivedHeaders.Store(r.Header.Clone())
+
+				// Read and discard body
+				_, _ = io.ReadAll(r.Body)
+				r.Body.Close()
+
+				// Send proper response
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(http.StatusOK)
+				w.Write([]byte(`[{"status": 202}]`))
+			}))
+			t.Cleanup(capturingHeadersServer.Close)
 
-	event := &types.Event{
-		Context:    context.Background(),
-		APIHost:    server.URL,
-		APIKey:     "test-api-key",
-		Dataset:    "test-dataset",
-		SampleRate: 1,
-		Timestamp:  time.Now().UTC(),
-		Data:       eventData,
+			// Start the test DirectTransmission
+			err := dt.Start()
+			require.NoError(t, err)
+			t.Cleanup(func() { dt.Stop() })
+
+			// Send a test event
+			eventData := types.NewPayload(&config.MockConfig{}, map[string]any{"test": "value"})
+			eventData.ExtractMetadata()
+
+			event := &types.Event{
+				Context:    context.Background(),
+				APIHost:    capturingHeadersServer.URL,
+				APIKey:     "test-additional-headers",
+				Dataset:    "test-dataset",
+				SampleRate: 1,
+				Timestamp:  time.Now().UTC(),
+				Data:       eventData,
+			}
+			dt.EnqueueEvent(event)
+
+			// Wait for the server to receive the request
+			require.Eventually(t, func() bool {
+				return receivedHeaders.Load() != nil
+			}, 5*time.Second, 50*time.Millisecond, "request with headers should have been received")
+
+			// Validate headers
+			headers := receivedHeaders.Load().(http.Header)
+			if len(tC.additionalHeaders) > 0 {
+				for customHeaderName, customHeaderValue := range tC.additionalHeaders {
+					assert.Equal(t, customHeaderValue, headers.Get(customHeaderName),
+						"Additional header '%s' has unexpected value", customHeaderName)
+				}
+			}
+			assert.Equal(t, "test-additional-headers", headers.Get("X-Honeycomb-Team"))
+			assert.Equal(t, "application/msgpack", headers.Get("Content-Type"))
+			assert.Contains(t, headers.Get("User-Agent"), tC.desc+"-test")
+		})
 	}
-	dt.EnqueueEvent(event)
-
-	// Wait for the event to be sent
-	require.Eventually(t, func() bool {
-		headerMutex.Lock()
-		defer headerMutex.Unlock()
-		return capturedHeaders != nil
-	}, 5*time.Second, 50*time.Millisecond, "headers should be captured")
-
-	// Verify standard Honeycomb headers are present
-	headerMutex.Lock()
-	defer headerMutex.Unlock()
-	assert.Equal(t, "test-api-key", capturedHeaders.Get("X-Honeycomb-Team"))
-	assert.Equal(t, "application/msgpack", capturedHeaders.Get("Content-Type"))
-
-	// Custom headers should not be present
-	assert.Empty(t, capturedHeaders.Get("FORWARD_TO_URL"))
 }