Add HeaderFunc to allow modifying headers before every request (#298)

Adds a new HeaderFunc to the StartSettings that allows for dynamically editing the headers before each HTTP request made by the OpAMP library.

Closes #297

client/clientimpl_test.go

@@ -340,6 +340,79 @@ func TestConnectWithHeader(t *testing.T) {
 	})
 }
 
+func TestConnectWithHeaderFunc(t *testing.T) {
+	testClients(t, func(t *testing.T, client OpAMPClient) {
+		// Start a server.
+		srv := internal.StartMockServer(t)
+		var conn atomic.Value
+		srv.OnConnect = func(r *http.Request) {
+			authHdr := r.Header.Get("Authorization")
+			assert.EqualValues(t, "Bearer 12345678", authHdr)
+			userAgentHdr := r.Header.Get("User-Agent")
+			assert.EqualValues(t, "custom-agent/1.0", userAgentHdr)
+			conn.Store(true)
+		}
+
+		hf := func(header http.Header) http.Header {
+			header.Set("Authorization", "Bearer 12345678")
+			header.Set("User-Agent", "custom-agent/1.0")
+			return header
+		}
+
+		// Start a client.
+		settings := types.StartSettings{
+			OpAMPServerURL: "ws://" + srv.Endpoint,
+			HeaderFunc:     hf,
+		}
+		startClient(t, settings, client)
+
+		// Wait for connection to be established.
+		eventually(t, func() bool { return conn.Load() != nil })
+
+		// Shutdown the Server and the client.
+		srv.Close()
+		_ = client.Stop(context.Background())
+	})
+}
+
+func TestConnectWithHeaderAndHeaderFunc(t *testing.T) {
+	testClients(t, func(t *testing.T, client OpAMPClient) {
+		// Start a server.
+		srv := internal.StartMockServer(t)
+		var conn atomic.Value
+		srv.OnConnect = func(r *http.Request) {
+			authHdr := r.Header.Get("Authorization")
+			assert.EqualValues(t, "Bearer 12345678", authHdr)
+			userAgentHdr := r.Header.Get("User-Agent")
+			assert.EqualValues(t, "custom-agent/1.0", userAgentHdr)
+			conn.Store(true)
+		}
+
+		baseHeader := http.Header{}
+		baseHeader.Set("User-Agent", "custom-agent/1.0")
+
+		hf := func(header http.Header) http.Header {
+			header.Set("Authorization", "Bearer 12345678")
+			return header
+		}
+
+		// Start a client.
+		settings := types.StartSettings{
+			OpAMPServerURL: "ws://" + srv.Endpoint,
+			Header:         baseHeader,
+			HeaderFunc:     hf,
+		}
+		startClient(t, settings, client)
+
+		// Wait for connection to be established.
+		eventually(t, func() bool { return conn.Load() != nil })
+
+		// Shutdown the Server and the client.
+		srv.Close()
+		_ = client.Stop(context.Background())
+	})
+}
+
 func TestConnectWithTLS(t *testing.T) {
 	testClients(t, func(t *testing.T, client OpAMPClient) {
 		// Start a server.

client/httpclient.go

@@ -44,7 +44,7 @@ func (c *httpClient) Start(ctx context.Context, settings types.StartSettings) er
 	c.opAMPServerURL = settings.OpAMPServerURL
 
 	// Prepare Server connection settings.
-	c.sender.SetRequestHeader(settings.Header)
+	c.sender.SetRequestHeader(settings.Header, settings.HeaderFunc)
 
 	// Add TLS configuration into httpClient
 	c.sender.AddTLSConfig(settings.TLSConfig)

client/internal/httpsender.go

@@ -59,7 +59,7 @@ type HTTPSender struct {
 	compressionEnabled bool
 
 	// Headers to send with all requests.
-	requestHeader http.Header
+	getHeader func() http.Header
 
 	// Processor to handle received messages.
 	receiveProcessor receivedProcessor
@@ -75,7 +75,7 @@ func NewHTTPSender(logger types.Logger) *HTTPSender {
 		pollingIntervalMs: defaultPollingIntervalMs,
 	}
 	// initialize the headers with no additional headers
-	h.SetRequestHeader(nil)
+	h.SetRequestHeader(nil, nil)
 	return h
 }
 
@@ -121,12 +121,26 @@ func (h *HTTPSender) Run(
 
 // SetRequestHeader sets additional HTTP headers to send with all future requests.
 // Should not be called concurrently with any other method.
-func (h *HTTPSender) SetRequestHeader(header http.Header) {
-	if header == nil {
-		header = http.Header{}
+func (h *HTTPSender) SetRequestHeader(baseHeaders http.Header, headerFunc func(http.Header) http.Header) {
+	if baseHeaders == nil {
+		baseHeaders = http.Header{}
+	}
+
+	if headerFunc == nil {
+		headerFunc = func(h http.Header) http.Header {
+			return h
+		}
+	}
+
+	h.getHeader = func() http.Header {
+		requestHeader := headerFunc(baseHeaders.Clone())
+		requestHeader.Set(headerContentType, contentTypeProtobuf)
+		if h.compressionEnabled {
+			requestHeader.Set(headerContentEncoding, encodingTypeGZip)
+		}
+
+		return requestHeader
 	}
-	h.requestHeader = header
-	h.requestHeader.Set(headerContentType, contentTypeProtobuf)
 }
 
 // makeOneRequestRoundtrip sends a request and receives a response.
@@ -255,7 +269,7 @@ func (h *HTTPSender) prepareRequest(ctx context.Context) (*requestWrapper, error
 		return nil, err
 	}
 
-	req.Header = h.requestHeader
+	req.Header = h.getHeader()
 	return &req, nil
 }
 
@@ -295,9 +309,10 @@ func (h *HTTPSender) SetPollingInterval(duration time.Duration) {
 	atomic.StoreInt64(&h.pollingIntervalMs, duration.Milliseconds())
 }
 
+// EnableCompression enables compression for the sender.
+// Should not be called concurrently with Run.
 func (h *HTTPSender) EnableCompression() {
 	h.compressionEnabled = true
-	h.requestHeader.Set(headerContentEncoding, encodingTypeGZip)
 }
 
 func (h *HTTPSender) AddTLSConfig(config *tls.Config) {

client/types/startsettings.go

@@ -17,6 +17,11 @@ type StartSettings struct {
 	// Optional additional HTTP headers to send with all HTTP requests.
 	Header http.Header
 
+	// Optional function that can be used to modify the HTTP headers
+	// before each HTTP request.
+	// Can modify and return the argument or return the argument without modifying.
+	HeaderFunc func(http.Header) http.Header
+
 	// Optional TLS config for HTTP connection.
 	TLSConfig *tls.Config
 

client/wsclient.go

@@ -31,7 +31,7 @@ type wsClient struct {
 	url *url.URL
 
 	// HTTP request headers to use when connecting to OpAMP Server.
-	requestHeader http.Header
+	getHeader func() http.Header
 
 	// Websocket dialer and connection.
 	dialer    websocket.Dialer
@@ -86,7 +86,21 @@ func (c *wsClient) Start(ctx context.Context, settings types.StartSettings) erro
 	}
 	c.dialer.TLSClientConfig = settings.TLSConfig
 
-	c.requestHeader = settings.Header
+	headerFunc := settings.HeaderFunc
+	if headerFunc == nil {
+		headerFunc = func(h http.Header) http.Header {
+			return h
+		}
+	}
+
+	baseHeader := settings.Header
+	if baseHeader == nil {
+		baseHeader = http.Header{}
+	}
+
+	c.getHeader = func() http.Header {
+		return headerFunc(baseHeader.Clone())
+	}
 
 	c.common.StartConnectAndRun(c.runUntilStopped)
 
@@ -142,7 +156,7 @@ func (c *wsClient) SendCustomMessage(message *protobufs.CustomMessage) (messageS
 // by the Server.
 func (c *wsClient) tryConnectOnce(ctx context.Context) (retryAfter sharedinternal.OptionalDuration, err error) {
 	var resp *http.Response
-	conn, resp, err := c.dialer.DialContext(ctx, c.url.String(), c.requestHeader)
+	conn, resp, err := c.dialer.DialContext(ctx, c.url.String(), c.getHeader())
 	if err != nil {
 		if c.common.Callbacks != nil && !c.common.IsStopping() {
 			c.common.Callbacks.OnConnectFailed(ctx, err)