Repository: open-telemetry/opamp-go (go)

Issue:
Support dynamically generating headers for each client request
I would like to use the headers for token-based authentication, e.g. oAuth2. In this case, tokens may expire and need to be refreshed.

However, opamp-go currently only allows specifying a fixed set of headers in the start settings for the client. 

Instead, it would be useful to provide a function that can dynamically determine headers, so that tokens could be refreshed for authentication, e.g.:

```go
// StartSettings defines the parameters for starting the OpAMP Client.
type StartSettings struct {
  // Current settings remain unchanged ...
 
  HeaderFunc func(base http.Header) http.Header
}
```

Here, HeaderFunc is a function that takes in the "base" static headers, and outputs a set of new headers. This would be called on every request sent to the server, and would allow for dynamic authentication headers in case of token refresh, for instance.

Pull request description:
Add HeaderFunc to allow modifying headers before every request
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

Patch:
diff --git a/client/httpclient.go b/client/httpclient.go
index 666747c..92259d5 100644
--- a/client/httpclient.go
+++ b/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)
diff --git a/client/internal/httpsender.go b/client/internal/httpsender.go
index e48e3ce..502bf7e 100644
--- a/client/internal/httpsender.go
+++ b/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) {
diff --git a/client/types/startsettings.go b/client/types/startsettings.go
index 857fd2a..b968c7f 100644
--- a/client/types/startsettings.go
+++ b/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
 
diff --git a/client/wsclient.go b/client/wsclient.go
index 22d434c..dc703dc 100644
--- a/client/wsclient.go
+++ b/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)


Test patch:
diff --git a/client/clientimpl_test.go b/client/clientimpl_test.go
index e41f455..76ca41a 100644
--- a/client/clientimpl_test.go
+++ b/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.