rpc: add SetHeader method to Client (#21392)

Resolves #20163

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/http.go

@@ -26,6 +26,7 @@ import (
 	"io/ioutil"
 	"mime"
 	"net/http"
+	"net/url"
 	"sync"
 	"time"
 )
@@ -40,9 +41,11 @@ var acceptedContentTypes = []string{contentType, "application/json-rpc", "applic
 
 type httpConn struct {
 	client    *http.Client
-	req       *http.Request
+	url       string
 	closeOnce sync.Once
 	closeCh   chan interface{}
+	mu        sync.Mutex // protects headers
+	headers   http.Header
 }
 
 // httpConn is treated specially by Client.
@@ -51,7 +54,7 @@ func (hc *httpConn) writeJSON(context.Context, interface{}) error {
 }
 
 func (hc *httpConn) remoteAddr() string {
-	return hc.req.URL.String()
+	return hc.url
 }
 
 func (hc *httpConn) readBatch() ([]*jsonrpcMessage, bool, error) {
@@ -102,16 +105,24 @@ var DefaultHTTPTimeouts = HTTPTimeouts{
 // DialHTTPWithClient creates a new RPC client that connects to an RPC server over HTTP
 // using the provided HTTP Client.
 func DialHTTPWithClient(endpoint string, client *http.Client) (*Client, error) {
-	req, err := http.NewRequest(http.MethodPost, endpoint, nil)
+	// Sanity check URL so we don't end up with a client that will fail every request.
+	_, err := url.Parse(endpoint)
 	if err != nil {
 		return nil, err
 	}
-	req.Header.Set("Content-Type", contentType)
-	req.Header.Set("Accept", contentType)
 
 	initctx := context.Background()
+	headers := make(http.Header, 2)
+	headers.Set("accept", contentType)
+	headers.Set("content-type", contentType)
 	return newClient(initctx, func(context.Context) (ServerCodec, error) {
-		return &httpConn{client: client, req: req, closeCh: make(chan interface{})}, nil
+		hc := &httpConn{
+			client:  client,
+			headers: headers,
+			url:     endpoint,
+			closeCh: make(chan interface{}),
+		}
+		return hc, nil
 	})
 }
 
@@ -131,7 +142,7 @@ func (c *Client) sendHTTP(ctx context.Context, op *requestOp, msg interface{}) e
 		if respBody != nil {
 			buf := new(bytes.Buffer)
 			if _, err2 := buf.ReadFrom(respBody); err2 == nil {
-				return fmt.Errorf("%v %v", err, buf.String())
+				return fmt.Errorf("%v: %v", err, buf.String())
 			}
 		}
 		return err
@@ -166,10 +177,18 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 	if err != nil {
 		return nil, err
 	}
-	req := hc.req.WithContext(ctx)
-	req.Body = ioutil.NopCloser(bytes.NewReader(body))
+	req, err := http.NewRequestWithContext(ctx, "POST", hc.url, ioutil.NopCloser(bytes.NewReader(body)))
+	if err != nil {
+		return nil, err
+	}
 	req.ContentLength = int64(len(body))
 
+	// set headers
+	hc.mu.Lock()
+	req.Header = hc.headers.Clone()
+	hc.mu.Unlock()
+
+	// do request
 	resp, err := hc.client.Do(req)
 	if err != nil {
 		return nil, err