rpc: support injecting HTTP headers through context (#26023)

This adds a way to specify HTTP headers per request.
Co-authored-by: Martin Holst Swende <martin@swende.se>
Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/context_headers.go

+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package rpc
+
+import (
+	"context"
+	"net/http"
+)
+
+type mdHeaderKey struct{}
+
+// NewContextWithHeaders wraps the given context, adding HTTP headers. These headers will
+// be applied by Client when making a request using the returned context.
+func NewContextWithHeaders(ctx context.Context, h http.Header) context.Context {
+	if len(h) == 0 {
+		// This check ensures the header map set in context will never be nil.
+		return ctx
+	}
+
+	var ctxh http.Header
+	prev, ok := ctx.Value(mdHeaderKey{}).(http.Header)
+	if ok {
+		ctxh = setHeaders(prev.Clone(), h)
+	} else {
+		ctxh = h.Clone()
+	}
+	return context.WithValue(ctx, mdHeaderKey{}, ctxh)
+}
+
+// headersFromContext is used to extract http.Header from context.
+func headersFromContext(ctx context.Context) http.Header {
+	source, _ := ctx.Value(mdHeaderKey{}).(http.Header)
+	return source
+}
+
+// setHeaders sets all headers from src in dst.
+func setHeaders(dst http.Header, src http.Header) http.Header {
+	for key, values := range src {
+		dst[http.CanonicalHeaderKey(key)] = values
+	}
+	return dst
+}

rpc/http.go

@@ -217,6 +217,8 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 	hc.mu.Lock()
 	req.Header = hc.headers.Clone()
 	hc.mu.Unlock()
+	setHeaders(req.Header, headersFromContext(ctx))
+
 	if hc.auth != nil {
 		if err := hc.auth(req.Header); err != nil {
 			return nil, err

rpc/http_test.go

@@ -17,6 +17,8 @@
 package rpc
 
 import (
+	"context"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -198,3 +200,43 @@ func TestHTTPPeerInfo(t *testing.T) {
 		t.Errorf("wrong HTTP.Origin %q", info.HTTP.UserAgent)
 	}
 }
+
+func TestNewContextWithHeaders(t *testing.T) {
+	expectedHeaders := 0
+	server := httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
+		for i := 0; i < expectedHeaders; i++ {
+			key, want := fmt.Sprintf("key-%d", i), fmt.Sprintf("val-%d", i)
+			if have := request.Header.Get(key); have != want {
+				t.Errorf("wrong request headers for %s, want: %s, have: %s", key, want, have)
+			}
+		}
+		writer.WriteHeader(http.StatusOK)
+		_, _ = writer.Write([]byte(`{}`))
+	}))
+	defer server.Close()
+
+	client, err := Dial(server.URL)
+	if err != nil {
+		t.Fatalf("failed to dial: %s", err)
+	}
+	defer client.Close()
+
+	newHdr := func(k, v string) http.Header {
+		header := http.Header{}
+		header.Set(k, v)
+		return header
+	}
+	ctx1 := NewContextWithHeaders(context.Background(), newHdr("key-0", "val-0"))
+	ctx2 := NewContextWithHeaders(ctx1, newHdr("key-1", "val-1"))
+	ctx3 := NewContextWithHeaders(ctx2, newHdr("key-2", "val-2"))
+
+	expectedHeaders = 3
+	if err := client.CallContext(ctx3, nil, "test"); err != ErrNoResult {
+		t.Error("call failed", err)
+	}
+
+	expectedHeaders = 2
+	if err := client.CallContext(ctx2, nil, "test"); err != ErrNoResult {
+		t.Error("call failed:", err)
+	}
+}