ipn/ipnlocal: proxy h2c grpc using net/http.Transport instead of x/net/http2

(Kinda related: #17351)

Updates #17305

Change-Id: I47df2612732a5713577164e74652bc9fa3cd14b3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/serve_test.go

@@ -15,6 +15,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
@@ -881,7 +882,7 @@ func mustCreateURL(t *testing.T, u string) url.URL {
 
 func newTestBackend(t *testing.T, opts ...any) *LocalBackend {
 	var logf logger.Logf = logger.Discard
-	const debug = true
+	const debug = false
 	if debug {
 		logf = logger.WithPrefix(tstest.WhileTestRunningLogger(t), "... ")
 	}
@@ -1085,3 +1086,88 @@ func TestEncTailscaleHeaderValue(t *testing.T) {
 		}
 	}
 }
+
+func TestServeGRPCProxy(t *testing.T) {
+	const msg = "some-response\n"
+	backend := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Path-Was", r.RequestURI)
+		w.Header().Set("Proto-Was", r.Proto)
+		io.WriteString(w, msg)
+	}))
+	backend.EnableHTTP2 = true
+	backend.Config.Protocols = new(http.Protocols)
+	backend.Config.Protocols.SetHTTP1(true)
+	backend.Config.Protocols.SetUnencryptedHTTP2(true)
+	backend.Start()
+	defer backend.Close()
+
+	backendURL := must.Get(url.Parse(backend.URL))
+
+	lb := newTestBackend(t)
+	rp := &reverseProxy{
+		logf:    t.Logf,
+		url:     backendURL,
+		backend: backend.URL,
+		lb:      lb,
+	}
+
+	req := func(method, urlStr string, opt ...any) *http.Request {
+		req := httptest.NewRequest(method, urlStr, nil)
+		for _, o := range opt {
+			switch v := o.(type) {
+			case int:
+				req.ProtoMajor = v
+			case string:
+				req.Header.Set("Content-Type", v)
+			default:
+				panic(fmt.Sprintf("unsupported option type %T", v))
+			}
+		}
+		return req
+	}
+
+	tests := []struct {
+		name      string
+		req       *http.Request
+		wantPath  string
+		wantProto string
+		wantBody  string
+	}{
+		{
+			name:      "non-gRPC",
+			req:       req("GET", "http://foo/bar"),
+			wantPath:  "/bar",
+			wantProto: "HTTP/1.1",
+		},
+		{
+			name:      "gRPC-but-not-http2",
+			req:       req("GET", "http://foo/bar", "application/grpc"),
+			wantPath:  "/bar",
+			wantProto: "HTTP/1.1",
+		},
+		{
+			name:      "gRPC--http2",
+			req:       req("GET", "http://foo/bar", 2, "application/grpc"),
+			wantPath:  "/bar",
+			wantProto: "HTTP/2.0",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rec := httptest.NewRecorder()
+			rp.ServeHTTP(rec, tt.req)
+
+			res := rec.Result()
+			got := must.Get(io.ReadAll(res.Body))
+			if got, want := res.Header.Get("Path-Was"), tt.wantPath; want != got {
+				t.Errorf("Path-Was %q, want %q", got, want)
+			}
+			if got, want := res.Header.Get("Proto-Was"), tt.wantProto; want != got {
+				t.Errorf("Proto-Was %q, want %q", got, want)
+			}
+			if string(got) != msg {
+				t.Errorf("got body %q, want %q", got, msg)
+			}
+		})
+	}
+}