net/http: add enforcement hook to Transport.RoundTrip, like our net ones

Updates #55
Updates tailscale/corp#12702

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

src/net/http/tailscale.go

@@ -0,0 +1,13 @@
+package http
+
+var roundTripEnforcer func(*Request) error
+
+func SetRoundTripEnforcer(f func(*Request) error) {
+	if f == nil {
+		panic("nil func")
+	}
+	if roundTripEnforcer != nil {
+		panic("already called")
+	}
+	roundTripEnforcer = f
+}

src/net/http/transport.go

@@ -511,6 +511,11 @@ func (t *Transport) alternateRoundTripper(req *Request) RoundTripper {
 
 // roundTrip implements a RoundTripper over HTTP.
 func (t *Transport) roundTrip(req *Request) (*Response, error) {
+	if roundTripEnforcer != nil {
+		if err := roundTripEnforcer(req); err != nil {
+			return nil, err
+		}
+	}
 	t.nextProtoOnce.Do(t.onceSetNextProtoDefaults)
 	ctx := req.Context()
 	trace := httptrace.ContextClientTrace(ctx)