net/http/httputil: don't panic in ReverseProxy unless running under a Server

Prior to the fix to #23643, the ReverseProxy didn't panic with
ErrAbortHandler when the copy to a client failed.

During Go 1.11 beta testing, we found plenty of code using
ReverseProxy in tests that were unprepared for a panic.

Change the behavior to only panic when running under the http.Server
that'll handle the panic.

Updates #23643

Change-Id: Ic1fa8405fd54c858ce8c797cec79d006833a9f7d
Reviewed-on: https://go-review.googlesource.com/122819Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -253,7 +253,11 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		defer res.Body.Close()
 		// Since we're streaming the response, if we run into an error all we can do
 		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
-		// on read error while copying body
+		// on read error while copying body.
+		if !shouldPanicOnCopyError(req) {
+			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
+			return
+		}
 		panic(http.ErrAbortHandler)
 	}
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer
@@ -271,6 +275,28 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 }
 
+var inOurTests bool // whether we're in our own tests
+
+// shouldPanicOnCopyError reports whether the reverse proxy should
+// panic with http.ErrAbortHandler. This is the right thing to do by
+// default, but Go 1.10 and earlier did not, so existing unit tests
+// weren't expecting panics. Only panic in our own tests, or when
+// running under the HTTP server.
+func shouldPanicOnCopyError(req *http.Request) bool {
+	if inOurTests {
+		// Our tests know to handle this panic.
+		return true
+	}
+	if req.Context().Value(http.ServerContextKey) != nil {
+		// We seem to be running under an HTTP server, so
+		// it'll recover the panic.
+		return true
+	}
+	// Otherwise act like Go 1.10 and earlier to not break
+	// existing tests.
+	return false
+}
+
 // removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
 // See RFC 7230, section 6.1
 func removeConnectionHeaders(h http.Header) {

src/net/http/httputil/reverseproxy_test.go

@@ -29,6 +29,7 @@ import (
 const fakeHopHeader = "X-Fake-Hop-Header-For-Test"
 
 func init() {
+	inOurTests = true
 	hopHeaders = append(hopHeaders, fakeHopHeader)
 }