net/http/httputil: add ModifyResponse to reverseProxy

Adds ModifyResponse, an optional func to ReverseProxy
that modifies a response in the backend, right before
the headers of the response are written to the internal
response writer.
If ModifyResponse returns an error, the proxy returns
a StatusBadGateway error.

Fixes #14237.

Change-Id: I8e03139e34dea0084512ccbd8cc49e941bf9fb5d
Reviewed-on: https://go-review.googlesource.com/32356
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/net/http/httputil/reverseproxy.go b/src/net/http/httputil/reverseproxy.go
index f18dd886cc..7867505708 100644
--- a/src/net/http/httputil/reverseproxy.go
+++ b/src/net/http/httputil/reverseproxy.go
@@ -52,6 +52,11 @@ type ReverseProxy struct {
 	// get byte slices for use by io.CopyBuffer when
 	// copying HTTP response bodies.
 	BufferPool BufferPool
+
+	// ModifyResponse is an optional function that
+	// modifies the Response from the backend.
+	// If it returns an error, the proxy returns a StatusBadGateway error.
+	ModifyResponse func(*http.Response) error
 }
 
 // A BufferPool is an interface for getting and returning temporary
@@ -216,6 +221,14 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		res.Header.Del(h)
 	}
 
+	if p.ModifyResponse != nil {
+		if err := p.ModifyResponse(res); err != nil {
+			p.logf("http: proxy error: %v", err)
+			rw.WriteHeader(http.StatusBadGateway)
+			return
+		}
+	}
+
 	copyHeader(rw.Header(), res.Header)
 
 	// The "Trailer" header isn't included in the Transport's response,