net/http/httputil: log err encountered during reverseproxy body copying

Fixes #16659

Change-Id: I13dd797e93e0b572eaf8726f1be594870d40183b
Reviewed-on: https://go-review.googlesource.com/30692
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -260,12 +260,40 @@ func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 	if p.BufferPool != nil {
 		buf = p.BufferPool.Get()
 	}
-	io.CopyBuffer(dst, src, buf)
+	p.copyBuffer(dst, src, buf)
 	if p.BufferPool != nil {
 		p.BufferPool.Put(buf)
 	}
 }
 
+func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
+	if len(buf) == 0 {
+		buf = make([]byte, 32*1024)
+	}
+	var written int64
+	for {
+		nr, rerr := src.Read(buf)
+		if rerr != nil && rerr != io.EOF {
+			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
+		}
+		if nr > 0 {
+			nw, werr := dst.Write(buf[:nr])
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if werr != nil {
+				return written, werr
+			}
+			if nr != nw {
+				return written, io.ErrShortWrite
+			}
+		}
+		if rerr != nil {
+			return written, rerr
+		}
+	}
+}
+
 func (p *ReverseProxy) logf(format string, args ...interface{}) {
 	if p.ErrorLog != nil {
 		p.ErrorLog.Printf(format, args...)

src/net/http/httputil/reverseproxy_test.go

@@ -10,6 +10,7 @@ import (
 	"bufio"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"log"
@@ -581,3 +582,44 @@ func TestReverseProxy_NilBody(t *testing.T) {
 		t.Errorf("status code = %v; want 502 (Gateway Error)", res.Status)
 	}
 }
+
+// Issue 16659: log errors from short read
+func TestReverseProxy_CopyBuffer(t *testing.T) {
+	backendServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		out := "this call was relayed by the reverse proxy"
+		// Coerce a wrong content length to induce io.UnexpectedEOF
+		w.Header().Set("Content-Length", fmt.Sprintf("%d", len(out)*2))
+		fmt.Fprintln(w, out)
+	}))
+	defer backendServer.Close()
+
+	rpURL, err := url.Parse(backendServer.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var proxyLog bytes.Buffer
+	rproxy := NewSingleHostReverseProxy(rpURL)
+	rproxy.ErrorLog = log.New(&proxyLog, "", log.Lshortfile)
+	frontendProxy := httptest.NewServer(rproxy)
+	defer frontendProxy.Close()
+
+	resp, err := http.Get(frontendProxy.URL)
+	if err != nil {
+		t.Fatalf("failed to reach proxy: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if _, err := ioutil.ReadAll(resp.Body); err == nil {
+		t.Fatalf("want non-nil error")
+	}
+	expected := []string{
+		"EOF",
+		"read",
+	}
+	for _, phrase := range expected {
+		if !bytes.Contains(proxyLog.Bytes(), []byte(phrase)) {
+			t.Errorf("expected log to contain phrase %q", phrase)
+		}
+	}
+}