net/http/cgi: allow CGI host to configure where child's stderr goes

Patch originally from Steven Hartland. Tweaked a bit & added a test.

Fixes #7197

Change-Id: I09012b4674e7c641dba31a24e9758cedb898d3ee
Reviewed-on: https://go-review.googlesource.com/21196Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/cgi/host.go

@@ -58,6 +58,7 @@ type Handler struct {
 	InheritEnv []string    // environment variables to inherit from host, as "key"
 	Logger     *log.Logger // optional log for errors or nil to use log.Print
 	Args       []string    // optional arguments to pass to child process
+	Stderr     io.Writer   // optional stderr for the child process; nil means os.Stderr
 
 	// PathLocationHandler specifies the root http Handler that
 	// should handle internal redirects when the CGI process
@@ -70,6 +71,13 @@ type Handler struct {
 	PathLocationHandler http.Handler
 }
 
+func (h *Handler) stderr() io.Writer {
+	if h.Stderr != nil {
+		return h.Stderr
+	}
+	return os.Stderr
+}
+
 // removeLeadingDuplicates remove leading duplicate in environments.
 // It's possible to override environment like following.
 //    cgi.Handler{
@@ -204,7 +212,7 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		Args:   append([]string{h.Path}, h.Args...),
 		Dir:    cwd,
 		Env:    env,
-		Stderr: os.Stderr, // for now
+		Stderr: h.stderr(),
 	}
 	if req.ContentLength != 0 {
 		cmd.Stdin = req.Body

src/net/http/cgi/host_test.go

@@ -8,6 +8,7 @@ package cgi
 
 import (
 	"bufio"
+	"bytes"
 	"fmt"
 	"io"
 	"net"
@@ -500,6 +501,23 @@ func TestEnvOverride(t *testing.T) {
 	runCgiTest(t, h, "GET /test.cgi HTTP/1.0\nHost: example.com\n\n", expectedMap)
 }
 
+func TestHandlerStderr(t *testing.T) {
+	check(t)
+	var stderr bytes.Buffer
+	h := &Handler{
+		Path:   "testdata/test.cgi",
+		Root:   "/test.cgi",
+		Stderr: &stderr,
+	}
+
+	rw := httptest.NewRecorder()
+	req := newRequest("GET /test.cgi?writestderr=1 HTTP/1.0\nHost: example.com\n\n")
+	h.ServeHTTP(rw, req)
+	if got, want := stderr.String(), "Hello, stderr!\n"; got != want {
+		t.Errorf("Stderr = %q; want %q", got, want)
+	}
+}
+
 func TestRemoveLeadingDuplicates(t *testing.T) {
 	tests := []struct {
 		env  []string

src/net/http/cgi/testdata/test.cgi

@@ -23,6 +23,10 @@ print "X-CGI-Pid: $$\r\n";
 print "X-Test-Header: X-Test-Value\r\n";
 print "\r\n";
 
+if ($params->{"writestderr"}) {
+    print STDERR "Hello, stderr!\n";
+}
+
 if ($params->{"bigresponse"}) {
     # 17 MB, for OS X: golang.org/issue/4958
     for (1..(17 * 1024)) {