http/cgi: clean up environment.

clean up duplicate environment for CGI.
overriding former by latter.
On windows, When there are duplicated environments like following,

SCRIPT_FILENAME=c:/progra~1/php/php-cgi.exe
SCRIPT_FILENAME=/foo.php

CreateProcess use first entry.

If make cgi.Handle like following,

        cgih = cgi.Handler{
                Path: "c:/strawberry/perl/bin/perl.exe",
                Dir:  "c:/path/to/webroot",
                Root: "c:/path/to/webroot",
                Args: []string{"foo.php"},
                Env:  []string{"SCRIPT_FILENAME=foo.php"},
        }

http/cgi should behave "SCRIPT_FILENAME is foo.php".
But currently, http/cgi is set duplicate environment entries.
So, browser show binary dump of "php-cgi.exe" that is specified indented
SCRIPT_FILENAME in first entry.
This change clean up duplicates, and use latters.

R=golang-dev, bradfitz, bradfitz
CC=golang-dev
https://golang.org/cl/5010044

src/pkg/http/cgi/host.go

@@ -69,6 +69,31 @@ type Handler struct {
 	PathLocationHandler http.Handler
 }
 
+// removeLeadingDuplicates remove leading duplicate in environments.
+// It's possible to override environment like following.
+//    cgi.Handler{
+//      ...
+//      Env: []string{"SCRIPT_FILENAME=foo.php"},
+//    }
+func removeLeadingDuplicates(env []string) (ret []string) {
+	n := len(env)
+	for i := 0; i < n; i++ {
+		e := env[i]
+		s := strings.SplitN(e, "=", 2)[0]
+		found := false
+		for j := i + 1; j < n; j++ {
+			if s == strings.SplitN(env[j], "=", 2)[0] {
+				found = true
+				break
+			}
+		}
+		if !found {
+			ret = append(ret, e)
+		}
+	}
+	return
+}
+
 func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	root := h.Root
 	if root == "" {
@@ -150,6 +175,8 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	env = removeLeadingDuplicates(env)
+
 	var cwd, path string
 	if h.Dir != "" {
 		path = h.Path

src/pkg/http/cgi/host_test.go

@@ -447,3 +447,32 @@ func TestDirWindows(t *testing.T) {
 	}
 	runCgiTest(t, h, "GET /test.cgi HTTP/1.0\nHost: example.com\n\n", expectedMap)
 }
+
+func TestEnvOverride(t *testing.T) {
+	cgifile, _ := filepath.Abs("testdata/test.cgi")
+
+	var perl string
+	var err os.Error
+	perl, err = exec.LookPath("perl")
+	if err != nil {
+		return
+	}
+	perl, _ = filepath.Abs(perl)
+
+	cwd, _ := os.Getwd()
+	h := &Handler{
+		Path: perl,
+		Root: "/test.cgi",
+		Dir:  cwd,
+		Args: []string{cgifile},
+		Env: []string{
+			"SCRIPT_FILENAME=" + cgifile,
+			"REQUEST_URI=/foo/bar"},
+	}
+	expectedMap := map[string]string{
+		"cwd": cwd,
+		"env-SCRIPT_FILENAME": cgifile,
+		"env-REQUEST_URI":     "/foo/bar",
+	}
+	runCgiTest(t, h, "GET /test.cgi HTTP/1.0\nHost: example.com\n\n", expectedMap)
+}