internal/play: support plain text response

Fixes golang/go#51181

Change-Id: I82f10320e86555a161cbfb5c5b79dfc331017003
Reviewed-on: https://go-review.googlesource.com/c/website/+/388016
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Jamal Carvalho <jamalcarvalho@google.com>

cmd/golangorg/testdata/godev.txt

@@ -43,12 +43,22 @@ GET https://go.dev/play/p/asdfasdf
 body contains The Go Playground
 body contains About the Playground
 
+GET https://go.dev/play/p/MAohLsrz7JQ.go
+header Content-Type == text/plain; charset=utf-8
+body !contains The Go Playground
+body !contains About the Playground
+body contains Hello, 世界
+
 GET https://golang.google.cn/play/p/asdfasdf
 code == 500
 body contains Sorry, but shared playground snippets are not visible in China.
 body !contains The Go Playground
 body !contains About the Playground
 
+GET https://golang.google.cn/play/p/MAohLsrz7JQ.go
+code == 500
+body contains Sorry, but shared playground snippets are not visible in China.
+
 # These $GOROOT/*.md files should not serve.
 GET https://go.dev/CONTRIBUTING
 code == 404

internal/play/play.go

@@ -35,6 +35,10 @@ func playHandler(site *web.Site) http.Handler {
 			site.ServeError(w, r, errors.New("Sorry, but shared playground snippets are not visible in China."))
 			return
 		}
+		if strings.HasSuffix(r.URL.Path, ".go") {
+			simpleProxy(w, r, "https://"+backend(r)+strings.TrimPrefix(r.URL.Path, "/play"))
+			return
+		}
 		site.ServePage(w, r, web.Page{
 			"URL":          r.URL.Path,
 			"layout":       "play",