dashboard/buildlet: make exec handler return process state in HTTP trailer

Requires Go tip to function, but compiles with old versions of Go, but
without returning the trailer, which we'll be able to detect in the
coordinator to make sure nobody used an old Go to cross-compile a
buildlet.

Change-Id: Ie5f71c0a4477563dea5f1526af10a49aa41536d6
Reviewed-on: https://go-review.googlesource.com/2159
Reviewed-by: Andrew Gerrand <adg@golang.org>

dashboard/buildlet/buildlet.go

@@ -154,16 +154,32 @@ func untar(r io.Reader, dir string) error {
 	return nil
 }
 
+// Process-State is an HTTP Trailer set in the /exec handler to "ok"
+// on success, or os.ProcessState.String() on failure.
+const hdrProcessState = "Process-State"
+
 func handleExec(w http.ResponseWriter, r *http.Request) {
 	if r.Method != "POST" {
 		http.Error(w, "requires POST method", http.StatusBadRequest)
 		return
 	}
+	if r.ProtoMajor*10+r.ProtoMinor < 11 {
+		// We need trailers, only available in HTTP/1.1 or HTTP/2.
+		http.Error(w, "HTTP/1.1 or higher required", http.StatusBadRequest)
+		return
+	}
+
+	w.Header().Set("Trailer", hdrProcessState) // declare it so we can set it
+
 	cmdPath := r.FormValue("cmd") // required
 	if !validRelPath(cmdPath) {
 		http.Error(w, "requires 'cmd' parameter", http.StatusBadRequest)
 		return
 	}
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
+	}
+
 	absCmd := filepath.Join(*scratchDir, filepath.FromSlash(cmdPath))
 	cmd := exec.Command(absCmd, r.PostForm["cmdArg"]...)
 	cmd.Dir = filepath.Dir(absCmd)
@@ -171,9 +187,16 @@ func handleExec(w http.ResponseWriter, r *http.Request) {
 	cmd.Stdout = cmdOutput
 	cmd.Stderr = cmdOutput
 	err := cmd.Run()
-	log.Printf("Run = %v", err)
+	state := "ok"
-	// TODO: put the exit status in the HTTP trailer,
+	if err != nil {
-	// once https://golang.org/issue/7759 is fixed.
+		if ps := cmd.ProcessState; ps != nil {
+			state = ps.String()
+		} else {
+			state = err.Error()
+		}
+	}
+	w.Header().Set(hdrProcessState, state)
+	log.Printf("Run = %s", state)
 }
 
 // flushWriter is an io.Writer wrapper that writes to w and