avoid write-after-end by checking res.writableEnded

new in 12.9, but falsy undefined should be harmless for old nodes,
they just won't get the improved behavior

Author: minrk

lib/configproxy.js

@@ -36,13 +36,15 @@ function argumentsArray(args) {
 function fail(req, res, code, msg) {
   // log a failure, and finish the HTTP request with an error code
   msg = msg || "";
+  res._logMsg = msg;
+
+  if (res.writableEnded) return; // response already done
   if (res.writeHead) res.writeHead(code);
   if (res.write) {
     if (!msg) {
       msg = http.STATUS_CODES[code];
     }
     res.write(msg);
-    res._logMsg = msg;
   }
   if (res.end) res.end();
 }
@@ -410,6 +412,7 @@ class ConfigurableProxy extends EventEmitter {
   _handleProxyErrorDefault(code, kind, req, res) {
     // called when no custom error handler is registered,
     // or is registered and doesn't work
+    if (res.writableEnded) return; // response already done
     if (!res.headersSent && res.writeHead) res.writeHead(code);
     if (res.write) res.write(http.STATUS_CODES[code]);
     if (res.end) res.end();
@@ -468,13 +471,14 @@ class ConfigurableProxy extends EventEmitter {
       }
 
       var errorRequest = (secure ? https : http).request(target, function (upstream) {
+        if (res.writableEnded) return; // response already done
         ["content-type", "content-encoding"].map(function (key) {
           if (!upstream.headers[key]) return;
           if (res.setHeader) res.setHeader(key, upstream.headers[key]);
         });
         if (res.writeHead) res.writeHead(code);
         upstream.on("data", (data) => {
-          if (res.write) res.write(data);
+          if (res.write && !res.writableEnded) res.write(data);
         });
         upstream.on("end", () => {
           if (res.end) res.end();
@@ -503,6 +507,7 @@ class ConfigurableProxy extends EventEmitter {
           this._handleProxyErrorDefault(code, kind, req, res);
           return;
         }
+        if (res.writableEnded) return; // response already done
         if (res.writeHead) res.writeHead(code, { "Content-Type": "text/html" });
         if (res.write) res.write(data);
         if (res.end) res.end();