http: split set-cookie when using setHeaders

lib/_http_outgoing.js

@@ -728,8 +728,23 @@ OutgoingMessage.prototype.setHeaders = function setHeaders(headers) {
     throw new ERR_INVALID_ARG_TYPE('headers', ['Headers', 'Map'], headers);
   }
 
-  for (const key of headers.keys()) {
-    this.setHeader(key, headers.get(key));
+  // Headers object joins multiple cookies with a comma when using
+  // the getter to retrieve the value,
+  // unless iterating over the headers directly.
+  // We also cannot safely split by comma.
+  // To avoid setHeader owerwriting the previous value we push
+  // set-cookie values in array and set them all at once.
+  const cookies = [];
+
+  for (const { 0: key, 1: value } of headers.entries()) {
+    if (key === 'set-cookie') {
+      cookies.push(value);
+      continue;
+    }
+    this.setHeader(key, value);
+  }
+  if (cookies.length) {
+    this.setHeader('set-cookie', cookies);
   }
 
   return this;

test/parallel/test-http-response-setheaders.js

@@ -129,3 +129,25 @@ const assert = require('assert');
     });
   }));
 }
+
+{
+  const server = http.createServer({ requireHostHeader: false }, common.mustCall((req, res) => {
+    const headers = new Headers();
+    headers.append('Set-Cookie', 'a=b');
+    headers.append('Set-Cookie', 'c=d');
+    res.setHeaders(headers);
+    res.end();
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    http.get({ port: server.address().port }, (res) => {
+      assert(Array.isArray(res.headers['set-cookie']));
+      assert.strictEqual(res.headers['set-cookie'].length, 2);
+      assert.strictEqual(res.headers['set-cookie'][0], 'a=b');
+      assert.strictEqual(res.headers['set-cookie'][1], 'c=d');
+      res.resume().on('end', common.mustCall(() => {
+        server.close();
+      }));
+    });
+  }));
+}