http: fix drain event with cork/uncork

lib/_http_outgoing.js

@@ -323,6 +323,12 @@ OutgoingMessage.prototype.uncork = function uncork() {
 
   this[kChunkedBuffer].length = 0;
   this[kChunkedLength] = 0;
+
+  // If we had a pending drain and flushed all data, emit the drain event.
+  if (this[kNeedDrain] && this.writableLength === 0) {
+    this[kNeedDrain] = false;
+    this.emit('drain');
+  }
 };
 
 OutgoingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {

test/parallel/test-http-response-drain-cork.js

@@ -0,0 +1,64 @@
+'use strict';
+const common = require('../common');
+const http = require('http');
+const assert = require('assert');
+
+// Test that drain event is emitted correctly when using cork/uncork
+// with ServerResponse and the write buffer is full
+
+const server = http.createServer(common.mustCall(async (req, res) => {
+  res.cork();
+
+  // Write small amount - won't need drain
+  assert.strictEqual(res.write('1'.repeat(10)), true);
+
+  // Write enough to exceed highWaterMark (set in 'connection' listener)
+  assert.strictEqual(res.write('2'.repeat(1000)), false);
+
+  // Verify writableNeedDrain is set
+  assert.strictEqual(res.writableNeedDrain, true);
+
+  // Wait for drain event after uncorking
+  const drainPromise = new Promise((resolve) => {
+    res.once('drain', common.mustCall(() => {
+      // After drain, writableNeedDrain should be false
+      assert.strictEqual(res.writableNeedDrain, false);
+      resolve();
+    }));
+  });
+
+  // Uncork should trigger drain
+  res.uncork();
+  await drainPromise;
+
+  res.end();
+}));
+
+server.on('connection', common.mustCall((socket) => {
+  // Set high water mark large enough to cover HTTP overhead + first
+  // small content batch, but not enough to cover second batch.
+  socket._writableState.highWaterMark = 1000;
+}));
+
+server.listen(0, common.localhostIPv4, common.mustCall(() => {
+  http.get({
+    host: common.localhostIPv4,
+    port: server.address().port,
+  }, common.mustCall((res) => {
+    let data = '';
+    res.setEncoding('utf8');
+
+    res.on('data', (chunk) => {
+      data += chunk;
+    });
+
+    res.on('end', common.mustCall(() => {
+      // Verify we got all the data
+      assert.strictEqual(data.length, 10 + 1000);
+      assert.strictEqual(data.substring(0, 10), '1'.repeat(10));
+      assert.strictEqual(data.substring(10), '2'.repeat(1000));
+
+      server.close(common.mustCall());
+    }));
+  }));
+}));