http: fix drain event with cork/uncork in ServerResponse

lib/_http_outgoing.js

@@ -287,14 +287,20 @@ OutgoingMessage.prototype.uncork = function uncork() {
       callbacks.push(buf[n + 2]);
     }
   }
-  this._send(crlf_buf, null, callbacks.length ? (err) => {
+  const ret = this._send(crlf_buf, null, callbacks.length ? (err) => {
     for (const callback of callbacks) {
       callback(err);
     }
   } : null);
 
   this[kChunkedBuffer].length = 0;
   this[kChunkedLength] = 0;
+
+  // If we successfully flushed and had pending drain, emit it
+  if (ret && this[kNeedDrain]) {
+    this[kNeedDrain] = false;
+    this.emit('drain');
+  }
 };
 
 OutgoingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {

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

@@ -0,0 +1,70 @@
+'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(100)), true);
+
+  // Write large amount that should require drain
+  const needsDrain = !res.write('2'.repeat(1000000));
+
+  if (needsDrain) {
+    // 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 if needed
+    res.uncork();
+
+    await drainPromise;
+
+    // Cork again for next write
+    res.cork();
+  }
+
+  // Write more data
+  res.write('3'.repeat(100));
+
+  // Final uncork and end
+  res.uncork();
+  res.end();
+}));
+
+server.listen(0, common.mustCall(() => {
+  http.get({
+    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, 100 + 1000000 + 100);
+      assert.strictEqual(data.substring(0, 100), '1'.repeat(100));
+      assert.strictEqual(data.substring(100, 1000100), '2'.repeat(1000000));
+      assert.strictEqual(data.substring(1000100), '3'.repeat(100));
+
+      server.close();
+    }));
+  }));
+}));
+