http2: fix endless loop when write an empty string

lib/internal/http2/core.js

@@ -1622,6 +1622,11 @@ class Http2Stream extends Duplex {
     if (!this.headersSent)
       this[kProceed]();
 
+    if (!data.length) {
+      cb();
+      return;
+    }
+
     const handle = this[kHandle];
     const req = new WriteWrap();
     req.stream = this[kID];
@@ -1659,6 +1664,11 @@ class Http2Stream extends Duplex {
     if (!this.headersSent)
       this[kProceed]();
 
+    if (!data.length) {
+      cb();
+      return;
+    }
+
     const handle = this[kHandle];
     const req = new WriteWrap();
     req.stream = this[kID];

test/parallel/test-http2-client-write-empty-string.js

@@ -0,0 +1,48 @@
+'use strict';
+
+const assert = require('assert');
+const http2 = require('http2');
+
+const common = require('../common');
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+
+const server = http2.createServer();
+server.on('stream', common.mustCall((stream, headers, flags) => {
+  stream.respond({ 'content-type': 'text/html' });
+
+  let data = '';
+  stream.on('data', common.mustNotCall((chunk) => {
+    data += chunk.toString();
+  }));
+  stream.on('end', common.mustCall(() => {
+    stream.end(`"${data}"`);
+  }));
+}));
+
+server.listen(0, common.mustCall(() => {
+  const port = server.address().port;
+  const client = http2.connect(`http://localhost:${port}`);
+
+  const req = client.request({
+    ':method': 'POST',
+    ':path': '/'
+  });
+
+  req.on('response', common.mustCall((headers) => {
+    assert.strictEqual(headers[':status'], 200);
+    assert.strictEqual(headers['content-type'], 'text/html');
+  }));
+
+  let data = '';
+  req.setEncoding('utf8');
+  req.on('data', common.mustCallAtLeast((d) => data += d));
+  req.on('end', common.mustCall(() => {
+    assert.strictEqual(data, '""');
+    server.close();
+    client.close();
+  }));
+
+  req.write('');
+  req.end();
+}));