[v12.x] http: fix crash for sync write errors during header parsing

lib/_http_client.js

@@ -431,8 +431,12 @@ function socketErrorListener(err) {
 
   const parser = socket.parser;
   if (parser) {
-    parser.finish();
-    freeParser(parser, req, socket);
+    // Use process.nextTick() on v12.x since 'error' events might be
+    // emitted synchronously from e.g. a failed write() call on the socket.
+    process.nextTick(() => {
+      parser.finish();
+      freeParser(parser, req, socket);
+    });
   }
 
   // Ensure that no further data will come out of the socket

test/parallel/test-http-sync-write-error-during-continue.js

@@ -0,0 +1,53 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const MakeDuplexPair = require('../common/duplexpair');
+
+// Regression test for the crash reported in
+// https://github.com/nodejs/node/issues/15102 (httpParser.finish() is called
+// during httpParser.execute()):
+
+{
+  const { clientSide, serverSide } = MakeDuplexPair();
+
+  serverSide.on('data', common.mustCall((data) => {
+    assert.strictEqual(data.toString('utf8'), `\
+GET / HTTP/1.1
+Expect: 100-continue
+Host: localhost:80
+Connection: close
+
+`.replace(/\n/g, '\r\n'));
+
+    setImmediate(() => {
+      serverSide.write('HTTP/1.1 100 Continue\r\n\r\n');
+    });
+  }));
+
+  const req = http.request({
+    createConnection: common.mustCall(() => clientSide),
+    headers: {
+      'Expect': '100-continue'
+    }
+  });
+  req.on('continue', common.mustCall((res) => {
+    let sync = true;
+
+    clientSide._writev = null;
+    clientSide._write = common.mustCall((chunk, enc, cb) => {
+      assert(sync);
+      // On affected versions of Node.js, the error would be emitted on `req`
+      // synchronously (i.e. before commit f663b31cc2aec), which would cause
+      // parser.finish() to be called while we are here in the 'continue'
+      // callback, which is inside a parser.execute() call.
+
+      assert.strictEqual(chunk.length, 0);
+      clientSide.destroy(new Error('sometimes the code just doesn’t work'), cb);
+    });
+    req.on('error', common.mustCall());
+    req.end();
+
+    sync = false;
+  }));
+}