Symbolicate stack traces off the main process

Summary:
Moves stack trace symbolication to a worker process.

The worker process is spawned laziliy, and is treated as an exclusive resource (requests are queued).

This helps keeping the server process responsive when symbolicating.

Reviewed By: cpojer

Differential Revision: D4602722

fbshipit-source-id: 5da97e53afd9a1ab981c5ba4b02a7d1d869dee71

packages/metro-bundler/package.json

@@ -17,6 +17,7 @@
     "babel-register": "^6.18.0",
     "babylon": "^6.14.1",
     "chalk": "^1.1.1",
+    "concat-stream": "^1.6.0",
     "core-js": "^2.2.2",
     "debug": "^2.2.0",
     "denodeify": "^1.2.1",
@@ -38,6 +39,7 @@
     "throat": "^3.0.0",
     "uglify-js": "^2.6.2",
     "worker-farm": "^1.3.1",
-    "write-file-atomic": "^1.2.0"
+    "write-file-atomic": "^1.2.0",
+    "xpipe": "^1.0.5"
   }
 }

packages/metro-bundler/src/Server/__tests__/Server-test.js

@@ -10,11 +10,14 @@
 
 jest.disableAutomock();
 
-jest.setMock('worker-farm', function() { return () => {}; })
+jest.mock('worker-farm', () => () => () => {})
-    .setMock('timers', { setImmediate: (fn) => setTimeout(fn, 0) })
+    .mock('timers', () => ({ setImmediate: (fn) => setTimeout(fn, 0) }))
-    .setMock('uglify-js')
+    .mock('uglify-js')
-    .setMock('crypto')
+    .mock('crypto')
-    .setMock('source-map', { SourceMapConsumer: function(fn) {}})
+    .mock(
+      '../symbolicate',
+      () => ({createWorker: jest.fn().mockReturnValue(jest.fn())}),
+    )
     .mock('../../Bundler')
     .mock('../../AssetServer')
     .mock('../../lib/declareOpts')
@@ -23,14 +26,14 @@ jest.setMock('worker-farm', function() { return () => {}; })
     .mock('../../lib/GlobalTransformCache');
 
 describe('processRequest', () => {
-  let SourceMapConsumer, Bundler, Server, AssetServer, Promise;
+  let Bundler, Server, AssetServer, Promise, symbolicate;
   beforeEach(() => {
     jest.resetModules();
-    SourceMapConsumer = require('source-map').SourceMapConsumer;
     Bundler = require('../../Bundler');
     Server = require('../');
     AssetServer = require('../../AssetServer');
     Promise = require('promise');
+    symbolicate = require('../symbolicate');
   });
 
   let server;
@@ -69,7 +72,7 @@ describe('processRequest', () => {
       Promise.resolve({
         getModules: () => [],
         getSource: () => 'this is the source',
-        getSourceMap: () => {},
+        getSourceMap: () => ({version: 3}),
         getSourceMapString: () => 'this is the source map',
         getEtag: () => 'this is an etag',
       }));
@@ -464,60 +467,38 @@ describe('processRequest', () => {
   });
 
   describe('/symbolicate endpoint', () => {
+    let symbolicationWorker;
+    beforeEach(() => {
+      symbolicationWorker = symbolicate.createWorker();
+      symbolicationWorker.mockReset();
+    });
+
     it('should symbolicate given stack trace', () => {
-      const body = JSON.stringify({stack: [{
+      const inputStack = [{
         file: 'http://foo.bundle?platform=ios',
         lineNumber: 2100,
         column: 44,
         customPropShouldBeLeftUnchanged: 'foo',
-      }]});
+      }];
-
+      const outputStack = [{
-      SourceMapConsumer.prototype.originalPositionFor = jest.fn((frame) => {
-        expect(frame.line).toEqual(2100);
-        expect(frame.column).toEqual(44);
-        return {
         source: 'foo.js',
         line: 21,
         column: 4,
-        };
+      }];
+      const body = JSON.stringify({stack: inputStack});
+
+      expect.assertions(2);
+      symbolicationWorker.mockImplementation(stack => {
+        expect(stack).toEqual(inputStack);
+        return outputStack;
       });
 
       return makeRequest(
         requestHandler,
         '/symbolicate',
-        { rawBody: body }
+        {rawBody: body},
-      ).then(response => {
+      ).then(response =>
-        expect(JSON.parse(response.body)).toEqual({
+        expect(JSON.parse(response.body)).toEqual({stack: outputStack}));
-          stack: [{
-            file: 'foo.js',
-            lineNumber: 21,
-            column: 4,
-            customPropShouldBeLeftUnchanged: 'foo',
-          }]
-        });
-      });
-    });
-
-    it('ignores `/debuggerWorker.js` stack frames', () => {
-      const body = JSON.stringify({stack: [{
-        file: 'http://localhost:8081/debuggerWorker.js',
-        lineNumber: 123,
-        column: 456,
-      }]});
-
-      return makeRequest(
-        requestHandler,
-        '/symbolicate',
-        { rawBody: body }
-      ).then(response => {
-        expect(JSON.parse(response.body)).toEqual({
-          stack: [{
-            file: 'http://localhost:8081/debuggerWorker.js',
-            lineNumber: 123,
-            column: 456,
-          }]
-        });
-      });
     });
   });
 

packages/metro-bundler/src/Server/index.js

@@ -15,12 +15,12 @@ const AssetServer = require('../AssetServer');
 const getPlatformExtension = require('../node-haste').getPlatformExtension;
 const Bundler = require('../Bundler');
 const MultipartResponse = require('./MultipartResponse');
-const SourceMapConsumer = require('source-map').SourceMapConsumer;
 
 const declareOpts = require('../lib/declareOpts');
 const defaults = require('../../defaults');
 const mime = require('mime-types');
 const path = require('path');
+const symbolicate = require('./symbolicate');
 const terminal = require('../lib/terminal');
 const url = require('url');
 
@@ -34,6 +34,7 @@ import type Bundle from '../Bundler/Bundle';
 import type {Reporter} from '../lib/reporting';
 import type {GetTransformOptions} from '../Bundler';
 import type GlobalTransformCache from '../lib/GlobalTransformCache';
+import type {SourceMap, Symbolicate} from './symbolicate';
 
 const {
   createActionStartEntry,
@@ -201,6 +202,7 @@ class Server {
   _debouncedFileChangeHandler: (filePath: string) => mixed;
   _hmrFileChangeListener: (type: string, filePath: string) => mixed;
   _reporter: Reporter;
+  _symbolicateInWorker: Symbolicate;
 
   constructor(options: Options) {
     this._opts = {
@@ -281,6 +283,8 @@ class Server {
       }
       this._informChangeWatchers();
     }, 50);
+
+    this._symbolicateInWorker = symbolicate.createWorker();
   }
 
   end(): mixed {
@@ -802,45 +806,27 @@ class Server {
 
       // In case of multiple bundles / HMR, some stack frames can have
       // different URLs from others
-      const urlIndexes = {};
+      const urls = new Set();
-      const uniqueUrls = [];
       stack.forEach(frame => {
         const sourceUrl = frame.file;
         // Skip `/debuggerWorker.js` which drives remote debugging because it
         // does not need to symbolication.
         // Skip anything except http(s), because there is no support for that yet
-        if (!urlIndexes.hasOwnProperty(sourceUrl) &&
+        if (!urls.has(sourceUrl) &&
             !sourceUrl.endsWith('/debuggerWorker.js') &&
             sourceUrl.startsWith('http')) {
-          urlIndexes[sourceUrl] = uniqueUrls.length;
+          urls.add(sourceUrl);
-          uniqueUrls.push(sourceUrl);
         }
       });
 
-      const sourceMaps = uniqueUrls.map(
+      const mapPromises =
-        sourceUrl => this._sourceMapForURL(sourceUrl)
+        Array.from(urls.values()).map(this._sourceMapForURL, this);
-      );
+
-      return Promise.all(sourceMaps).then(consumers => {
+      debug('Getting source maps for symbolication');
-        return stack.map(frame => {
+      return Promise.all(mapPromises).then(maps => {
-          const sourceUrl = frame.file;
+        debug('Sending stacks and maps to symbolication worker');
-          if (!urlIndexes.hasOwnProperty(sourceUrl)) {
+        const urlsToMaps = zip(urls.values(), maps);
-            return frame;
+        return this._symbolicateInWorker(stack, urlsToMaps);
-          }
-          const idx = urlIndexes[sourceUrl];
-          const consumer = consumers[idx];
-          const original = consumer.originalPositionFor({
-            line: frame.lineNumber,
-            column: frame.column,
-          });
-          if (!original) {
-            return frame;
-          }
-          return Object.assign({}, frame, {
-            file: original.source,
-            lineNumber: original.line,
-            column: original.column,
-          });
-        });
       });
     }).then(
       stack => {
@@ -858,16 +844,13 @@ class Server {
     );
   }
 
-  _sourceMapForURL(reqUrl: string): Promise<SourceMapConsumer> {
+  _sourceMapForURL(reqUrl: string): Promise<SourceMap> {
     const options = this._getOptionsFromUrl(reqUrl);
     const building = this._useCachedOrUpdateOrCreateBundle(options);
-    return building.then(p => {
+    return building.then(p => p.getSourceMap({
-      const sourceMap = p.getSourceMap({
       minify: options.minify,
       dev: options.dev,
-      });
+    }));
-      return new SourceMapConsumer(sourceMap);
-    });
   }
 
   _handleError(res: ServerResponse, bundleID: string, error: {
@@ -990,4 +973,16 @@ function contentsEqual(array: Array<mixed>, set: Set<mixed>): boolean {
   return array.length === set.size && array.every(set.has, set);
 }
 
+function* zip<X, Y>(xs: Iterable<X>, ys: Iterable<Y>): Iterable<[X, Y]> {
+  //$FlowIssue #9324959
+  const ysIter: Iterator<Y> = ys[Symbol.iterator]();
+  for (const x of xs) {
+    const y = ysIter.next();
+    if (y.done) {
+      return;
+    }
+    yield [x, y.value];
+  }
+}
+
 module.exports = Server;

packages/metro-bundler/src/Server/symbolicate/__tests__/symbolicate-test.js

@@ -0,0 +1,106 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+jest.disableAutomock();
+jest.mock('child_process');
+jest.mock('net');
+
+const EventEmitter = require('events');
+const {Readable} = require('stream');
+const {createWorker} = require('../');
+
+let childProcess, socketResponse, socket, worker;
+
+beforeEach(() => {
+  childProcess = Object.assign(new EventEmitter(), {send: jest.fn()});
+  require('child_process').fork.mockReturnValueOnce(childProcess);
+  setupCommunication();
+
+  socketResponse = '{"error": "no fake socket response set"}';
+  socket = Object.assign(new Readable(), {
+    _read() {
+      this.push(socketResponse);
+      this.push(null);
+    },
+    end: jest.fn(),
+    setEncoding: jest.fn(),
+  });
+  require('net').createConnection.mockImplementation(() => socket);
+
+  worker = createWorker();
+});
+
+it('sends a socket path to the child process', () => {
+  socketResponse = '{}';
+  return worker([], fakeSourceMaps())
+    .then(() => expect(childProcess.send).toBeCalledWith(expect.any(String)));
+});
+
+it('fails if the child process emits an error', () => {
+  const error = new Error('Expected error');
+  childProcess.send.mockImplementation(() =>
+    childProcess.emit('error', error));
+
+  expect.assertions(1);
+  return worker([], fakeSourceMaps())
+    .catch(e => expect(e).toBe(error));
+});
+
+it('fails if the socket connection emits an error', () => {
+  const error = new Error('Expected error');
+  socket._read = () => socket.emit('error', error);
+
+  expect.assertions(1);
+  return worker([], fakeSourceMaps())
+    .catch(e => expect(e).toBe(error));
+});
+
+it('sends the passed in stack and maps over the socket', () => {
+  socketResponse = '{}';
+  const stack = ['the', 'stack'];
+  return worker(stack, fakeSourceMaps())
+    .then(() =>
+      expect(socket.end).toBeCalledWith(JSON.stringify({
+        maps: Array.from(fakeSourceMaps()),
+        stack,
+      })));
+});
+
+it('resolves to the `result` property of the message returned over the socket', () => {
+  socketResponse = '{"result": {"the": "result"}}';
+  return worker([], fakeSourceMaps())
+    .then(response => expect(response).toEqual({the: 'result'}));
+});
+
+it('rejects with the `error` property of the message returned over the socket', () => {
+  socketResponse = '{"error": "the error message"}';
+
+  expect.assertions(1);
+  return worker([], fakeSourceMaps())
+    .catch(error => expect(error).toEqual(new Error('the error message')));
+});
+
+it('rejects if the socket response cannot be parsed as JSON', () => {
+  socketResponse = '{';
+
+  expect.assertions(1);
+  return worker([], fakeSourceMaps())
+    .catch(error => expect(error).toBeInstanceOf(SyntaxError));
+});
+
+function setupCommunication() {
+  childProcess.send.mockImplementation(() =>
+    process.nextTick(() => childProcess.emit('message')));
+}
+
+function* fakeSourceMaps() {
+  yield [1, {}];
+  yield [2, {}];
+}

packages/metro-bundler/src/Server/symbolicate/__tests__/util-test.js

@@ -0,0 +1,157 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+jest.disableAutomock();
+
+const {LazyPromise, LockingPromise} = require('../util');
+
+describe('Lazy Promise', () => {
+  let factory;
+  const value = {};
+
+  beforeEach(() => {
+    factory = jest.fn();
+    factory.mockReturnValue(Promise.resolve(value));
+  });
+
+  it('does not run the factory by default', () => {
+    new LazyPromise(factory); // eslint-disable-line no-new
+    expect(factory).not.toBeCalled();
+  });
+
+  it('calling `.then()` returns a promise', () => {
+    expect(new LazyPromise(factory).then()).toBeInstanceOf(Promise);
+  });
+
+  it('does not invoke the factory twice', () => {
+    const p = new LazyPromise(factory);
+    p.then(x => x);
+    p.then(x => x);
+    expect(factory).toHaveBeenCalledTimes(1);
+  });
+
+  describe('value and error propagation', () => {
+    it('resolves to the value provided by the factory', () => {
+      expect.assertions(1);
+      return new LazyPromise(factory)
+        .then(v => expect(v).toBe(value));
+    });
+
+    it('passes through errors if not handled', () => {
+      const error = new Error('Unhandled');
+      factory.mockReturnValue(Promise.reject(error));
+
+      expect.assertions(1);
+      return new LazyPromise(factory)
+        .then()
+        .catch(e => expect(e).toBe(error));
+    });
+
+    it('uses rejection handlers passed to `then()`', () => {
+      const error = new Error('Must be handled');
+      factory.mockReturnValue(Promise.reject(error));
+
+      expect.assertions(1);
+      return new LazyPromise(factory)
+        .then(() => {}, e => expect(e).toBe(error));
+    });
+
+    it('uses rejection handlers passed to `catch()`', () => {
+      const error = new Error('Must be handled');
+      factory.mockReturnValue(Promise.reject(error));
+
+      expect.assertions(1);
+      return new LazyPromise(factory)
+        .catch(e => expect(e).toBe(error));
+    });
+  });
+});
+
+describe('Locking Promise', () => {
+  it('resolves to the value of the passed-in promise', () => {
+    const value = {};
+
+    expect.assertions(1);
+    return new LockingPromise(Promise.resolve(value))
+      .then(v => expect(v).toBe(value));
+  });
+
+  it('passes through rejections', () => {
+    const error = new Error('Rejection');
+
+    expect.assertions(1);
+    return new LockingPromise(Promise.reject(error))
+      .then()
+      .catch(e => expect(e).toBe(error));
+  });
+
+  it('uses rejection handlers passed to `then()`', () => {
+    const error = new Error('Must be handled');
+
+    expect.assertions(1);
+    return new LockingPromise(Promise.reject(error))
+      .then(x => x, e => expect(e).toBe(error));
+  });
+
+  it('uses rejection handlers passed to `catch()`', () => {
+    const error = new Error('Must be handled');
+
+    expect.assertions(1);
+    return new LockingPromise(Promise.reject(error))
+      .catch(e => expect(e).toBe(error));
+  });
+
+  describe('locking', () => {
+    const value = Symbol;
+    let locking;
+    beforeEach(() => {
+      locking = new LockingPromise(Promise.resolve(value));
+    });
+
+
+    it('only allows one handler to access the promise value', () => {
+      const deferred = defer();
+      const secondHandler = jest.fn();
+      locking.then(() => deferred.promise);
+      locking.then(secondHandler);
+      return Promise.resolve() // wait for the next tick
+        .then(() => expect(secondHandler).not.toBeCalled());
+    });
+
+    it('allows waiting handlers to access the value after the current handler resolves', () => {
+      let counter = 0;
+
+      const deferred = defer();
+      const x = locking.then(v => {
+        const result = [++counter, v];
+        return deferred.promise.then(() => result);
+      });
+      const y = locking.then(v => [++counter, v]);
+      const z = locking.then(v => [++counter, v]);
+
+      deferred.resolve();
+
+      return Promise.all([x, y, z])
+        .then(([first, second, third]) => {
+          expect(first).toEqual([1, value]);
+          expect(second).toEqual([2, value]);
+          expect(third).toEqual([3, value]);
+        });
+    });
+  });
+});
+
+
+function defer() {
+  let resolve;
+  const promise = new Promise(res => { resolve = res; });
+  return {promise, resolve};
+}

packages/metro-bundler/src/Server/symbolicate/__tests__/worker-test.js

@@ -0,0 +1,111 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+jest.disableAutomock();
+
+const SourceMapGenerator = require('../../../Bundler/source-map/Generator');
+const {symbolicate} = require('../worker');
+
+let connection;
+beforeEach(() => {
+  connection = {end: jest.fn()};
+});
+
+it('symbolicates stack frames', () => {
+  const mappings = [
+    {
+      from: {file: 'bundle1.js', lineNumber: 1, column: 2},
+      to: {file: 'apples.js', lineNumber: 12, column: 34},
+    },
+    {
+      from: {file: 'bundle2.js', lineNumber: 3, column: 4},
+      to: {file: 'bananas.js', lineNumber: 56, column: 78},
+    },
+    {
+      from: {file: 'bundle1.js', lineNumber: 5, column: 6},
+      to: {file: 'clementines.js', lineNumber: 90, column: 12},
+    },
+  ];
+
+  const stack = mappings.map(m => m.to);
+  const maps =
+    Object.entries(groupBy(mappings, m => m.from.file))
+    .map(([file, ms]) => [file, sourceMap(file, ms)]);
+
+  return symbolicate(connection, makeData(stack, maps))
+    .then(() =>
+      expect(connection.end).toBeCalledWith(
+        JSON.stringify({result: mappings.map(m => m.to)})
+      )
+    );
+});
+
+it('ignores stack frames without corresponding map', () => {
+  const frame = {
+    file: 'arbitrary.js',
+    lineNumber: 123,
+    column: 456,
+  };
+
+  return symbolicate(connection, makeData([frame], [['other.js', emptyMap()]]))
+    .then(() =>
+      expect(connection.end).toBeCalledWith(
+        JSON.stringify({result: [frame]})
+      )
+    );
+});
+
+it('ignores `/debuggerWorker.js` stack frames', () => {
+  const frame = {
+    file: 'http://localhost:8081/debuggerWorker.js',
+    lineNumber: 123,
+    column: 456,
+  };
+
+  return symbolicate(connection, makeData([frame]))
+    .then(() =>
+      expect(connection.end).toBeCalledWith(
+        JSON.stringify({result: [frame]})
+      )
+    );
+});
+
+function makeData(stack, maps = []) {
+  return JSON.stringify({maps, stack});
+}
+
+function sourceMap(file, mappings) {
+  const g = new SourceMapGenerator();
+  g.startFile(file, null);
+  mappings.forEach(({from, to}) =>
+    g.addSourceMapping(to.lineNumber, to.column, from.lineNumber, from.column));
+  return g.toMap();
+}
+
+function groupBy(xs, key) {
+  const grouped = {};
+  xs.forEach(x => {
+    const k = key(x);
+    if (k in grouped) {
+      grouped[k].push(x);
+    } else {
+      grouped[k] = [x];
+    }
+  });
+  return grouped;
+}
+
+function emptyMap() {
+  return {
+    version: 3,
+    sources: [],
+    mappings: '',
+  };
+}

packages/metro-bundler/src/Server/symbolicate/package.json

@@ -0,0 +1 @@
+{"main": "./symbolicate.js"}

packages/metro-bundler/src/Server/symbolicate/symbolicate.js

@@ -0,0 +1,76 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+const concat = require('concat-stream');
+const debug = require('debug')('RNP:Symbolication');
+const net = require('net');
+const temp = require('temp');
+const xpipe = require('xpipe');
+
+const {LazyPromise, LockingPromise} = require('./util');
+const {fork} = require('child_process');
+
+export type {SourceMap as SourceMap};
+import type {MixedSourceMap as SourceMap} from '../../lib/SourceMap';
+
+export type Stack = Array<{file: string, lineNumber: number, column: number}>;
+export type Symbolicate =
+  (Stack, Iterable<[string, SourceMap]>) => Promise<Stack>;
+
+const affixes = {prefix: 'metro-bundler-symbolicate', suffix: '.sock'};
+const childPath = require.resolve('./worker');
+
+exports.createWorker = (): Symbolicate => {
+  const socket = xpipe.eq(temp.path(affixes));
+  const child = new LockingPromise(new LazyPromise(() => startupChild(socket)));
+
+  return (stack, sourceMaps) =>
+    child
+      .then(() => connectAndSendJob(socket, message(stack, sourceMaps)))
+      .then(JSON.parse)
+      .then(response =>
+        'error' in response
+          ? Promise.reject(new Error(response.error))
+          : response.result
+      );
+};
+
+function startupChild(socket) {
+  const child = fork(childPath);
+  return new Promise((resolve, reject) => {
+    child
+      .once('error', reject)
+      .once('message', () => {
+        child.removeAllListeners();
+        resolve(child);
+      });
+      child.send(socket);
+  });
+}
+
+function connectAndSendJob(socket, data) {
+  const job = new Promise((resolve, reject) => {
+    debug('Connecting to worker');
+    const connection = net.createConnection(socket);
+    connection.setEncoding('utf8');
+    connection.on('error', reject);
+    connection.pipe(concat(resolve));
+    debug('Sending data to worker');
+    connection.end(data);
+  });
+  job.then(() => debug('Received response from worker'));
+  return job;
+}
+
+function message(stack, sourceMaps) {
+  return JSON.stringify({maps: Array.from(sourceMaps), stack});
+}

packages/metro-bundler/src/Server/symbolicate/util.js

@@ -0,0 +1,86 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+
+'use strict';
+
+type PromiseLike<R> = {
+  catch<U>(onReject?: (error: any) => ?Promise<U> | U): Promise<U>,
+  then<U>(
+    fulfilled?: R => Promise<U> | U,
+    rejected?: (error: any) => Promise<U> | U,
+  ): Promise<U>,
+};
+
+/**
+ * A promise-like object that only creates the underlying value lazily
+ * when requested.
+ */
+exports.LazyPromise = class LazyPromise<T> {
+  _promise: PromiseLike<T>;
+
+  constructor(factory: () => PromiseLike<T>) {
+    //$FlowIssue #16209141
+    Object.defineProperty(this, '_promise', {
+      configurable: true,
+      enumerable: true,
+      get: () => (this._promise = factory()),
+      set: value => Object.defineProperty(this, '_promise', {value}),
+    });
+  }
+
+  then<U>(
+    fulfilled?: (value: T) => Promise<U> | U,
+    rejected?: (error: any) => Promise<U> | U
+  ): Promise<U> {
+    return this._promise.then(fulfilled, rejected);
+  }
+
+  catch<U>(
+    rejected?: (error: any) => ?Promise<U> | U
+  ): Promise<U> {
+    return this._promise.catch(rejected);
+  }
+};
+
+/**
+ * A promise-like object that allows only one `.then()` handler to access
+ * the wrapped value simultaneously. Can be used to lock resources that do
+ * asynchronous work.
+ */
+exports.LockingPromise = class LockingPromise<T> {
+  _gate: PromiseLike<any>
+  _promise: PromiseLike<T>
+
+  constructor(promise: PromiseLike<T>) {
+    this._gate = this._promise = promise;
+  }
+
+  then<U>(
+    fulfilled?: (value: T) => Promise<U> | U,
+    rejected?: (error: any) => Promise<U> | U
+  ): Promise<U> {
+    const whenUnlocked = () => {
+      const promise = this._promise.then(fulfilled, rejected);
+      this._gate = promise.then(empty); // avoid retaining the result of promise
+      return promise;
+    };
+
+    return this._gate.then(whenUnlocked, whenUnlocked);
+  }
+
+  catch<U>(
+    rejected?: (error: any) => ?Promise<U> | U
+  ): Promise<U> {
+    return this._promise.catch(rejected);
+  }
+};
+
+function empty() {}

packages/metro-bundler/src/Server/symbolicate/worker.js

@@ -0,0 +1,77 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+// RUNS UNTRANSFORMED IN NODE >= v4
+// NO FANCY FEATURES, E.G. DESTRUCTURING, PLEASE!
+
+const SourceMapConsumer = require('source-map').SourceMapConsumer;
+const concat = require('concat-stream');
+const net = require('net');
+
+process.once('message', socket => {
+  net.createServer({allowHalfOpen: true}, connection => {
+    connection.setEncoding('utf8');
+    connection.pipe(concat(data =>
+      symbolicate(connection, data)
+        .catch(console.error) // log the error as a last resort
+    ));
+  }).listen(socket, () => process.send(null));
+});
+
+function symbolicate(connection, data) {
+  return Promise.resolve(data)
+    .then(JSON.parse)
+    .then(symbolicateStack)
+    .then(JSON.stringify)
+    .catch(makeErrorMessage)
+    .then(message => connection.end(message));
+}
+
+function symbolicateStack(data) {
+  const consumers = new Map(data.maps.map(mapToConsumer));
+  return {
+    result: data.stack.map(frame => mapFrame(frame, consumers)),
+  };
+}
+
+function mapFrame(frame, consumers) {
+    const sourceUrl = frame.file;
+    const consumer = consumers.get(sourceUrl);
+    if (consumer == null) {
+      return frame;
+    }
+    const original = consumer.originalPositionFor({
+      line: frame.lineNumber,
+      column: frame.column,
+    });
+    if (!original) {
+      return frame;
+    }
+    return Object.assign({}, frame, {
+      file: original.source,
+      lineNumber: original.line,
+      column: original.column,
+    });
+}
+
+function makeErrorMessage(error) {
+  return JSON.stringify({
+    error: String(error && error.message || error),
+  });
+}
+
+function mapToConsumer(tuple) {
+  tuple[1] = new SourceMapConsumer(tuple[1]);
+  return tuple;
+}
+
+// for testing
+exports.symbolicate = symbolicate;