refactor: rewire the desktop capturer API to remove race conditions (#18029)

We now create a new instance of atom::api::DesktopCapturer for every
request instead of weirdly re-using the same instance and queuing
requests.  This means there is now a 1:1 relationship between request
and DesktopCapturer so there isn't a race condition between the observer
for one request calling back before the observer of another.  This is an
issue ever since the backing APIs moved to worker threads.

This also does a few things to ensure memory management
* Only ever listen to one event per-request, after that we wipe the emit
function to ignore all future events
* Ensures we clean up the window_capturer_, screen_capturer_ and
captured_sources_ in native land once the request is over.

This _in theory_ fixes a flake we've been seeing on CI where we try to
resolve the promise for a request that no longerr exists.

atom/browser/api/atom_api_desktop_capturer.cc

@@ -226,7 +226,7 @@ void Initialize(v8::Local<v8::Object> exports,
                 void* priv) {
   v8::Isolate* isolate = context->GetIsolate();
   mate::Dictionary dict(isolate, exports);
-  dict.Set("desktopCapturer", atom::api::DesktopCapturer::Create(isolate));
+  dict.SetMethod("createDesktopCapturer", &atom::api::DesktopCapturer::Create);
 }
 
 }  // namespace

lib/browser/desktop-capturer.js

@@ -2,13 +2,12 @@
 
 const ipcMainUtils = require('@electron/internal/browser/ipc-main-internal-utils')
 
-const { desktopCapturer } = process.electronBinding('desktop_capturer')
+const { createDesktopCapturer } = process.electronBinding('desktop_capturer')
 const eventBinding = process.electronBinding('event')
 
 const deepEqual = (a, b) => JSON.stringify(a) === JSON.stringify(b)
 
-// A queue for holding all requests from renderer process.
-let requestsQueue = []
+let currentlyRunning = []
 
 ipcMainUtils.handle('ELECTRON_BROWSER_DESKTOP_CAPTURER_GET_SOURCES', (event, captureWindow, captureScreen, thumbnailSize, fetchWindowIcons) => {
   const customEvent = eventBinding.createWithSender(event.sender)
@@ -18,38 +17,70 @@ ipcMainUtils.handle('ELECTRON_BROWSER_DESKTOP_CAPTURER_GET_SOURCES', (event, cap
     return []
   }
 
-  return new Promise((resolve, reject) => {
-    const request = {
-      options: {
+  const options = {
     captureWindow,
     captureScreen,
     thumbnailSize,
     fetchWindowIcons
+  }
+
+  for (const running of currentlyRunning) {
+    if (deepEqual(running.options, options)) {
+      // If a request is currently running for the same options
+      // return that promise
+      return running.getSources
+    }
+  }
+
+  const getSources = new Promise((resolve, reject) => {
+    const stopRunning = () => {
+      // Remove from currentlyRunning once we resolve or reject
+      currentlyRunning = currentlyRunning.filter(running => running.options !== options)
+    }
+    const request = {
+      options,
+      resolve: (value) => {
+        stopRunning()
+        resolve(value)
       },
-      resolve,
-      reject
-    }
-    requestsQueue.push(request)
-    if (requestsQueue.length === 1) {
-      desktopCapturer.startHandling(captureWindow, captureScreen, thumbnailSize, fetchWindowIcons)
+      reject: (err) => {
+        stopRunning()
+        reject(err)
+      },
+      capturer: createDesktopCapturer()
     }
+    request.capturer.emit = createCapturerEmitHandler(request.capturer, request)
+    request.capturer.startHandling(captureWindow, captureScreen, thumbnailSize, fetchWindowIcons)
 
     // If the WebContents is destroyed before receiving result, just remove the
-    // reference from requestsQueue to make the module not send the result to it.
+    // reference to resolve, emit and the capturer itself so that it never dispatches
+    // back to the renderer
     event.sender.once('destroyed', () => {
       request.resolve = null
+      delete request.capturer.emit
+      delete request.capturer
+      stopRunning()
     })
   })
+
+  currentlyRunning.push({
+    options,
+    getSources
+  })
+
+  return getSources
 })
 
-desktopCapturer.emit = (event, name, sources, fetchWindowIcons) => {
-  // Receiving sources result from main process, now send them back to renderer.
-  const handledRequest = requestsQueue.shift()
-  const unhandledRequestsQueue = []
+const createCapturerEmitHandler = (capturer, request) => {
+  return function handlEmitOnCapturer (event, name, sources, fetchWindowIcons) {
+    // Ensure that this capturer instance can only ever receive a single event
+    // if we get more than one it is a bug but will also cause strange behavior
+    // if we still try to handle it
+    delete capturer.emit
 
     if (name === 'error') {
       const error = sources
-    handledRequest.reject(error)
+      request.reject(error)
       return
     }
 
@@ -63,25 +94,8 @@ desktopCapturer.emit = (event, name, sources, fetchWindowIcons) => {
       }
     })
 
-  if (handledRequest.resolve) {
-    handledRequest.resolve(result)
-  }
-
-  // Check the queue to see whether there is another identical request & handle
-  requestsQueue.forEach(request => {
-    if (deepEqual(handledRequest.options, request.options)) {
     if (request.resolve) {
       request.resolve(result)
     }
-    } else {
-      unhandledRequestsQueue.push(request)
-    }
-  })
-  requestsQueue = unhandledRequestsQueue
-
-  // If the requestsQueue is not empty, start a new request handling.
-  if (requestsQueue.length > 0) {
-    const { captureWindow, captureScreen, thumbnailSize, fetchWindowIcons } = requestsQueue[0].options
-    return desktopCapturer.startHandling(captureWindow, captureScreen, thumbnailSize, fetchWindowIcons)
   }
 }