[api-minor] Only use Workers when postMessage transfers are supported (PR 11123 follow-up)

Given that all modern browsers now support `postMessage` transfers, and have for years, it no longer seems necessary for the PDF.js library to support using Workers unless the `postMessage` transfers functionality is available.
This patch is a follow-up to PR 11123, which made it impossible to *manually* disable `postMessage` transfers for performance reasons (since it increases memory usage), which hasn't caused any bug reports as far as I know.[1]

Hence we'll now only support *proper* Worker implementations, with fully working `postMessage` transfers, and fallback to using "fake" Workers otherwise.

---
[1] At the time of that PR we still "supported" IE, which is why this code was left intact.

src/core/worker.js

@@ -75,16 +75,9 @@ class WorkerMessageHandler {
       }
       testMessageProcessed = true;
 
-      // check if Uint8Array can be sent to worker
-      if (!(data instanceof Uint8Array)) {
-        handler.send("test", null);
-        return;
-      }
-      // making sure postMessage transfers are working
-      const supportTransfers = data[0] === 255;
-      handler.postMessageTransfers = supportTransfers;
-
-      handler.send("test", { supportTransfers });
+      // Ensure that `TypedArray`s can be sent to the worker,
+      // and that `postMessage` transfers are supported.
+      handler.send("test", data instanceof Uint8Array && data[0] === 255);
     });
 
     handler.on("configure", function wphConfigure(data) {
@@ -156,10 +149,6 @@ class WorkerMessageHandler {
     const workerHandlerName = docParams.docId + "_worker";
     let handler = new MessageHandler(workerHandlerName, docId, port);
 
-    // Ensure that postMessage transfers are always correctly enabled/disabled,
-    // to prevent "DataCloneError" in browsers without transfers support.
-    handler.postMessageTransfers = docParams.postMessageTransfers;
-
     function ensureNotTerminated() {
       if (terminated) {
         throw new Error("Worker was terminated");

src/display/api.js

@@ -439,7 +439,6 @@ function getDocument(src) {
             workerId,
             worker.port
           );
-          messageHandler.postMessageTransfers = worker.postMessageTransfers;
           const transport = new WorkerTransport(
             messageHandler,
             task,
@@ -498,7 +497,6 @@ async function _fetchDocument(worker, source, pdfDataRangeTransport, docId) {
       },
       maxImageSize: source.maxImageSize,
       disableFontFace: source.disableFontFace,
-      postMessageTransfers: worker.postMessageTransfers,
       docBaseUrl: source.docBaseUrl,
       ignoreErrors: source.ignoreErrors,
       isEvalSupported: source.isEvalSupported,
@@ -2079,7 +2077,6 @@ class PDFWorker {
 
     this.name = name;
     this.destroyed = false;
-    this.postMessageTransfers = true;
     this.verbosity = verbosity;
 
     this._readyCapability = createPromiseCapability();
@@ -2188,13 +2185,10 @@ class PDFWorker {
             return; // worker was destroyed
           }
           if (data) {
-            // supportTypedArray
             this._messageHandler = messageHandler;
             this._port = worker;
             this._webWorker = worker;
-            if (!data.supportTransfers) {
-              this.postMessageTransfers = false;
-            }
+
             this._readyCapability.resolve();
             // Send global setting, e.g. verbosity level.
             messageHandler.send("configure", {
@@ -2222,7 +2216,7 @@ class PDFWorker {
         });
 
         const sendTest = () => {
-          const testObj = new Uint8Array([this.postMessageTransfers ? 255 : 0]);
+          const testObj = new Uint8Array([255]);
           // Some versions of Opera throw a DATA_CLONE_ERR on serializing the
           // typed array. Also, checking if we can use transfers.
           try {

src/shared/message_handler.js

@@ -83,7 +83,6 @@ class MessageHandler {
     this.comObj = comObj;
     this.callbackId = 1;
     this.streamId = 1;
-    this.postMessageTransfers = true;
     this.streamSinks = Object.create(null);
     this.streamControllers = Object.create(null);
     this.callbackCapabilities = Object.create(null);
@@ -180,7 +179,7 @@ class MessageHandler {
    * @param {Array} [transfers] - List of transfers/ArrayBuffers.
    */
   send(actionName, data, transfers) {
-    this._postMessage(
+    this.comObj.postMessage(
       {
         sourceName: this.sourceName,
         targetName: this.targetName,
@@ -204,7 +203,7 @@ class MessageHandler {
     const capability = createPromiseCapability();
     this.callbackCapabilities[callbackId] = capability;
     try {
-      this._postMessage(
+      this.comObj.postMessage(
         {
           sourceName: this.sourceName,
           targetName: this.targetName,
@@ -247,7 +246,7 @@ class MessageHandler {
             cancelCall: null,
             isClosed: false,
           };
-          this._postMessage(
+          comObj.postMessage(
             {
               sourceName,
               targetName,
@@ -322,7 +321,7 @@ class MessageHandler {
           this.sinkCapability = createPromiseCapability();
           this.ready = this.sinkCapability.promise;
         }
-        self._postMessage(
+        comObj.postMessage(
           {
             sourceName,
             targetName,
@@ -549,20 +548,6 @@ class MessageHandler {
     delete this.streamControllers[streamId];
   }
 
-  /**
-   * Sends raw message to the comObj.
-   * @param {Object} message - Raw message.
-   * @param transfers List of transfers/ArrayBuffers, or undefined.
-   * @private
-   */
-  _postMessage(message, transfers) {
-    if (transfers && this.postMessageTransfers) {
-      this.comObj.postMessage(message, transfers);
-    } else {
-      this.comObj.postMessage(message);
-    }
-  }
-
   destroy() {
     this.comObj.removeEventListener("message", this._onComObjOnMessage);
   }