Call generateRequest BEFORE pushing any segment with PlayReady

Another day, another PlayReady-specific issue :/

A partner signalled to us that they weren't able to play a mix of
unencrypted and encrypted content on any PlayReady devices.

After investigation, it seems that calling `generateRequest` for the
first time after clear segments are already present on a MSE
`SourceBuffer` associated to the MediaSource linked to the
corresponding media element immediately triggered an HTML5
`MEDIA_ERR_DECODE` error.

We tried A LOT of work-arounds:

  - patching clear segments with a `tenc` box with a `0x0` key id to
    incite the CDM to understand that encrypted contents may be pushed
    in the future

  - Rewriting the pssh sent through the EME `generateRequest` API so
    that it is barebone to limit weird PlayReady edge cases.

  - Replacing those stream clear segments' with those in our demo page,
    just to check that the clear segments were not at fault here

  - Waiting more time between the association of a MediaKeys to the
    media element and pushing the first segments.

None of those actions had an effect.

However, what had an effect, was to call the `generateRequest` API
BEFORE buffering any segment yet AFTER attaching the MediaKeys (and
perhaps MediaSource) to the media element.

So this commit does just that, communicating dummy initialization data
for a session that will be closed directly after.

Note that we already do a fake `generateRequest` on Edge Chromium with
Playready since #1434, yet this test was not sufficient, seemingly
because it is performed BEFORE MediaKeys attachment.

Note that this commit fixes the clear -> encrypted issues our partner
were having, but we're unsure yet of if it fixes the encrypted -> clear
issues (and I have good reasons to think it does not).

So, uh, yeah, PlayReady seems to keep being hard-at-work giving us
challenges and head-scratchers.

Author: peaBerberian

src/compat/should_call_generate_request_before_buffering_media.ts

@@ -0,0 +1,19 @@
+/**
+ * (2024-07-23) We noticed issues with most devices relying on PlayReady when
+ * playing some contents with mix encrypted and clear contents (not with
+ * Canal+ own contents weirdly enough, yet with multiple other contents
+ * encoded/packaged differently).
+ * The issue fixed itself when we called the
+ * `MediaKeySession.prototype.generateRequest` EME API **BEFORE** any segment
+ * was buffered.
+ * @param {string} keySystem - The key system in use.
+ * @returns {boolean}
+ */
+export default function shouldCallGenerateRequestBeforeBufferingMedia(
+  keySystem: string,
+): boolean {
+  if (keySystem.indexOf("playready") !== -1) {
+    return true;
+  }
+  return false;
+}

src/main_thread/decrypt/content_decryptor.ts

@@ -15,7 +15,12 @@
  */
 
 import type { ICustomMediaKeys, ICustomMediaKeySystemAccess } from "../../compat/eme";
-import eme, { getInitData } from "../../compat/eme";
+import eme, { closeSession, getInitData } from "../../compat/eme";
+import {
+  DUMMY_PLAY_READY_HEADER,
+  generatePlayReadyInitData,
+} from "../../compat/generate_init_data";
+import shouldCallGenerateRequestBeforeBufferingMedia from "../../compat/should_call_generate_request_before_buffering_media";
 import config from "../../config";
 import { EncryptedMediaError, OtherError } from "../../errors";
 import log from "../../log";
@@ -291,6 +296,25 @@ export default class ContentDecryptor extends EventEmitter<IContentDecryptorEven
           return;
         }
 
+        if (
+          shouldCallGenerateRequestBeforeBufferingMedia(mediaKeySystemAccess.keySystem)
+        ) {
+          try {
+            const session = mediaKeys.createSession();
+            const initData = generatePlayReadyInitData(DUMMY_PLAY_READY_HEADER);
+            await session.generateRequest("cenc", initData);
+            closeSession(session);
+          } catch (err) {
+            const error = err instanceof Error ? err : new Error("Unknown Error");
+            log.warn("DRM: unable to fully perform fake generateRequest call", error);
+          }
+        }
+
+        if (this._isStopped()) {
+          // We might be stopped since then
+          return;
+        }
+
         const prevState = this._stateData.state;
         this._stateData = {
           state: ContentDecryptorState.ReadyForContent,