fix(preload): Fix load interruption (#6271)

Interrupting load() was causing two concurrent sets of load() operations
to happen at once, which led the asset URI for the second operation to
be overwritten by the first.

This was exposed by a test failure on Safari. There is nothing special
about Safari, but the timing happened to work out such that the
concurrent load() calls would intefere with each other.

This fixes the issue by acquiring the mutex in load() for the
preloadManager.start() operation.

This issue did not affect any releases.

Closes #6225

lib/player.js

@@ -1440,11 +1440,13 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
         // "preload" here without prefetch.
         // That way, both a preload and normal load can follow the same code
         // paths.
+        // NOTE: await preloadInner_ can be outside the mutex because it should
+        // not mutate "this".
         preloadManager = await this.preloadInner_(
             assetUri, startTime, mimeType, /* standardLoad= */ true);
         if (preloadManager) {
           preloadManager.setEventHandoffTarget(this);
-          await preloadManager.start();
+          await mutexWrapOperation(() => preloadManager.start(), 'preload');
         }
       } else {
         // Hook up events, so any events emitted by the preloadManager will

test/player_load_graph_integration.js

@@ -756,6 +756,7 @@ describe('Player Load Graph', () => {
         'manifest-parser',
         'manifest',
         'drm-engine',
+        // Excludes 'unload'.
       ]);
 
       /** @type {!Set.<string>} */
@@ -774,6 +775,7 @@ describe('Player Load Graph', () => {
         await player.attach(video);
         player.load('test:sintel').catch(() => {});
       } else {
+        goog.asserts.assert(state == 'unload', 'Unrecognized testing state!');
         await player.attach(video);
         await player.load('test:sintel');
         player.unload().catch(() => {});