OPFSAdaptiveVFS: load where FileSystemSyncAccessHandle is unavailable

[lalexdotcom]

What happens

OPFSAdaptiveVFS.js reads globalThis.FileSystemSyncAccessHandle.prototype while the module is evaluated, to detect readwrite-unsafe support. Where the interface does not exist, importing the module throws a TypeError before anything else runs. That is the case:

• outside a dedicated worker: on a page's main thread, or in Node, for instance a unit test or server-side rendering that imports the module;
• on a page that is not a secure context (plain http, localhost aside), where browsers withhold OPFS;
• in browsers older than the interface: Chrome before 102, Chrome for Android before 109, Firefox before 111 (MDN browser-compat-data).

The VFS cannot run there anyway, but importing it should not fail. For example, an application bundles several VFS into one worker and picks one at runtime. Where this interface is missing, it cannot use MemoryVFS or an IndexedDB VFS either, because the whole worker fails to load.

The change

Optional chaining on that read: hasUnsafeAccessHandle is undefined where the interface is missing. Where it exists, nothing changes.

Test

test/OPFSAdaptiveVFS.test.js gains a test that imports the module from the test page, a main thread, where FileSystemSyncAccessHandle is undefined. On master it fails on Chromium and Firefox (Playwright), for example Expected 'can't access property "prototype", globalThis.FileSystemSyncAccessHandle is undefined' to be undefined. With the change, the file's 72 tests pass, 3 runs of 3, on both. The whole suite passes on Chromium: 6158 passed, 0 failed.

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