Crash `Current context must not be nil` in `recordSnapshot`

[finestructure]

I mentioned this crash to @stephencelis a couple of weeks ago when we first saw it with Xcode 16b2 but it had "disappeared" when I revisited our tests with Xcode 16b3.

Turns out it didn't really disappear but the conditions to trigger it changed in our project (SPI-Server). The conditions in that project are now:

• a test is recording a snapshot in a closure marked @Sendable
• the test must be be actually trying to record a snapshot (i.e. unless we had snapshot changes the crash wouldn't trigger)
• it only crashes when snapshots are created within Xcode (due to the __XCODE_BUILT_PRODUCTS_DIR_PATHS env variable check), not when the test are run via swift test

NB: we see the crash also with Xcode 15.4 and it seems to be related to the @Sendable changes we introduced while making the project ready for Swift 6. Reverting to a non-sendable closure makes the snapshot write succeed.

However, I've just set up a new project and it's much simpler to trigger the crash there: https://github.com/finestructure/snapshot-testing-crash

NB: This test crashes on both Xcode 15.4 and Xcode 16b3. The culprit seems to be the async attribute on the test. It does not crash without it:

    func test_crash() async throws {
        // This crashes with `Current context must not be nil`
        assertSnapshot(of: "foo", as: .lines)
    }

    func test_no_crash() throws {
        // This does not crash
        assertSnapshot(of: "foo", as: .lines)
    }

[finestructure]

I should add that this feels like a sendable/concurrency bug upstream but I thought I'd raise it here first in case you have some ideas where to best report/address it.

[finestructure]

Oh, never mind. I just saw this is a duplicate of #822 and that I should be marking those tests @MainActor!

[finestructure]

The issue is slightly more complicated than a missing @MainActor annotation and the solution might be helpful for others.

We had tests like the following:

    @MainActor
    func test_documentation_routes_current() async throws {
        ...
        try await app.test(.GET, "/owner/package/~/documentation/target") { @Sendable res in
            ...
            assertSnapshot(of: body, as: .html, named: "index")
            ...
        }
        ...
    }

and these were leading to crashes on when recording. The fix is to move the @MainActor annotation into the closure instead of the @Sendable:

    func test_documentation_routes_current() async throws {
        ...
        try await app.test(.GET, "/owner/package/~/documentation/target") { @MainActor res in
            ...
            assertSnapshot(of: body, as: .html, named: "index")
            ...
        }
        ...
    }

[mbrandonw]

Hey @finestructure, thanks for documenting this! We do have plans to overhaul the sendability of the library, and we even have an experimental async branch that we use on the Point-Free codebase. We just need to clean things up, and see if it's possible to do in a backwards compatible way.