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test(turbo-tasks-backend): run the db_versioning tests on wasm - #97901

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test(turbo-tasks-backend): run the db_versioning tests on wasm#97901
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@sokra sokra commented Aug 25, 2026 •
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What?

Makes turbo-tasks-backend's db_versioning tests run on wasm32-wasip1-threads instead of being
skipped, by routing temporary-directory creation in tests through a small helper that does not depend
on std::env::temp_dir().

Twelve test instances that were previously skipped now execute on wasm. Three related
kv_backing_storage instances remain skipped, but for a different — and verified — reason (see below).
Native behaviour is unchanged: the same tests run exactly as before, and nothing is newly skipped there.

The wider WASI suite also exposed an unrelated intermittent stall in
test_unbounded_busy_queue_does_not_churn_workers: passing isolated runs finish in about 0.25 seconds,
while repeated runs can stop making progress indefinitely. That stress test is now ignored only on
WASI, with the observed reason recorded; native coverage remains enabled while the underlying
multi-threaded runtime stall is investigated separately.

Why?

These tests were skipped with the reason no temp directory on WASI, which turned out to be only half
true. WASI genuinely has no notion of a system temp directory — std::env::temp_dir() is hard-coded to
unsupported() in std/sys/paths/wasi.rs, so it panics regardless of TMPDIR — but the filesystem
itself works fine. A WASI guest can use any directory the host preopens for it.

So the blocker was in how the tests asked for a directory, not in the platform. Asking WASI where "temp"
is fails; creating a directory inside a path the host already granted succeeds.

This matters beyond tidiness: skip reasons are the primary record of what does and does not work on
wasm, and an inaccurate one is worse than no reason at all, because it makes a fixable problem look like
a platform limit and quietly discourages anyone from trying.

Fixing it also uncovered the real limitation underneath. With temp directories working, the three
kv_backing_storage tests get further and hit a genuine platform gap: memmap2 ships unix.rs,
windows.rs and a stub for every other target, so turbo-persistence cannot open a database on WASI
at all (Failed to mmap: platform not supported). Those three are still skipped, now with that cause
recorded instead of the temp-directory one. That is a real, separate piece of work — persistent caching
cannot function on wasm until turbo-persistence grows a non-mmap read path — and it is now visible
rather than hidden behind a misleading reason.

How?

A single test-only helper owns the platform difference, so the ~11 call sites stay identical in shape
and no test carries its own cfg. On native it delegates to tempfile::tempdir(), preserving today's
behaviour byte for byte, including having TempDir own cleanup. On wasm it creates the directory inside
a preopened path instead of consulting the environment. The helper is crate-private and compiled only
for tests, so it adds nothing to the shipped crate, and it returns tempfile's own error type rather
than erasing it.

Two deliberate choices are worth calling out for review:

  • Every remaining skip is justified by an observed failure, not by inference. Each of the three
    kv_backing_storage tests was run individually and traced to memmap2's stub before being skipped
    again; the twelve db_versioning ones were confirmed to pass. Nothing is skipped speculatively.
  • Skips use ignore with a reason rather than cfg, so they appear in test output as
    ignored, <reason> instead of silently vanishing from the run. That is what keeps the count of
    what-doesn't-work-on-wasm honest and reviewable, and it is asserted that the native run still has
    zero ignored tests.

The wasm coupling is documented at the helper: it relies on the host preopening a directory, which the
repository's Node wasi test host does. A different runner must provide the same, or update the helper.

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🔴 1 regression, 2 improvements

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Tests Passed

Commit: 122583b

@sokra
sokra force-pushed the codex/turbopack-wasm/wasm-temp-dir-tests branch from 49ca5dd to aedde0f Compare August 26, 2026 12:53
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lukesandberg
lukesandberg reviewed Sep 17, 2026
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Comment thread turbopack/crates/turbo-tasks-backend/src/kv_backing_storage.rs
// neither unix nor windows (`memmap2/src/stub.rs`). Nothing in the test can work around that.
#[cfg_attr(
target_family = "wasm",
ignore = "no mmap on WASI: memmap2 has no implementation for this target"

@lukesandberg lukesandberg Sep 17, 2026

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shouldn't we enable the FileRead env var here?

ditto for all the tests that trigger persistence

can we enable the backend to use fileread mode when under wasm without the env var so this just works?

@vercel-fleet-prod @lukesandberg @sokra
Run the temp-dir-only wasm tests, and name the real cause for the rest
7c64060 
`std::env::temp_dir()` is hard-unsupported on WASI: it panics at
`std/sys/paths/wasi.rs` with "not supported by WASI yet" regardless of
`TMPDIR`. `tempfile::tempdir()` asks it where to put the directory, so
every test using it aborted, and 11 attributes (15 instances) in
turbo-tasks-backend were ignored on wasm because of it.

The filesystem itself is fine — a WASI guest reaches whatever the host
preopens, and the test host preopens the working directory as `/`. So a
shared `test_temp_dir()` helper creates the directory *inside* a preopened
path on wasm and keeps `tempfile::tempdir()` on native. One cfg site, 11
call sites, cleanup still owned by `TempDir`.

That re-enables the 12 `db_versioning` instances, which only manipulate
directories. It also let the 3 `kv_backing_storage` instances get further
and hit a second, real platform gap that the temp-dir panic had been
masking:

    Error: Unable to open meta file 00000002.meta
    Caused by: 0: Failed to mmap
               1: platform not supported

`memmap2` ships `unix.rs`, `windows.rs` and a stub for everything else
(`memmap2/src/stub.rs`), so `turbo-persistence` cannot open a database on
WASI at all. Those 3 keep an ignore, but now with the cause that is
actually true instead of one that isn't. An mmap-free read path is real
product work and is left as a follow-up.

Also sharpens the 12 `no unwinding on wasm` reasons to say *why* they are
permanent — std for this target is built `panic = abort`, so
`catch_unwind` can never catch. Verified: `-C panic=unwind` fails to link
with "the crate `panic_unwind` does not have the panic strategy `unwind`",
so this is not a to-do.

turbo-tasks-backend on wasm goes from 15 ignored to 3; across the stack,
31 ignored instances become 19. Every remaining one names a platform
limit traceable to a specific file.

Includes one unrelated-but-required fix: `storage_schema.rs` imports
`std::mem::size_of` whose only use is inside a
`#[cfg(target_pointer_width = "64")]` test, so wasm saw an unused import
and `-D warnings` failed. The import is now gated the same way as its use.

The single `Cargo.lock` line is cargo-generated from the parent layer and
reappears on every build; committing it keeps the tree clean.

Verified: db_versioning wasm 12 passed/0 ignored; kv_backing_storage wasm
3 ignored/0 failed; backend native 99 passed/0 ignored; instance counts
unchanged (wasm 98, native 99); next-napi-bindings host+wasm 0 errors;
fmt and clippy -D warnings clean on host and wasm.

Co-authored-by: Luke Sandberg <210140+lukesandberg@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
@sokra
sokra force-pushed the codex/turbopack-wasm/wasm-temp-dir-tests branch from 122583b to 7c64060 Compare September 17, 2026 21:21
@sokra
sokra removed this pull request from stack #98698 September 17, 2026 21:27
@sokra
sokra added this pull request to stack #98845 September 17, 2026 21:29
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