RuntimeError: unreachable

not ok 244 wasi/test-wasi-pthread

duration_ms: 55929.95800
severity: fail
exitcode: 1
stack: |-
Testing with --turbo-fast-api-calls: --target=wasm32-wasip1-threads pthread
Testing with --no-turbo-fast-api-calls: --target=wasm32-wasip1-threads pthread
[process 425298]: --- stderr ---
(node:425298) ExperimentalWarning: WASI is an experimental feature and might change at any time
(Use node26 --trace-warnings ... to show where the warning was created)
fd_write(2, 69600, 2, 69596)
Assertion failed: r == 0 (c/pthread.c: main: 17)
wasm://wasm/00020abe:1

RuntimeError: unreachable
    at wasm://wasm/00020abe:wasm-function[26]:0x892
    at wasm://wasm/00020abe:wasm-function[27]:0x8da
    at wasm://wasm/00020abe:wasm-function[13]:0x46a
    at wasm://wasm/00020abe:wasm-function[11]:0x328
    at WASI.start (node:wasi:138:7)
    at /home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/test/fixtures/wasi-preview-1.js:191:20

Node.js v26.3.1

[process 425298]: --- stdout ---

[process 425298]: status = 1, signal = null
/home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/test/common/child_process.js:112
    throw error;
    ^

Error: - process terminated with status 1, expected 0
    at testWasiPreview1 (/home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/test/common/wasi.js:29:3)
    at Object.<anonymous> (/home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/test/wasi/test-wasi-pthread.js:7:1)
    at Module._compile (node:internal/modules/cjs/loader:1873:14)
    at Object..js (node:internal/modules/cjs/loader:2013:10)
    at Module.load (node:internal/modules/cjs/loader:1596:32)
    at Module._load (node:internal/modules/cjs/loader:1398:12)
    at wrapModuleLoad (node:internal/modules/cjs/loader:255:19)
    at Module.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:154:5)
    at node:internal/main/run_main_module:33:47 {
  options: { env: { NODE_DEBUG_NATIVE: 'wasi', NODE_PLATFORM: 'linux' } },
  command: '/home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/out/Release/node26 --no-turbo-fast-api-calls /home/abuild/rpmbuild/BUILD/nodejs26-26.3.1-build/node-v26.3.1/test/fixtures/wasi-preview-1.js --target=wasm32-wasip1-threads pthread'
}

Node.js v26.3.1

This has now reproduced in two nodejs/node PR CIs targeting main, on non-Linux platforms, with the same signature as #59146:

macOS 15 x64: PR lib: load fewer builtins when bootstrapping without a snapshot #65329, https://ci.nodejs.org/job/node-test-pull-request/76142/
Windows 10 x64 / VS2022 clang: PR fs: use sized reads for large files in readFileUtf8 #65328, https://ci.nodejs.org/job/node-test-pull-request/76144/

Both failures include:

fd_write(2, 69600, 2, 69596)
Assertion failed: r == 0 (c/pthread.c: main: 17)
RuntimeError: unreachable

They are grouped in the 2026-08-22 reliability report:

https://github.com/nodejs/reliability/blob/main/reports/2026-08-22.md

Linux is already marked PASS, FLAKY under #59146. Since the exact signature now spans Linux, macOS, and Windows, the status mark should also cover macOS and Windows.

test: fix the thread-spawn handshake in the WASI threads fixture