[Next.js/dylib] tests/test_next_app_route_dylib.sh aborts on the first request: Rust provider-host + flat_namespace bind the stdlib image's __rust_dealloc to the System shim (mimalloc buffer freed by libsystem) #8205

Symptom

tests/test_next_app_route_dylib.sh (the #8161 gate behind next-app-route.yml, which has never had a run yet) fails on cold start 1, first request, on clean main 3c95020f8:

PERRY_NEXT_APP_ROUTE_READY http://127.0.0.1:3180
tests/test_next_app_route_dylib.sh: line 273: 37574 Abort trap: 6   … "$host" "$runtime_library" "$stdlib_library" "$app"

Node oracle PASS, app-only dylib compiles, provider images link, ABI check passes (missing-symbols empty), host reaches READY, then the readiness curl aborts the process. Measured 2026-08-16 on the bench mini (macOS 26.5.1, LLVM 22.1.8, perry-dev, script defaults). The same commit, same compiler, same app source, served 500 verifier batches through routeModule.handle minutes later under tests/release/packages/next-app-route/fixture.sh (see #8163 / #8040 comments), so this is the fixture layout, not the compiler or the route.

Crash report (~/Library/Logs/DiagnosticReports/provider-host-2026-08-16-030353.ips)

libsystem_malloc  ___BUG_IN_CLIENT_OF_LIBMALLOC_POINTER_BEING_FREED_WAS_NOT_ALLOCATED
libperry_stdlib.dylib  <alloc::vec::IntoIter<i64> as Drop>::drop   (perry_ffi)
libperry_stdlib.dylib  js_node_http_server_process_pending
libperry_stdlib.dylib  js_stdlib_process_pending
libperry_runtime.dylib js_run_stdlib_pump
provider-host          provider_host::main

Mechanism (symbol evidence, nm on the kept images)

• libperry_runtime.dylib defines the rustc allocator shim and it is mimalloc: __RNv…7___rustc14___rust_dealloc: b _mi_free (_mi_free is a local t in the runtime image).
• libperry_stdlib.dylib imports __rust_alloc/__rust_dealloc/__rust_alloc_zeroed (U), as it should.
• provider-host (built from provider-host.rs, a Rust binary) ALSO defines __RNv…7___rustc14___rust_dealloc — the System-allocator shim every Rust executable carries.
• provider-linker.sh links the provider with -Wl,-flat_namespace -Wl,-interposable. In a flat namespace an undefined symbol binds to the first image that defines it, and the main executable is first — so the stdlib image's __rust_dealloc binds to the host's System shim while allocations that came out of the runtime image were made by mimalloc. The first cross-image Vec<i64> drop (js_node_http_server_process_pending draining the pending-request list) hands a mimalloc pointer to libsystem free() → abort.

tests/release/packages/next-app-route/ does not have this problem: its host is C (provider-host.c, no Rust shims) and its images are linked with -undefined dynamic_lookup (two-level namespace), so __rust_dealloc in the stdlib image resolves to the runtime's mimalloc shim.

Fix options (either is small; both are in tests/fixtures/next-app-route/)

1. Make the host C (or link provider-host.rs with -C prefer-dynamic/no std shims — awkward); the release-tier provider-host.c is drop-in.
2. Drop -flat_namespace -interposable from provider-linker.sh and let two-level namespace bind __rust_* to the runtime image (keep the exported-symbols list).

Either way the gate then needs a first green run before next-app-route.yml means anything; today its first scheduled run (03:17 UTC 2026-08-17) will be red for this reason. Longer term the two copies of the #8034 fixture should converge on one layout (the release one is the one with production evidence; the tests/fixtures/ one has the armed bypass guard — porting the guard over is a one-file copy, verified working in the runs above).