fix(sys): disable mimalloc v3 FIXED_SLOT TLS on macOS#67
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See napi-rs/mimalloc-safe#67 & napi-rs/mimalloc-safe#68 If `oxlint` and `oxfmt` plan to adopt `mimalloc v3` in the future, this upgrade would be highly worthwhile and could bring meaningful performance and memory management improvements.
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mimalloc v3 on macOS defaults to MI_TLS_MODEL_FIXED_SLOT, which stores the per-thread theap pointer in TCB[108]/[109] via inline assembly, bypassing pthread_setspecific. The slot is hardcoded and shared across all mimalloc instances in the same process.
In Node.js processes that dlopen multiple napi addons each statically linking mimalloc-safe (e.g. a CLI binding plus a separately loaded rolldown binding), each instance writes its own theap to the same TCB slot, corrupting the other's heap metadata. On tokio runtimes the most visible symptom is BlockingTask's Option discriminant byte getting clobbered, panicking with "blocking task ran twice".
Define MI_HAS_TLS_SLOT=0 for macOS targets when the v3 source tree is selected. This makes the platform cascade in prim.h skip the fixed-slot branch and fall through to MI_TLS_MODEL_DYNAMIC_PTHREADS, which uses pthread_getspecific — each dylib gets its own pthread_key, so multiple mimalloc instances coexist safely. The cost is an indirect function call per default-heap access (~3 ns -> ~10 ns); negligible in practice.
See c_src/mimalloc3/include/mimalloc/prim.h:345-349 caveat and the "@apple: it would be great to get 2 official slots for custom allocators" comment at line 374.