Dedupe extracted embedded native modules in compiled binaries - #29587

Fixes #29585.

Problem

A bun build --compile binary that loads an embedded native library (bun:ffi dlopen or process.dlopen on a .node) extracted a fresh copy to /tmp on every call and never cleaned it up. Each Worker re-extracted on its first dlopen, so Worker-heavy servers leaked fast (the reporter measured ~14 GB/hour). The cause: resolve_embedded_file_to_buf named each tmpfile with nanoTimestamp() plus a per-VM counter, so no two calls ever produced the same path.

Fix

Extraction is now stateless and idempotent. The on-disk file is the cache.

• The name is {tmpdir}/.bun-{euid}-{wyhash(contents):x}.{ext}. Same content gives the same path, so all dlopens, Workers, and restarts of the binary share one file. The euid keeps users on a shared /tmp apart.
• Reuse check: lstatat the canonical name and reuse it if it is a regular file, owned by our euid, with the expected size. lstatat (not fstatat) makes a planted symlink fail the ISREG check instead of being followed.
• Extraction writes a unique scratch file, then rename(2)s it onto the canonical name. Racing Workers converge on one valid file. If the rename fails (a foreign owner squats the name on sticky /tmp), that call uses its scratch file.
• The post-dlopen deletion in BunProcess.cpp is removed on both platforms: deleting the shared file would defeat the dedup, and the Windows delete-on-reboot required admin rights and only ever marked the file in active use. The orphaned Bun__unlink export is deleted with its last caller.

Verification

• test/regression/issue/29585.test.ts: one file after 10 dlopens, after a second process run, after deleting the file (re-extracts), and across 5 concurrent Workers. Fails pre-fix with Expected: 1 / Received: 10.
• test/napi/napi.test.ts (--compile): the .node / process.dlopen path keeps a stable extracted count across two runs, on all platforms (verified on Windows Server 2019 x64).

no test proof · iteration 55 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/napi/napi.test.ts test/regression/issue/29585.test.ts