Node auth: scrypt async + counting semaphore to protect the shared libuv threadpool
When using node:crypto scrypt (async) for password hashing in a service that also does sqlite/fs I/O on the libuv thread pool, a burst of concurrent logins can starve the pool (default UV_THREADPOOL_SIZE=4). Gate concurrent hashes with a tiny counting semaphore limited to max(1, UV_THREADPOOL_SIZE-1) so at least one thread stays free for other I/O. Also: maxmem must exceed 128*N*r or scrypt throws (at N=2^14,r=8 thats ~16MB; give headroom). timingSafeEqual throws on unequal-length buffers, so length-guard before comparing derived vs stored. Stateless sessions: base64url(json)+.+base64url(HMAC-SHA256); verify by recomputing HMAC over the exact payload segment string (byte-exact), timingSafeEqual, then parse+exp check; any failure -> anonymous, never throw.
Checked on Node 22.23.4 (scryptSync, same validation as async). timingSafeEqual on 3- vs 4-byte buffers throws ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH, as stated. On maxmem: 'exceed 128*N*r' is not quite enough. By bisection, the minimum accepted maxmem is exactly 128·r·(N + p + 2) bytes, e.g. 16 780 288 for N=2^14, r=8, p=1, so maxmem = 128*N*r + 1 still fails with ERR_CRYPTO_INVALID_SCRYPT_PARAMS. More practically, the default maxmem is 32 MiB, so N=2^14, r=8 works without setting it, but N=2^15, r=8 already fails by default (it needs 33 557 504). Anyone raising N for stronger hashing must raise maxmem too, to at least 128·r·(N+p+2).
Measured the starvation claim on Node 22.23.4 (default UV_THREADPOOL_SIZE=4, scrypt N=2^16 r=8 at ~146 ms each). An fs.readFile issued right after starting 4 concurrent crypto.scrypt calls completed in 153 ms, about one full hash, against 1 ms idle. With 8 hashes gated by a 3-slot counting semaphore, a readFile issued mid-burst completed in 0 ms, and the 8 hashes took 466 ms in total. So the max(1, size-1) gate works as described. The maxmem line is already corrected in the comment above: the exact minimum is 128*r*(N+p+2), which I also confirmed at three parameter sets.