crypto: avoid reusing randomInt() cache bytes

[gengjiawen]

When an asynchronous crypto.randomInt() call finds the cache empty, it queues itself and refills the shared cache with randomFill(). A synchronous call made before the refill callback runs refills the same buffer with randomFillSync() and takes values from offset 0. The callback then reset the offset to 0, so the queued call and later calls got the bytes that synchronous calls had already returned. The threadpool job and randomFillSync() could also write the buffer at the same time.

The asynchronous refill now fills a separate buffer, allocated on first use and reused after that. The callback copies it into the cache before resetting the offset. Synchronous calls and the threadpool never touch the same memory, and resetting the offset only hands out bytes nobody has used. The synchronous path is unchanged. The asynchronous path does one extra 6 KiB copy per refill (every 1024 values).

The new test makes the first randomInt() call in a process asynchronous, waits for the refill job to finish, makes synchronous calls, and checks that the async value and the values after the callback differ from the synchronous ones. Without the fix it failed in every run I tried.

Fixes: #66595
Refs: #35110

AI disclosure: written with the help of Claude (claude:opus-5.5). I checked the bug against the source and the latest nightly, and reviewed the fix and the test.

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (c3189d4) to head (59e0b84).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66597      +/-   ##
==========================================
+ Coverage   90.45%   90.46%   +0.01%     
==========================================
  Files         791      791              
  Lines      276562   276571       +9     
  Branches    53111    53118       +7     
==========================================
+ Hits       250154   250191      +37     
+ Misses      16801    16782      -19     
+ Partials     9607     9598       -9     

Files with missing lines	Coverage Δ
lib/internal/crypto/random.js	96.45% <100.00%> (+0.04%)	⬆️

... and 24 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.