buffer: add mask() and unmask()

[jasnell]

Add buffer.mask(source, mask, output[, offset[, length]]) and buffer.unmask(buffer, mask), which XOR data with a repeating 4-byte key. This is the masking that WebSocket clients apply to every frame they send (RFC 6455, Section 5.3), and that servers undo on every frame they receive.

undici and ws (without optional dependencies) both need mask and unmask. The bufferutil native addon makes it faster but it's an optional dependency that appears to only be installed with ws about 3% of the time. So almost all users run the JS loop.

Since we're shipping undici and WebSocket now, it's worth making this faster.

I put this on buffer because it's an operation on a buffer but it's likely only useful for WebSocket, so we could just as easily expose it via http. I have no particular preference.

For inputs < 24, the JS loop is still faster because we hit the floor of the C++ binding, even with fast apis. There's not much we can do about that. At larger sizes the perf boost is significant.

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[jasnell]

@lpinca

[codecov]

Codecov Report

❌ Patch coverage is 91.86992% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.37%. Comparing base (cd908df) to head (03fe080).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	85.71%	0 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66335      +/-   ##
==========================================
- Coverage   90.37%   90.37%   -0.01%     
==========================================
  Files         792      792              
  Lines      275324   275447     +123     
  Branches    52764    52797      +33     
==========================================
+ Hits       248828   248933     +105     
- Misses      16918    16920       +2     
- Partials     9578     9594      +16     

Files with missing lines	Coverage Δ
lib/buffer.js	99.75% <100.00%> (+<0.01%)	⬆️
src/node_buffer.cc	71.17% <85.71%> (+0.67%)	⬆️

... and 27 files with indirect coverage changes

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[pipobscure]

First off: I like it! (LGTM)

Question: unmask is just mask(buffer,mask,buffer) to mask in place and as you said likely only useful to people that understand that well-enough to actually implement websocket. (And masking is the easy part. Ask me how I know). Is that really worth a separate function? (And yes this is an extremely nitpicking thing that can well be ignored)

[jasnell]

I kept it as a separate function to keep the API shape the same as bufferutil. Happy to coalesce but would like to see what @lpinca thinks

[lpinca]

The discussion about whether to add this feature to the Node.js core dates back to 2015 (#1010 (comment), #1010 (comment), #1202) and I think it would be a valuable addition. Every WebSocket implementation would benefit from this just like buffer.isUtf8() which was originally added for the same reason.

I don't have a strong option about unmask(). I think it is nice sugar and the client only masks while the server only unmasks even though it is basically the same thing. Shipping only mask() is also ok.

Some tests are hard to follow.

[jasnell]

I think there's value in matching the current API so that there's just less for someone migrating from bufferutil to think about.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78001/

[lpinca]

RSLGTM