http2: add new connectionWindowSize option

[pimterry]

#64623 massively increased the default HTTP/2 connection window size from 64KB bytes to 32MB.

This is definitely a good thing for bandwidth in common scenarios, but there's no way for users to opt-out of this change or to shrink the window themselves for other use cases.

That limitation is because the only way to change this today is through setLocalWindowSize, which is only available for dynamic changes after connection setup. Unfortunately, once an HTTP/2 connection is set up it's impossible to shrink the initial window back below the advertised amount (impossible in the protocol, not just our implementation), so changes only apply once the initial window has been used up. With a default of 32MB per session, that can take a while.

Shrinking the window can be useful as a quick workaround to preserve old behaviour, or to optimize for low-latency connections or constrained memory environments, where you want smaller buffers & more responsive backpressure instead of a huge window optimized for throughput over high-latency.

This PR adds a new HTTP/2 connectionWindowSize option to let you configure the initial connection window. If set, it overrides the initial window that's configured on nghttp2 before the session connects.

En route this also commonizes some option validation into validateSessionOptions, deduplicating some existing validation and giving a single place to add the validation for this new option.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/http2
• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (d6e67a5) to head (137d777).
⚠️ Report is 281 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65619      +/-   ##
==========================================
- Coverage   90.07%   90.06%   -0.01%     
==========================================
  Files         751      751              
  Lines      254881   254932      +51     
  Branches    48111    48124      +13     
==========================================
+ Hits       229582   229603      +21     
- Misses      16486    16496      +10     
- Partials     8813     8833      +20     

Files with missing lines	Coverage Δ
lib/internal/http2/core.js	95.10% <100.00%> (+0.02%)	⬆️
lib/internal/http2/util.js	92.93% <100.00%> (+0.04%)	⬆️
src/node_http2.cc	81.87% <100.00%> (+0.03%)	⬆️
src/node_http2.h	92.39% <100.00%> (+0.16%)	⬆️
src/node_http2_state.h	100.00% <ø> (ø)

... and 42 files with indirect coverage changes

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[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77187/

[nodejs-github-bot]

Landed in b12688c

[aduh95]

This does land cleanly on v26.x-staging, can you please open a manual backport PR?

[aduh95]

Something I'm noticing while working on the changelog for the new release: we usually prefer add instead of add new in the commit message, new is not only redundant but also a JS keyword so I find the wording confusing.