http: add 'sendingHeaders' event for response header manipulation

[bjohansebas]

Okay, after so much time I can finally find time for this. Meanwhile, I’m still testing locally because building Node is quite heavy.

In summary, this serves as a replacement for on-headers, which is a package that does monkey-patching. The idea is to have an event so that middlewares like compression or express-session can use it to modify headers, and for now also, in the case of compression, to transform it into a gzip stream and send the data. However, we will still need two more events to fully remove the monkey-patching of res.write and res.end used in compression, express-session, and many other middlewares

cc: @blakeembrey

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[bjohansebas]

Okay, continuing with this, Express would have an interesting situation here. While on-headers would run first and then this new event would run afterward, it could cause issues with things like compression, since it might overwrite work that was already done internally by on-headers, potentially breaking applications.

In other words, this would affect compression and other middleware. From the Express side that might not be a huge problem, but I'm not sure about the rest of the ecosystem. That's why I'm not sure whether this should be considered a semver-major change @nodejs/http @nodejs/tsc?

[Ethan-Arrowood]

Nice work. I'm +1 on landing something like this to eliminate the monkey-patching in on-headers package.

I think this should be semver-minor btw.

[bjohansebas]

Okay, I added the safeguard you mentioned—good catch. Also, statusMessage is now assigned after the hook, so it now respects whatever the developer sets.

[bjohansebas]

This is to prevent regressions while making the changes

[codecov]

Codecov Report

❌ Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.25%. Comparing base (3ac95f2) to head (7b8a916).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/_http_server.js	92.10%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63996      +/-   ##
==========================================
+ Coverage   90.23%   90.25%   +0.01%     
==========================================
  Files         741      741              
  Lines      241692   241742      +50     
  Branches    45541    45558      +17     
==========================================
+ Hits       218097   218178      +81     
+ Misses      15113    15103      -10     
+ Partials     8482     8461      -21     

Files with missing lines	Coverage Δ
lib/internal/http2/compat.js	97.25% <100.00%> (+0.03%)	⬆️
lib/_http_server.js	96.38% <92.10%> (-0.12%)	⬇️

... and 33 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.