http3: sniff Content-Type when flushing the ResponseWriter

[WeidiDeng]

When caddy log is enabled, it can be observed Content-Type sniffing no longer works for http3, but works for h1/2. This is caused because when logging is enabled, caddy wraps all response writers with responseRecorder which will call WriteHeader first.

Stdlib will try set Content-Type header in the same logic, for http1, *chunkWriter is wrapped by *response to write the actual data. http2 will set this header in the same way.

Because this header is set once, it won't be overwritten automatically

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 84.78%. Comparing base (083ceb4) to head (0ff1f51).
Report is 1 commits behind head on master.

Files	Patch %	Lines
http3/response_writer.go	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4412      +/-   ##
==========================================
- Coverage   84.79%   84.78%   -0.01%     
==========================================
  Files         154      154              
  Lines       14645    14647       +2     
==========================================
  Hits        12418    12418              
- Misses       1715     1716       +1     
- Partials      512      513       +1     

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[marten-seemann]

@WeidiDeng The large refactor for HTTP Datagram support (#4452) was merged. Do you want to update this PR?

[marten-seemann]

This looks a lot cleaner now.

Do I understand correctly that the only thing that this PR changes is that the sniffing algorithm is now also run when flushing, whereas before it was only run on Write?

[marten-seemann]

This PR is the only thing left for the v0.43 release.

[marten-seemann]

Do I understand correctly that the only thing that this PR changes is that the sniffing algorithm is now also run when flushing, whereas before it was only run on Write?

@WeidiDeng Can you confirm this?

[WeidiDeng]

@marten-seemann Yes, I confirm that's the change.

[marten-seemann]

Thank you @WeidiDeng!