Support trailers

[TimothyGu]

Fixes #330.

[codecov-io]

Codecov Report

Merging #331 into master will increase coverage by 1.78%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #331      +/-   ##
========================================
+ Coverage   98.21%   100%   +1.78%     
========================================
  Files           6      6              
  Lines         503    433      -70     
  Branches      153    133      -20     
========================================
- Hits          494    433      -61     
+ Misses          5      0       -5     
+ Partials        4      0       -4

Impacted Files	Coverage Δ
src/headers.js	100% <100%> (+3.81%)	⬆️
src/response.js	100% <100%> (ø)	⬆️
src/index.js	100% <100%> (+4.87%)	⬆️
src/request.js	100% <0%> (ø)	⬆️
src/body.js	100% <0%> (ø)	⬆️

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[bitinn]

while this look fine, I haven't used trailer header before; need another pair of eyes to come in and check this is legit.

[TimothyGu]

@jkantr? @zkat?

[jkantr]

LGTM

(sorry for the tardy response, i slipped and rather fell out of our plane of existence for a while)

[bitinn]

I am honestly happy to merge.

But it seems like major browsers are resisting this feature:

whatwg/fetch#473 (see linked bug tracker)

[TimothyGu]

Switched to standard pending per #331 (comment).

[phamann]

@TimothyGu What is the status of this?

Whilst yes, Chrome etc have not implemented Trailer support in their fetch implementations. It is still been accepted as part of the spec and useful within server-side environments regardless of browser implementation status.

Trailers have become more useful since this PR was opened due to their adoption within the Server-Timing API as a mechanism to return timing information about a response after it is closed, such as the final round-trip time etc.

Firefox have also recently implemented trailer support specifically for Server-Timing: https://bugzilla.mozilla.org/show_bug.cgi?id=1413999

Therefore, would you consider reopening and merging this PR? I am happy to do the work to fix the merge conflicts etc if so :)

[xxczaki]

Closing this, as #690 is on hold.