otelhttptrace: handle missing getconn hook without panic

[tonistiigi]

We have many reports that end() gets called without the span being defined in start() and causes a panic.

Ref moby/buildkit#4377
Ref docker/buildx#2232

I have not not fully debugged in what condition this happens in stdlib (I assume some keepalive pool case) but I don't see any other possible explanation for these panic cases.

Note that there are other httptrace hooks that also use .root without validation. I don't atm. have any proof that these could be called without GetConn() being called first as well so didn't add extra validation to these.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 61.3%. Comparing base (45226f8) to head (d12420f).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5187     +/-   ##
=======================================
- Coverage   61.3%   61.3%   -0.1%     
=======================================
  Files        185     185             
  Lines      13883   13887      +4     
=======================================
+ Hits        8512    8513      +1     
- Misses      5171    5173      +2     
- Partials     200     201      +1     

Files	Coverage Δ
...on/net/http/httptrace/otelhttptrace/clienttrace.go	78.1% <25.0%> (-0.9%)	⬇️

[dmathieu]

This change only handles missing span in end() if useSpans is false.
It seems like that could also happen with the use of sub spans as well.

Also, I know this is hard to reproduce, and this package doesn't really have any tests at the moment, but would it be possible to write a test?

[tonistiigi]

This change only handles missing span in end() if useSpans is false.
It seems like that could also happen with the use of sub spans as well.

What place/variable do you have in mind?

Also, I know this is hard to reproduce, and this package doesn't really have any tests at the moment, but would it be possible to write a test?

These calls are made from stdlib and I don't know the condition when the specific call order appears.

[pellared]

These calls are made from stdlib and I don't know the condition when the specific call order appears.

I think the main usage of httptrace.ClientTrace is here: https://github.com/golang/go/blob/master/src/net/http/transport.go