fix: workaround curl_multi_poll returning an error on EINTR

[pitrou]

Fixes #11647.

---

This change is 

[pitrou]

For now this implements a workaround in CurlImpl::WaitForHandles. Once we agree on that, I'll have to port the workaround to CurlDownloadRequest::WaitForHandles (or factor out the logic into a shared helper routine).

@coryan

[pitrou]

I'll note that I can't test whether this actually fixes the underlying issue. At least it doesn't seem to break the test suite on my machine.

[pitrou]

By the way, this fix assumes we want to retry on EINTR, which seems to make sense but is not the case currently.
In any case, it seems PerformWorkUntil loops if WaitForHandles returns with success. Thoughts?

[coryan]

Thanks very much for the PR.

For now this implements a workaround in CurlImpl::WaitForHandles. Once we agree on that, I'll have to port the workaround to CurlDownloadRequest::WaitForHandles (or factor out the logic into a shared helper routine).

In all likelihood we will remove that code in a couple of months. It is there just as an emergency hatch in case the new code does not work for some people, and so far it looks like it is not needed.

The code looks good to me, though it is a bit sad to have so many #if branches.

Let's run the build so see if the robots have something to say.

[coryan]

/gcbrun

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -2.45 ⚠️

Comparison is base (f2eaddb) 93.77% compared to head (63b8702) 91.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11649      +/-   ##
==========================================
- Coverage   93.77%   91.32%   -2.45%     
==========================================
  Files        1824     1404     -420     
  Lines      164431   108689   -55742     
==========================================
- Hits       154190    99261   -54929     
+ Misses      10241     9428     -813     

Impacted Files	Coverage Δ
google/cloud/internal/curl_impl.cc	90.62% <100.00%> (-0.25%)	⬇️

... and 509 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[coryan]

The failure with checkers-pr is just formatting, you can see what the robots want at the end of:

https://github.com/googleapis/google-cloud-cpp/pull/11649/checks?check_run_id=13593517862

[coryan]

/gcbrun

[coryan]

Sorry about the guessing game for formatting, I should have pointed you to https://github.com/googleapis/google-cloud-cpp/blob/main/doc/contributor/README.md#formatting

It looks good now. I will approve and merge once the builds pass.