mark recoverability of DNS errors correctly

[tanmaykm]

This changes HTTP client to not retry on certain DNS errors. From the list of possible DNS errors only EAI_AGAIN may be recoverable. This also changes the error code comparison to use UV_EAI_AGAIN, because that is what a DNSError instance would contain.

fixes: #1085

[codecov-commenter]

Codecov Report

Merging #1088 (f4bd74b) into master (10182c0) will decrease coverage by 0.02%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
- Coverage   82.33%   82.32%   -0.02%     
==========================================
  Files          32       32              
  Lines        3040     3043       +3     
==========================================
+ Hits         2503     2505       +2     
- Misses        537      538       +1     

Files Changed	Coverage Δ
src/clientlayers/RetryRequest.jl	79.16% <75.00%> (-0.84%)	:arrow_down:

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[nickrobinson251]

can we add a test please?

[tanmaykm]

We had faced this in a likely stressed condition in our application. I was testing this with a modified julia binary to artificially throw the DNS error. I found it hard to replicate the error otherwise.

I could add a unit test for isrecoverable, maybe that would be useful?

[tanmaykm]

@nickrobinson251 Actually I think I can add some relevant tests. And I also missed out some more changes to isrecoverable. PR in a bit.

[nickrobinson251]

I think I can add some relevant tests

Thanks!

a unit test for isrecoverable, maybe that would be useful?

yeah, i think unit tests would be helpful