blob/azureblob: Do not panic if Content-Length and Content-Range are missing

[chancez]

I found that when we're downloading an a gzipped object with Content-Encoding: gzip, AZB is transparently sending back an uncompressed object rather than the gzipped object, and when this happens, I also discovered that go-cloud fails and panics because the Content-Length and the Content-Range fields are not set in this situation.

To gracefully handle this, check if the ContentLength field is nil before using it, and return 0 if both ContentRange and ContentLength are missing. This unfortunately does mean Size() may return 0 in some cases, but I don't see a good way to fix that without a bigger change in design. With this patch, I can confirm our issue is resolved.

Unfortunately, I did not see a good reference for how I could write a unit test for this. It seems existing conformance tests rely on a live bucket, and there's no unit tests that mock the underlying response or server for any of the existing drivers that I could reference either. If you have a suggestion for testing, let me know, but currently no new tests are added as part of this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.43%. Comparing base (ba58ec7) to head (8799b3b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3445      +/-   ##
==========================================
+ Coverage   73.40%   73.43%   +0.02%     
==========================================
  Files         113      113              
  Lines       14948    14950       +2     
==========================================
+ Hits        10973    10978       +5     
+ Misses       3201     3198       -3     
  Partials      774      774              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.