clear cache of the result of GCECredentials.on_gce? when GCECredentials.on_gce? returns false.

[serihiro]

What will this PR change ?

• Modify to clear the cache of the result of GCECredentials.on_gce? if GCECredentials.on_gce? returns false.
• This change resolves the problem which GCECredentials.on_gce? keeps returning false even though GCECredentials::COMPUTE_CHECK_URI is active, as I explained in the following.

Why is this change needed?

• GCECredentials.on_gce? caches the result as a Class instance variable. So, once GCECredentials.on_gce? is called, this method keeps returning a same value in a same ruby process.
• But, GCECredentials.on_gce? returns false occasionally even though COMPUTE_CHECK_URI is active.
• So, if GCECredentials.on_gce? returns false in the first time call, GCECredentials.on_gce? keeps returning false within a same ruby process.
  • I faced this problem when I developed an ETL tool which accesses to BigQuery via google-cloud-ruby and uses GCE instance auth.
• The following is my just experiment.
  • By calling Google::Auth::GCECredentials.on_gce? without caching in 30 times at GCE, I got
    false once in 30 times.

Note: Google::Auth::GCECredentials._unmemoized_on_gce? is a method which is defined by memoist implicitly, and is executed without caching.

irb(main):055:0> 30.times {pp Google::Auth::GCECredentials._unmemoized_on_gce? }
true
true
true
true
true
true
true
true
true
true
true
true
true
true
false
true
true
true
true
true
true
true
true
true
true
true
true
true
true
true

• To avoid this problem, I think Google:: Auth#get_application_default should clear the cache of the result of GCECredentials.on_gce? when GCECredentials.on_gce? returns false.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[coveralls]

Coverage increased (+1.3%) to 98.508% when pulling 6d6bcd1 on serihiro:clear-cache-when-on_gce-returns-false into 18611aa on google:master.

[dazuma]

This is a good catch: there definitely is the possibility of false negatives since this call depends on a network connection, and we shouldn't memoize those. Ideally we'd still want to memoize any "emphatic" negatives, but I can't think of a common case that would generate those. So LGTM.

[serihiro]

Thanks for reviewing and merging! 🍻