Verifies that client is available before sending breadcrumbs

[lomefin]

Description

During build, we have encountered situations where Sentry wants to send
a breadcrumb but it fails to accomplish the task because of
NoMethodError: undefined method configuration' for nil:NilClass`.
After one of this error is triggered, Sentry fails to work at all,
passing from 0 to several hundred errors in our pipeline, and then back
to zero once we start rebuilding.

Our pipeline tests using parallelism and there is a ton of concurrency,
so we believe that has something to do with it.

The backtrace points out to Hub#add_breadcrumb and inside of it to
configuration. It is possible that while the new "local" hub is
available, the client is not and fails to work.

We have observed that several other method have checks
in place to verify that the client is really available but not on
add_breadcrumb so we added a verification before starting the rest
of the process.

[st0012]

It is possible that while the new "local" hub is
available, the client is not and fails to work.

Can you elaborate more on this? I think this indicates a bigger problem either in your application's Sentry setup OR in the SDK itself.

I'd like to check a few things like:

• Does the app itself create and manage Hubs? For example, by calling Hub#new_from_top or Hub#clone
• Does any part of it call Hub#pop_scope?
  • @sl0thentr0py @solnic Although it may not be the cause of this issue, I feel like pop_scope is a very dangerous API to expose as it pops client as well. I think when there's only one layer left, pop_scope should retain the layer but just replace the scope?

[sl0thentr0py]

@st0012 this is just part of the design problem of the older hub/scopes, we will eventually consolidate into a single OTEL-like context so there will be no more pop_scope.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.67%. Comparing base (5980531) to head (038fee8).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2394      +/-   ##
==========================================
- Coverage   98.68%   98.67%   -0.01%     
==========================================
  Files         210      210              
  Lines       13923    13923              
==========================================
- Hits        13740    13739       -1     
- Misses        183      184       +1     

Components	Coverage Δ
sentry-ruby	99.05% <100.00%> (-0.01%)	⬇️
sentry-rails	97.34% <ø> (ø)
sentry-sidekiq	97.07% <ø> (ø)
sentry-resque	96.79% <ø> (ø)
sentry-delayed_job	98.92% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/hub.rb	99.40% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes