Use IO#readline_nonblock instead of IO#readline when reading output from CPI sub-processes.

[yatzek]

IO.#readline, being a blocking method, can cause a dealock when the cpi
sends output without a line break. We had such issues with azure cpi.

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[rkoster]

Thanks for the PR, seems like a great improvement. However, when testing test change locally I found that changing back to the original implementation did not make the test fail.

[rkoster]

With the latest changes I'm getting:

........................................................................................................................An exception occurred running RSpec::Core::Example:
#<NoMethodError: undefined method `asset' for #<RSpec::ExampleGroups::BoshCloudsExternalCpi::FixForADealockScenarioWhenACpiSubProcessSendsAnInclompleteLineMissingNToSTDOUT "does not deadlock" (./bosh-director/spec/unit/cloud/external_cpi_spec.rb:658)>>

Test directory: /Users/rubenk/workspace/bosh/src/tmp/integration-tests-workspace/pid-99152/spec-20211108-99152-ikaeaa

Sandbox directory: /Users/rubenk/workspace/bosh/src/tmp/integration-tests-workspace/pid-99152
F

Failures:

  1) Bosh::Clouds::ExternalCpi Fix for a dealock scenario when a cpi sub-process sends an inclomplete line (missing "\n") to STDOUT does not deadlock
     Failure/Error: before { allow(File).to receive(:executable?).with(asset("bin/dummy_cpi")).and_return(true) }

     NoMethodError:
       undefined method `asset' for #<RSpec::ExampleGroups::BoshCloudsExternalCpi::FixForADealockScenarioWhenACpiSubProcessSendsAnInclompleteLineMissingNToSTDOUT "does not deadlock" (./bosh-director/spec/unit/cloud/external_cpi_spec.rb:658)>
     # ./bosh-director/spec/unit/cloud/external_cpi_spec.rb:656:in `block (3 levels) in <top (required)>'

Finished in 3.13 seconds (files took 3.25 seconds to load)
841 examples, 1 failure

Failed examples:

rspec ./bosh-director/spec/unit/cloud/external_cpi_spec.rb:658 # Bosh::Clouds::ExternalCpi Fix for a dealock scenario when a cpi sub-process sends an inclomplete line (missing "\n") to STDOUT does not deadlock

[beyhan]

My summary:

• all unit tests ran successful on my MacOS.
• leaving the new does not deadlock unit test and reverting to the old code failed the test

@rkoster is the failure with the unit test reproducible on your machine?

[rkoster]

Thanks @yatzek!