Make access to ArrayDeque synchronized

[Mahoney]

ArrayDeque specifies that:

Array deques ... are not thread-safe; in the absence of external
synchronization, they do not support concurrent access by multiple
threads.

marshalerPool is concurrently added to by the OkHttp threadpool without synchronization, along with all threads that end spans (with synchronisation in SimpleSpanProcessor.exportLock, which is not used to synchronize with the OkHttp threadpool).

Just making the ArrayQueue synchronous internally removes all need to worry about upstream locks.

Fixes #7019

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Mahoney / name: Robert Elliot (6d32714)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.95%. Comparing base (c8da020) to head (6d32714).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7027   +/-   ##
=========================================
  Coverage     89.95%   89.95%           
  Complexity     6636     6636           
=========================================
  Files           745      745           
  Lines         20010    20015    +5     
  Branches       1962     1962           
=========================================
+ Hits          17999    18004    +5     
  Misses         1415     1415           
  Partials        596      596           

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[laurit]

the same construct is also used in

opentelemetry-java/exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/logs/LogReusableDataMarshaler.java

Line 23 in c8da020

private final Deque<LowAllocationLogsRequestMarshaler> marshalerPool = new ArrayDeque<>();

and

opentelemetry-java/exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/metrics/MetricReusableDataMarshaler.java

Line 23 in c8da020

private final Deque<LowAllocationMetricsRequestMarshaler> marshalerPool = new ArrayDeque<>();

did you consider just using a different queue implementation such as ConcurrentLinkedDeque?

[jack-berg]

Let's do this.

Also, this change is applicable to MetricReusableDataMarshaler and LogReusableDataMarshaler as well.

[Mahoney]

did you consider just using a different queue implementation such as ConcurrentLinkedDeque?

No, I didn't even remember ConcurrentLinkedDeque existed, and I'm not familiar with its guarantees and performance characteristics. Might well be a better and lighter option.