Prevent crashes caused by threading issues

[ejensen]

Overview

This PR addresses a few crashes caused by threading issues in array mutation. The symptoms of these crashes are EXC_BREAKPOINT, SIGABRT ABORT, and EXC_BAD_ACCESS KERN_INVALID_ADDRESS exceptions on the spc_sync_array thread. The exceptions have the following details:

• Object of class SynchronizedArray deallocated with non-zero retain count 3. This object's deinit, or something called from it, may have created a strong reference to self which outlived deinit, resulting in a dangling reference.

• malloc: Non-aligned pointer being freed (2)

• malloc: double free for ptr

Issues being addressed

1. Instances of the SychronizedArray class were being replaced rather than updated. Replacing the instances of SychronizedArray removes the safety the SychronizedArray type provides. It also results in the spc_sync_array dispatch queue owned by SychronizedArray becoming unowned when the SychronizedArray instance deallocates, causing SIGABRT ABORT and EXC_BAD_ACCESS KERN_INVALID_ADDRESS exceptions when the operations of the queue execute.
   • This issue is addressed by making SychronizedArray instances lets and using the synchronized functions of SychronizedArray to update the underlying array variable
2. The sendBuffer array was being mutated on multiple threads, causing EXC_BREAKPOINT exceptions
   • This issue is addressed by using SychronizedArray

[ejensen]

@dsrees when you get a chance, can you take a look at these changes?

[dsrees]

@ejensen Thanks! These threading issues are always hard to track down in the wild so I appreciate the contribution

[ejensen]

Thank you @dsrees! Do you think this fix can be included in a 5.3.2 bug fix version release?

[dsrees]

done