PYTHON-5671

Changes in this PR

Based on investigation in PYTHON-5007, using fine-grained locks improved thread efficiency by about 20% in a small read scenario. This PR seeks to implement the suggestions.

Previously, code in pool.py was protected by the sole lock self.lock and its two conditionals. Now, there are six individual locks that are the sole protectors of the following:

• self._conns_lock
  Protects self.conns
• self._active_contexts_lock
  Protects self.active_contexts as well as the subsequent updates to self.__pinned_sockets, self.active_sockets, next_connection_id, self.ncursors, and self.ntxns
• self.size_cond
  Protects self.requests and self.gen
• self._max_connecting_cond
  Protects self._pending
• self._operation_count_lock
  Protects self.operation_count
• self.lock
  Protects self.state

Other strategies used to mitigate lock contention include:

• Moving lines that do not require a lock to be performed before lock acquisition
• Use of temporary, local variables to avoid nested lock contention
• Removing lock acquisition from update_is_writable() as its sole caller already has the lock
• Copying self.active_contexts to update its references when additional additions/deletions to the set should not be operated upon or their absence does not modify behavior, avoids errors that arise if calling on the original and it's length/size changes

Test Plan

Tests already exist in main/test/test_pooling.py and test/asynchronous/test_pooling.py. Since this was only a code refactor, no additional tests were added.

Checklist

Checklist for Author

• [yes] Did you update the changelog (if necessary)?
• [yes] Is there test coverage?
• [n/a] Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?