SQLite cache: Use SQLite write-ahead logging for better concurrency

[amotl]

Problem

@edgarasg reported there is still a problem with the SQLite-based cache and concurrent access.

• Database is locked in thread ... error #111 (comment)

Solution

@JWCook informed us about the wal=True argument to requests-cache' Session class/mixin, effectively enabling SQLite write-ahead logging, allowing read operations to not block writes.

• Database is locked in thread ... error #111 (comment)

[1] https://sqlite.org/wal.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.17%. Comparing base (756e395) to head (3fa8a80).

❗ Current head 3fa8a80 differs from pull request most recent head 47348c7. Consider uploading reports for the commit 47348c7 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   87.17%   87.17%           
=======================================
  Files           9        9           
  Lines        1076     1076           
=======================================
  Hits          938      938           
  Misses        138      138           

Flag	Coverage Δ
unittests	87.17% <ø> (ø)

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