Fairly serious thread-safety issues

[brettwooldridge]

I plugged your pool into the HikariCP benchmark harness because I was curious how DBPool performed. The benchmark uses Oracle's JMH benchmarking framework, and the primary connection test simply performs DataSource.getConnection() then connection.close() in multiple threads, as fast as possible.

Unfortunately, DBPool blows-up within the first few milliseconds. Because the open and closing members of CacheConnection are not volatile there are quickly cross-thread visibility issues (exceptions) that cause the connection to be declared invalid almost immediately and ejected from the pool.

I note also the complete lack of unit tests for this library, which should have quickly uncovered this issue. Applications that are using DBPool are either not under significant load or are "getting lucky" by crossing a Java memory barrier that happens to flush the CPU cache-lines before cross-thread visibility issues arise.

[gwinstanley]

Interesting, thanks for letting me know. When I get a chance I'll take a look at this in more detail and see how to handle it for the next release. Regarding tests, I have written a suite of tests for DBPool, but they're not currently committed, and evidently need updating, as you're issue demonstrates. I'll update once I've analysed the situation.