fix concurrency issues of parallel transactions (#327)

[goteguru]

This is the proposed solution for async transaction execution failure (see #327 )

Tests are running fine for postgres, mysql, sqlite.

I had to change the test tests/test_databases.py because it incorrectly expects database.transaction() to use the same connection as the connection context around it, which is certainly not true (and actually the part of the problem). If I acquire a connection, I have to use that connection for a transaction. (Requesting the database directly may give any other connection from the pool).

All comments are welcome.

[ToGoBananas]

looks good, no?

[erakli]

@tomchristie take a look at this PR please

[tomchristie]

@tomchristie take a look at this PR please

I'm happy to help out with expanding the maintainers team as needed, but I don't have any time to invest in databases myself.

[aminalaee]

@goteguru Please run linting/formatting scripts in the scripts folder.

[goteguru]

@tomchristie take a look at this PR please

I'm happy to help out with expanding the maintainers team as needed, but I don't have any time to invest in databases myself.

Thx for the honest reply. In the meantime I've replaced databases with direct asyncpg calls in my project (I don't like if local changes of dependencies unmerged to upstream), but I believe my future (concurrent) project can profit from this fix.

Maybe it would be nice to list the active project maintainers (w/ commit right) in the project page.

[goteguru]

@aminalaee

@goteguru Please run linting/formatting scripts in the scripts folder.

I already did manually (all tests are passed), but I run them again for isort.
BTW: some windows commit corrupted the line endings, therefore the scripts will fail on linux using dash for sh (eg. all ubuntu).

Please consider recommended best practices while working from a windows machine:
git:formatting and whitespace

[aminalaee]

Thank you @goteguru

[gertvdijk]

Cool. Looks like this could fix #294, #230 and #176 too.

Although, I have switched away from using databases too, in favor of sqlalchemy + asyncpg.