Connection not closed when database name is incorrect #173 fix

[parMaster]

#173 describes a bug when a database name is provided that does not exist, but the connections remain open and db.Close() doesn't help to close them.
This PR is supposed to fix this issue by closing connection before returning error. The connection is unusable despite the message from the server like Cannot open database "shmaster" that was requested by the login. Using the user default database "master" instead - db.PingContext returns an error, but connections remain open

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.83%. Comparing base (02deabf) to head (baba640).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   74.62%   74.83%   +0.20%     
==========================================
  Files          32       32              
  Lines        6396     6397       +1     
==========================================
+ Hits         4773     4787      +14     
+ Misses       1335     1327       -8     
+ Partials      288      283       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[shueybubbles]

thx for the PR

Please add a test that fails without your change.

[parMaster]

thx for the PR

Please add a test that fails without your change.

You're right.
As you can see in the original issue, we can monitor the opened connections with a netstat -an | grep '1.1433' cmd, but I think it's not right to call exec.Command("netstat", "-an") and check the output.
So, I've decided to query a sys.dm_exec_connections system view which Returns information about the connections established to this instance of the database engine and check if there are leaked connections left even after explicit conn.Close().
The test fails without my change and shows the number of leaked connections.

[parMaster]

@microsoft-github-policy-service agree company="Amoniac OÜ"