Fix machine_type unique constraint to support a machine_type for multiple sites

[giffels]

This pull request fixes a bug in the database schema of the SqliteRegistry plugin. In the current schema it was not possible to have the very same MachineTypes on different Sites, since the unique constraints put on the database table were too strict. This pull request introduces a more relaxed unique constraint, that requires only the combination (MachineType, Site) to be unique.

In addition, this pull request introduces a more relaxed unique constraint on the Resources table, too. Now it is possible that the same remote_resource_uuid can be used at different sites within the same table. This was not mentioned in #219, but could lead to further trouble in the future. For example, two HTCondor batch systems at different sites can have the same job number used as remote_resource_uuid.

In addition, the unittest of the SqliteRegistry have been improved by adding both scenarios mentioned above and de-duplicate some of the test code.

Fixes #219

[codecov-commenter]

Codecov Report

Merging #220 (62ad6d5) into master (c532a19) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   99.55%   99.56%           
=======================================
  Files          41       41           
  Lines        1810     1823   +13     
=======================================
+ Hits         1802     1815   +13     
  Misses          8        8           

Impacted Files	Coverage Δ
tardis/plugins/sqliteregistry.py	100.00% <100.00%> (ø)

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[maxfischer2781]

Thanks for catching and fixing both issues!

I have some comments/requests on code health but nothing major.

[maxfischer2781]

By the way, my SQL is quite a bit rusty. I think the INSERT OR ROLLBACK is fine but I'm not sure if it's actually needed for site/machinetype, seeing how you check these before attempting the insert.

[giffels]

By the way, my SQL is quite a bit rusty. I think the INSERT OR ROLLBACK is fine but I'm not sure if it's actually needed for site/machinetype, seeing how you check these before attempting the insert.

Actually, it is not needed. The code is only called when the Site or machine_type is not present in the DB. However, in case of any unexpected failure ROLLBACK will throw an exception after the DB rollback. This is an improvement to the IGNORE which was used before, that just silently ignored the errors.

[maxfischer2781]

INSERT INTO tardis