Add Backend method like TimeSeries.unlock()

[OFR-IIASA]

When a script fails and a scenario is checked out, then there is no possibility to unlock the scenario. Previously, there used to be a possibility to do this using the command mp.._jobj.unlockRunid(). This is of course not ideal as it was more of a backdoor solution, but this feature would be nice for expert users, which is no longer available since switching to the new backend.

[khaeru]

Backend API docs, for reference.

Some thoughts:

• The .check_out()/.commit() methods are in TimeSeries, rather than Scenario.
• I've always felt these were a weird hybrid of SQL and git semantics.
  • In SQL, there is nothing like CHECK_OUT, only COMMIT, ROLLBACK, and some others.
    • ixmp lacks anything like ROLLBACK.
  • git checkout modifies or reverts the working tree.
    • In most workflows, it is not needed to call it before modifying files.
    • Often, when used, its meaning is actually closer to SQL ROLLBACK: git checkout -- file.txt rolls back file.txt to its content as of the last commit.
  • Neither includes a notion of locking.
• .check_out() and .commit() actually do THREE things:
  • Manage transactions.
  • Locking.
  • .check_out() raises an exception if .has_solution() is True. (cf. Incorrect semantics in Platform, TimeSeries, Scenario #113 under “Methods in the wrong place”)
• The 'locked' status of a TimeSeries seems to mean 'locked for other users'.
  • ixmp.Platform and base.Backend don't explicitly discuss the (possibly) multi-user nature of Backends.
  • In SQL, if a transaction fails or is rolled back, it is simply not committed, and nothing prevents another set of modifications from starting immediately.

[khaeru]

Duplicate of #30, which is about 1.5 years older.