Pluggable signature storage

[takluyver]

This makes signature storage pluggable at NotebookNotary.store, and provides two implementations:

• SQLiteSignatureStore is the default
• MemorySignatureStore is a fallback and an example for other implementations.

I'm experimenting with configuring the store as an instance rather than the factory pattern we use elsewhere. This relies on using Python config files, and means that configuration would look like this:

from sqlsigs import MySQLSignatureStore
c.NotebookNotary.store = MySQLSignatureStore('127.0.0.1', database='nbsignatures')

# Rather than:
c.NotebookNotary.store_type = 'sqlsigs.MySQLSignatureStore'
c.MySQLSignatureStore.server = '127.0.0.1'
c.MySQLSignatureStore.database = 'nbsignatures'

[minrk]

this needs to be configurable, if users are to set it in config files

[minrk]

Awesome! Looks very nice. 👍 on the configurable Instance idea vs the factory approach.

[minrk]

Are you okay holding this for 4.3? I'd like to push out 4.2 with the json output stuff that's ready to go.

[takluyver]

Yep, no problem with holding this for 4.3.

[minrk]

Do you want to add a test to exercise notary loading a store instance from config?

[takluyver]

I have added such a test, but it highlighted something unexpected: traitlets deepcopy-s config values before they're applied, so the store is actually a copy of the one you create in config. I can't immediately see a problem with that, but it seems like the kind of weird thing that could bite someone and be really hard to figure out. So I'm wondering about reverting to the usual factory pattern configurable.

[takluyver]

Finally got round to looking at this again. I have switched the configuration to a more conventional factory-pattern approach to avoid the deepcopy issue.

[minrk]

Nice