association_table: add "default_bank" column, change plugin's default bank behavior

[cmoussa1]

Background

As discussed in #123, the "default" bank support added in #122 currently treats the first bank the user belonged to in the flux-accounting database as the default bank. It cannot be changed once the user gets added to the DB. It would be good to add more robust default bank support in both the flux-accounting database and the priority plugin, where a user's default bank could be changed by a sys admin even after they are initially added.

Implementation

This PR adds a default_bank column to the association_table in the flux-accounting database. When a user/bank combination gets added to the database, a lookup is performed to see if the user already exists in the table and, if they do, what their default bank is. It will set the default_bank to the value of the first bank they are added to; however, this value can now be changed like most other fields in the association_table with the edit-user subcommand.

The default_bank column is now also added to the payload information that is sent to the multi-factor priority plugin. When the plugin extracts this data, it will look at the default_bank key-value pair and compare it to the bank key-value pair in the same payload using strcmp. If the values are the same, it will add the fairshare information to the "default" key in the map data structure in the plugin.

This is similar to the behavior previously present in the plugin, except now, if there is ever an update to a user's default bank in the flux-accounting DB, that change should be reflected in 1) the new payloads sent by bulk_update.py, and 2) the map data structure that caches user-bank-fairshare information.

Fixes #123

[codecov]

Codecov Report

Merging #130 (848b4c6) into master (fb4fd16) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
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  Coverage   85.53%   85.53%           
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  Files          18       18           
  Lines         892      892           
=======================================
  Hits          763      763           
  Misses        129      129           

Impacted Files	Coverage Δ
src/plugins/mf_priority.cpp	94.23% <100.00%> (ø)

[grondo]

This looks good to me! Nice and simple approach.

I made one comment inline, but after re-reviewing the code I noted that it seems fine as is. I left the comment just to get us thinking about what a real "bulk" update protocol might look like.

[grondo]

Just an idea/question: It might be slightly awkward here that the protocol requires that the default_bank is sent along with every bank update. Unless I've missed something below, if default_bank != bank, then default_bank is unused.

Another idea would be to have default_bank be a boolean, but I'm not sure if this would make the client side more difficult.

Also, I realize you have an issue open to turn this simpler protocol into a "bulk update" protocol, so that you can update multiple users in one RPC. If that is the case, then maybe a cleanup of the RPC design could be done at that time. No reason to fix the current design if it works for now.

[grondo]

Ok, actually I see now above that this protocol simplifies the current bulk_update.py client, so it is probably fine to leave as is for now.

[cmoussa1]

Thanks for the review @grondo - I should copy your thoughts over to #133 so that I don't forget them, and then I will set MWP.