Always pass exported functions to Ra queries

[dumbbell]

Why

Using fun() is fragile if it has to be executed on a remote node. Indeed, the remote node may have another version of the module hosting the function and the function reference may be invalid.

How

There are two parts:

1. We always use an exported function inside Khepri.
2. We ensure that the caller of folding APIs passes MFA if they request leader or consistent queries. fun() are only accepted for local queries.

Fixes #238.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b75785f) 88.74% compared to head (f02a92b) 89.37%.

Files	Patch %	Lines
src/khepri_machine.erl	95.23%	2 Missing :warning:
src/khepri.erl	96.55%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   88.74%   89.37%   +0.63%     
==========================================
  Files          20       21       +1     
  Lines        2896     3079     +183     
==========================================
+ Hits         2570     2752     +182     
- Misses        326      327       +1     

Flag	Coverage Δ
erlang-24	88.34% <98.66%> (+0.66%)	:arrow_up:
erlang-25	88.34% <98.66%> (+0.66%)	:arrow_up:
erlang-26	88.98% <98.66%> (+0.62%)	:arrow_up:
os-ubuntu-latest	89.34% <98.66%> (+0.60%)	:arrow_up:
os-windows-latest	89.08% <98.66%> (+0.68%)	:arrow_up:

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[dumbbell]

The patch is finished but I would like to test it with RabbitMQ before merging it.

[dumbbell]

I have second thoughts on the fact that MFAs are good enough as a solution to execute code remotely. Yes, with the current patch, we ensure that a query won’t break because the function reference is invalid on another node. But that doesn’t mean that the function exported remotely is the same as the local one. This is quite unintuitive.

Thinking out loud here; if a leader query was requested, what about we check the cursor on the leader, wait for the local cursor to catch up, then execute the query function locally? Don’t we have the same consistency guaranty? It may take more time to answer the query though, because the catch up can be longer than a remote execution.

[dumbbell]

This pull request was superseded by #260.