Add stored procedures and triggers

[dumbbell]

There are two parts in this patch:

Stored procedures

We leverage the khepri_fun framework used to extract anonymous functions (used for transactions) to extract and store one as the payload of a Khepri tree node. The function will be replicated on all Khepri nodes in the cluster and therefore can be executed anywhere, regardless of the availability of the initial module(s) the function was extracted from.

To store a function, we introduce a new payload record:

Payload = #kpayload_sproc{sproc = fun() -> ... end}.

This payload record can be passed to khepri_machine:put/3 and higher-level functions in khepri.

To execute a stored procedure, there is a new khepri_machine public API:

khepri_machine:run_sproc(StoredId, StoredProcPath, Args).

StoredProcPath is the path where the function was stored in the tree and Args is a (possibly empty) list of terms, like one would pass to erlang:apply/2.

Stored procedures may do whatever they want and have side effects, unlike transaction functions. Stored procedures don't have direct access to the tree and can't modify it directly, again unlike transaction functions. However, they are free to use the Khepri API to update the tree as any caller.

The return value of khepri_machine:run_sproc/3 is the return value of the stored procedure.

When a path is queried using e.g. khepri_machine:get/2, the node properties map has a sproc member instead of a data one.

Triggers

A trigger is a way to associate an event with a stored procedure: when the event occurs, the stored procedure is executed.

This commit supports only one type of event: changes to the tree (the database). Whenever a tree node is added, modified or deleted, the list of triggers is evaluated to see if one or more are interested in this event. If some are, their associated stored procedure is executed.

To register a new trigger, one first defines an event filter, stores a procedure and finally registers the association between this event filter and the stored procedure (i.e. the trigger):

EventFilter = #kevf_tree{path = PathPattern,
                         props = #{on_actions => [create, update]}},
khepri_machine:register_trigger(
  StoredId, TriggerId, EventFilter, StoredProcPath).

An event filter is evaluted using:

• the PathPattern which can contains path conditions;
• the event's action, i.e. whether a node was created, updated or
  removed.

The return value of the stored procedure is ignored when executed in the context of a trigger.

See #16.

[coveralls]

Pull Request Test Coverage Report for Build 325

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Details

• 142 of 190 (74.74%) changed or added relevant lines in 8 files are covered.
• 34 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.8%) to 83.681%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/khepri_utils.erl	15	16	93.75%
src/khepri_app.erl	1	3	33.33%
src/khepri_tx.erl	17	20	85.0%
src/khepri_event_handler.erl	14	20	70.0%
src/khepri_sproc.erl	9	15	60.0%
src/khepri.erl	4	12	33.33%
src/khepri_machine.erl	77	99	77.78%

Files with Coverage Reduction	New Missed Lines	%
src/khepri_utils.erl	1	97.44%
src/khepri_machine.erl	33	86.25%

Totals
Change from base Build 309:	-1.8%
Covered Lines:	1405
Relevant Lines:	1679

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💛 - Coveralls