Adding Requisites

Requisites are a basic language feature of idem, they allow for a definition of how to perform a dependency check. The simplest requisite is require. The require requisite simply mandates that the required ref has been processed and returned a result of True.

There are 2 plugin subsystems that are used in the resolution of requisites. The idem.req sub and the idem.rules sub. These are simple subs that allow the definition of the requisite as well as the rules that create the requisite check.

An idem.req plugin has a function called define which is used to define what rules are run by the requisite and how those rules are checked. This allows the rules to be re-usable for multiple requsite definitions. The simple example of the require requisite is this:

def define(hub):
    """
    Return the definition used by the runtime to insert the conditions of the
    given requisite
    """
    return {
        "result": [True, None],
    }

This return states that only one rule needs to be checked, the result rule. The value, in this case, that is passed to the result rule is the list [True, None]. This value is loaded into the result rule as the condition argument.

The rules are a little more complicated. They need to do the work to verify that for all of the required ID refs that the rule is followed. Here is the result rule:

def check(hub, condition, reqret, chunk):
    """
    Check to see if the result is True
    """
    if isinstance(condition, list):
        if reqret["ret"]["result"] in condition:
            return {}
    if reqret["ret"]["result"] is condition:
        return {}
    else:
        return {
            "errors": [
                f'Result of require {reqret["r_tag"]} is "{reqret["ret"]["result"]}", not "{condition}"'
            ]
        }

A rule plugin needs a function called check that is used to check the rule. It takes 3 arguments. These arguments are condition, reqret, and chunk. These are objects that are internal to idem. The condition is the data passed in by the define function in the idem.req plugin. The reqret is the return information for a single required ID that has already been executed. The chunk is the ID declaration with the assigned requisite. The return from this function defines