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8f1caa6a90
... with associated calling periods and use the new API in `basic_test`.
Previously `ticker` would use a single function, `on_tick`, which it
called in a loop with yields in-between. In `basic_test` we would use
this to tick every object in synchrony.
However, to closely simulate a production environment, we may want the
tick ratios to be different. For example Raft servers should be ticked
rarely compared to the network.
We may also want to give the Seastar reactor more space between the
function calls (e.g. if they cause a bunch of work to be created for the
reactor that needs more than one tick to complete).
To support these use cases we generalize `ticker` to take a set of
functions with associated numbers. These numbers are the call periods of
their corresponding functions: given {n, f}, `f` will be called each
`n`th tick.
We also modify `basic_test` to use this new approach: we tick Raft
servers once per 10 network ticks (in particular, once per 10 reactor
yields).
This commit effectively reverts 01b6a2eb38
which caused the ticker to call `on_tick` only when the Seastar reactor
had no work to do. This approach is unfortunately incompatible with the
approach taken there. We *do* want the ticker to race with other work,
potentially producing more work while already scheduled work is executing,
and we want to see in tests what happens when we adjust the ticking ratios
of different subsystems.
The previous approach also had a problem where if there was an infinite task
loop executing, the ticker wouldn't ever tick.
The previous fix was introduced since the ticker caused too much work to
be produced (so the reactor couldn't keep up) due to ticking the Raft
servers too often (after each yield). This commit deals with the problem
in a different way, by ticking the servers rarely, which also resembles
"real-life" scenarios better.
With this change we must also wait a bit longer for the first node to
elect itself as a leader at the beginning of the test.