test: check hotreload

[ArtDu]

The class of problems that this test catches is the expirationd integration with cartridge. When hot-reloading roles (that is, when call require("cartridge.roles").reload() ), expirationd should exit gracefully. All fibers should terminate and the task list should be cleared.

These tests will also be used as an example of using hot reload and expirationd

tests:

• check how expiraiond will behave when reloading a role without writing additional code in role stop
• transactions will be processed correctly if we specify the use of batch transactions in expirationd.start
• reloading the role with saving the iteration state and starting the expirationd task from the moment the last role ended

Closes #54

[akudiyar]

The changes must be validated with @Totktonada

[Totktonada]

What property of expirationd you verify here? I don't understand how all this code is relevant to expirationd.

[Totktonada]

There should be a well defined hypothesis about a class of problems we can catch this way (plus understanding why we can't do it in a more obvious and simple way) to actually add and support this testing code within the expirationd repository.

[Totktonada]

Please, take me right: I encourage using of smart testing techniques, where usual unit / functional testing are not powerful enough. Say, when a functional testing is able to catch particular set of highly specialized problems, but well implemented integration testing is able to catch almost any problem of given class. Or, say, the typical situation, when a functional testing unable to catch a real world problem, which only appears with data from a previous version of a module / an application.

I understand what cartridge/hotreload.lua doing (as well as moonlibs/module-reload). But I still don't catch the idea of this test. The questions that I ask for myself are:

• What is the class of problems it should catch?

• Whether it is beyond the class of problems that will be catched by the following code:

  local expirationd = require('expirationd')
  
  <..create a space..>
  <..fill the space..>
  expirationd.start(<...>)
  <..check all tuples are expired..>
  
  expirationd.kill(<...>)
  <..fill the space..>
  <..check that tuples are not expiring..>
  
  package.loaded.expirationd = nil
  
  expirationd = require('expirationd')
  expirationd.start(<...>)
  <..check all tuples are expired..>

• Even more, whether it is beyoud problems we able to catch this way:

  require('expirationd')
  package.loaded.expirationd = nil
  require('expirationd')

[ArtDu]

in this case, @no1seman asked to make a test, which will clearly show that when call require("cartridge.roles").reload(), correct processing of the expirationd module is possible

[vrogach2020]

After discussion with @ligurio and @ArtDu there are some follow-ups:

• split test cases into unit tests and integration tests
• keep unit tests in this repo
• if integration tests will require cartridge dependency probably we will move them to other repo (checking bundle integrations)

[ligurio]

After discussion with @ligurio and @ArtDu there are some follow-ups:

So I'll close PR. Reopen it when changes will be ready to review.