From: shevegen@...
Date: 2018-11-04T15:25:29+00:00
Subject: [ruby-core:89707] [Ruby trunk Feature#15277] at_exec

Issue #15277 has been updated by shevegen (Robert A. Heiler).


> I imagine that the need of calling at_exec is rare though

I think something similar could be said about at_exit. While I
think it is fine that it exists, I found that in larger projects
it can be quite annoying if some other project uses at_exit.

For example, this old project makes use of at_exit:

    https://rubygems.org/gems/cgi-exception

It prints exceptions that a .cgi script has onto the web-page,
a bit similar as to how .php files work.

I copy/paste the code so that it is easier to read:

    ## when process exit, print exception if exception raised
      at_exit do
        if $!
          ex = $!
          CGIExceptionPrinter.new($_header_printed).handle(ex)
          if defined?(EditorKicker)
            EditorKicker.handle(ex) #if ENV['EDITOR_KICKER']
          end
        end
      end

The display of where an error line occurs is nice, but I found
that at_exit events can be difficult to control and lead to
some more complexity. It may not always be easy to find out
which at_exit is triggered first and where. This is largely
a reason why I avoid at_exit, even if it may be useful.

While I am neutral on the proposal here, thus not having a pro
or con opinion, I think we can generalize this a bit by looking
not only at at_exit, but also autoload. Autoload is similar in
some ways in that it "says": "if a constant is first used and
unknown, load these files". So it also is a bit like an at-"event"
or a hook. There are other hooks/events in ruby; catch-sigint
or the included, inherited etc...

Perhaps it may be useful to easily specific "at" events.

So for example, rather than a pre-defined at_exec method,
there could be a simpler "build" API that allows such events
to happen. Like the various attr APIs like attr_reader,
attr_writer and so on.

Perhaps like this way:

    define_at :exec { puts "Bye!" }
    at_exec { puts "Bye!" }

(I guess the first line needs (), but I only wanted to demonstrate
the idea here. This would generate at_* methods, without having
them pre-defined.)

Of course I don't know if Marc-Andre agrees to any of this - I only
wanted to give this as a rough idea what I mean. :)

You could then think of at_exit as being similarly defined. And any
other methods that a ruby hacker may want to use, via an "at_" prefix.

But anyway, I do not want to distract from the suggestion. Marc-Andre
also gave a use case and I think even if one may not like the special
case of at_exec, I think the usefulness of the described use case is
still a possibility.

Ultimately the hook/events/conditional_code execution outside of 
more "classical" if/else checks is something that matz may have to
think about. (On an unimportant side note, the old programming
language LPC, used for some old text-MUDs, also had certain events
and hooks that could be "attached" onto mobs/monsters/npcs, to 
enable certain additional ad-hoc functionality or behaviour, without
this having to necessarily be written into the files that would be
used to create the initial object at hand. Of course ruby is a lot
more flexible than LPC, but I wanted to mention this since such 
conditional hooks also existed in older languages, at the least
to some extent.)

----------------------------------------
Feature #15277: at_exec
https://bugs.ruby-lang.org/issues/15277#change-74750

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
There's currently no easy way to have code executed before a subsequent call to `exec`. One has to monkey-patch the builtin method.

I'd like to propose a new method `at_exec` that would be very similar to `at_exit`, except that the callbacks are triggered before the current process is replaced by the external command.

```
# This would output "Hello", "Bye", and "Foo"
at_exec { puts "Bye!" }
puts "Hello"
exec "echo Foo"
```

Use case: we roll our own in `DeepCover`. Some test suites will call `exec`, and we need to store our counters before that happens.



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