From: nobu@...
Date: 2020-01-02T01:01:13+00:00
Subject: [ruby-core:96620] [Ruby master Feature#16471] Two feature requests for WeakRef: get original object, callback feature

Issue #16471 has been updated by nobu (Nobuyoshi Nakada).

Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN)
ruby -v deleted (ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu])
Description updated
Tracker changed from Bug to Feature

> First, add the ability to pull the original object out of the WeakRef object,

`WeakRef#__getobj__`.

> Second, add a callback feature for when a WeakRef's object is being purged by GC.
> ```ruby
>             # Here's where we set the callback
>             @cache[pk].on_garbage do |wr|
>                 wr.db.cache.delete wr.pk
>             end

This usage would be impossible even if the callback were provided.
The callback will be called **after** `wr` was collected.
Probably you may be interested in [Feature #16038].


----------------------------------------
Feature #16471: Two feature requests for WeakRef: get original object, callback feature
https://bugs.ruby-lang.org/issues/16471#change-83591

* Author: Snappingturtle (Mike O'Sullivan)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I'd like to request two features for `WeakRef`. I'll explain what I want, then provide a real world use case.

First, add the ability to pull the original object out of the `WeakRef` object, something like this:

```ruby
myhash = {}
myhash = WeakRef.new(myhash)
myhash = myhash.original_object()
```

Second, add a callback feature for when a WeakRef's object is being purged by GC. It would work something like this:

```ruby
wr = WeakRef.new(myobject)

wr.on_garbage() do |wr|
    puts 'trashing'
end

# time goes by...
# myobject goes out of scope
# outputs "trashing"
```

Here's the specific use case I would want it for. I'm developing a database system which includes a class called `Node`. A `Node` object holds a reference to a database handle and the primary key of a record in that database. It also has methods for getting and setting values in that record. So a simplified version looks something like this:

```ruby
class Node
    attr_reader :dbh
    attr_reader :pk
    
    def initialize(dbh, pk)
        @dbh = dbh
        @pk = pk
    end
    
    def set(fieldname, value)
        # a bunch of SQL to set the value
    end
    
    def get(fieldname)
        # a bunch of SQL to get the value
    end
end

node = Node.new dbh, 'abc'
node.set 'name', 'Fred'
```

The database object will have a node method, so you would usually get nodes like this:

```ruby
node = dbh.node('abc')
```

Make sense so far? It's a pretty simple concept. It works, but I'd like to make a small improvement. (Whether or not it's actually an improvement is a judgement call... I expect some disagreement on this point. But work with me here.)

I'd like the database object to keep a cache of `Node` objects. However, the database doesn't keep those node objects alive: they can fall out of scope and get purged by the GC. However, if that cache is never purged of dead objects, it just grows bigger and bigger. I could occasionally just run a routine to work through the cache and purge dead references, but that seems very inefficient. It would be better to just have them purged as they die.

So I could implement it something like this:

```ruby
class DataBase
    attr_reader :cache
    
    def initialize
        @cache = {}
    end
    
    def node(pk)
        if @cache[pk]
            # Here's where we need to get at the original object
            return @cache[pk].original_object
        else
            new_node = Node.new(self, pk)
            
            @cache[pk] = WeakRef.new(new_node)
            
            # Here's where we set the callback
            @cache[pk].on_garbage do |wr|
                wr.db.cache.delete wr.pk
            end
            
            return new_node
        end
    end
end
```

So when a `Node` object is garbage collected, it's deleted from the cache. The cache stays clean of dead objects.

I'll be interested to hear what you think of this idea and how difficult it would be to implement it.



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