[ruby-core:103403] [Ruby master Feature#17795] `before_fork` and `after_fork` callback API

Issue #17795 has been updated by byroot (Jean Boussier).


> Afaik the proper way to do this is to close the connection after the fork.

No before. Otherwise the connection is "shared" and closing it in the children cause issues for the connections in the parent.

> I'd like to know where the idea that it's slow is coming from.

Maybe your glibc is quite old? https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal

```ruby
require 'benchmark/ips'

module Foo
  class << self
    attr_accessor :bar
  end
  @bar = 42
end

puts "#{RUBY_VERSION} #{RUBY_PLATFORM}"
Benchmark.ips do |x|
  x.report('Process.pid') { Process.pid }
  x.report('Module.attr') { Foo.bar }
  x.compare!
end
```

```
3.0.1 x86_64-darwin20
Warming up --------------------------------------
         Process.pid     1.914M i/100ms
         Module.attr     1.775M i/100ms
Calculating -------------------------------------
         Process.pid     19.144M (0.7%) i/s -     97.626M in   5.099666s
         Module.attr     17.820M (ア 0.4%) i/s -     90.530M in   5.080332s

Comparison:
         Process.pid: 19144498.7 i/s
         Module.attr: 17820085.3 i/s - 1.07x  (ア 0.00) slower
```

```
3.0.1 x86_64-linux
Warming up --------------------------------------
         Process.pid   698.792k i/100ms
         Module.attr     1.886M i/100ms
Calculating -------------------------------------
         Process.pid      6.862M (ア 1.5%) i/s -     34.940M in   5.092832s
         Module.attr     19.184M (ア 1.0%) i/s -     96.197M in   5.014902s

Comparison:
         Module.attr: 19183904.4 i/s
         Process.pid:  6862219.4 i/s - 2.80x  (ア 0.00) slower
```

So fast enough for things that are infrequently called, but slow enough that I see it sitting at `1-2%` of CPU profiles in real production workloads.

> So it seems to me this is not such a nice API.

It's not really intended as an actual API, but as a smaller change that would be more easily accepted by the core team.

----------------------------------------
Feature #17795: `before_fork` and `after_fork` callback API
https://bugs.ruby-lang.org/issues/17795#change-91497

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
Replaces: https://bugs.ruby-lang.org/issues/5446

### Context

Ruby code in production is very often running in a forking setup (puma, unicorn, etc), and it is common some types of libraries to need to know when the Ruby process was forked. For instance:

  - Most database clients, ORMs or other libraries keeping a connection pool might need to close connections before the fork happens.
  - Libraries relying on some kind of dispatcher thread might need to restart the thread in the forked children, and clear any internal buffer (e.g. statsd clients, newrelic_rpm).

**This need is only for forking the whole ruby process, extensions doing a `fork(2) + exec(2)` combo etc are not a concern, this aim at only catching `kernel.fork`, `Process.fork` and maybe `Process.daemon`.**.
The use case is for forks that end up executing Ruby code.

### Current solutions

Right now this use case is handled in several ways.

#### Rely on the integrating code to call a `before_fork` or `after_fork` callback.

Some libraries simply rely on documentation and require the user to use the hooks provided by their forking server.

Examples:

  - Sequel: http://sequel.jeremyevans.net/rdoc/files/doc/fork_safety_rdoc.html
  - Rails's Active Record: https://devcenter.heroku.com/articles/concurrency-and-database-connections#multi-process-servers
  - ScoutAPM (it tries to detect popular forking setup and register itself): https://github.com/scoutapp/scout_apm_ruby/blob/fa83793b9e8d2f9a32c920f59b57d7f198f466b8/lib/scout_apm/environment.rb#L142-L146
  - NewRelic RPM (similarly tries to register to popular forking setups): https://www.rubydoc.info/github/newrelic/rpm/NewRelic%2FAgent:after_fork


#### Continuously check `Process.pid`

Some libraries chose to instead keep the process PID in a variable, and to regularly compare it to `Process.pid` to detect forked children.
Unfortunately `Process.pid` is relatively slow on Linux, and these checks tend to be in tight loops, so it's not uncommon when using these libraries
to spend `1` or `2%` of runtime in `Process.pid`.

Examples:

  - Rails's Active Record used to check `Process.pid` https://github.com/Shopify/rails/blob/411ccbdab2608c62aabdb320d52cb02d446bb39c/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L946, it still does but a bit less: https://github.com/rails/rails/pull/41850
  - the `Typhoeus` HTTP client: https://github.com/typhoeus/typhoeus/blob/a345545e5e4ac0522b883fe0cf19e5e2e807b4b0/lib/typhoeus/pool.rb#L34-L42
  - Redis client: https://github.com/redis/redis-rb/blob/6542934f01b9c390ee450bd372209a04bc3a239b/lib/redis/client.rb#L384
  - Some older versions of NewRelic RPM: https://github.com/opendna/scoreranking-api/blob/8fba96d23b4d3e6b64f625079c184f3a292bbc12/vendor/gems/ruby/1.9.1/gems/newrelic_rpm-3.7.3.204/lib/new_relic/agent/harvester.rb#L39-L41

#### Continuously check `Thread#alive?`

Similar to checking `Process.pid`, but for the background thread use case. `Thread#alive?` is regularly checked, and if the thread is dead, it is assumed that the process was forked.
It's much less costly than a `Process.pid`, but also a bit less reliable as the thread could have died for other reasons. It also delays re-creating the thread to the next check rather than immediately upon forking.

Examples:

  - `statsd-instrument`: https://github.com/Shopify/statsd-instrument/blob/0445cca46e29aa48e9f1efec7c72352aff7ec931/lib/statsd/instrument/batched_udp_sink.rb#L63

#### Decorate `Kernel.fork` and `Process.fork`

Another solution is to prepend a module in `Process` and `Kernel`, to decorate the fork method and implement your own callback. It works well, but is made difficult by `Kernel.fork`.


Examples:

  - Active Support: https://github.com/rails/rails/blob/9aed3dcdfea6b64c18035f8e2622c474ba499846/activesupport/lib/active_support/fork_tracker.rb
  - `dd-trace-rb`: https://github.com/DataDog/dd-trace-rb/blob/793946146b4709289cfd459f3b68e8227a9f5fa7/lib/ddtrace/profiling/ext/forking.rb
  - To some extent, `nakayoshi_fork` decorates the `fork` method: https://github.com/ko1/nakayoshi_fork/blob/19ef5efc51e0ae51d7f5f37a0b785309bf16e97f/lib/nakayoshi_fork.rb

### Proposals

I see two possible features to improve this situation:

#### `Process.before_fork` and `Process.after_fork` callbacks

One solution would be for Ruby to expose a callback API for these two events, similar to `Kernel.at_exit`.

#### Make `Kernel.fork` a delegator

A simpler change would be to just make `Kernel.fork` a delegator to `Process.fork`. This would make it much easier to prepend a module on `Process` for each library to implement its own callback.

Proposed patch: https://github.com/ruby/ruby/pull/4361



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