[ruby-core:102310] [Ruby master Feature#17593] load_iseq_eval should override the ISeq path

Issue #17593 has been reported by byroot (Jean Boussier).

----------------------------------------
Feature #17593: load_iseq_eval should override the ISeq path
https://bugs.ruby-lang.org/issues/17593

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
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Full context in https://github.com/Shopify/bootsnap/pull/343

Consider the following script

```ruby
system('mkdir', '-p', '/tmp/build', '/tmp/app')
File.write('/tmp/app/a.rb', 'p ["app/a", __FILE__, __dir__]')
File.write('/tmp/app/b.rb', 'p ["app/b", __FILE__, __dir__]')
File.write('/tmp/build/a.rb', 'p ["build/a", __FILE__, __dir__]; require_relative "b"')

$iseq = RubyVM::InstructionSequence.compile_file('/tmp/build/a.rb')

class RubyVM::InstructionSequence
  def self.load_iseq(feature)
    if feature == "/tmp/app/a.rb"
      $iseq
    end
  end
end

require '/tmp/app/a.rb'
```

Current behavior:

```ruby
["build/a", "/tmp/build/a.rb", "/private/tmp/build"]
/tmp/build/a.rb:1:in `require_relative': cannot load such file -- /private/tmp/build/b (LoadError)
	from /tmp/build/a.rb:1:in `<main>'
	from <internal:/opt/rubies/3.0.0-pshopify2/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/opt/rubies/3.0.0-pshopify2/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /tmp/iseq_debug.rb:16:in `<main>'
```

Expected behavior

```ruby
["build/a", "/private/tmp/app/a.rb", "/private/tmp/app"]
["app/b", "/private/tmp/app/b.rb", "/private/tmp/app"]
```

### What's going on?

`RubyVM::InstructionSequence` instances have a `pathobj` property that is recorded when the source is parsed, and when the ISeq is later evaled, the VM use that `path` as if you were loading a `.rb` file located at that path.

So if that source use constructs such as `require_relative`, `__FILE__`, `__dir__`, etc, they will all happen relative to where the source was located upon compilation, not relative to the source was upon evaluation.

### Why is it a problem?

Some deployment strategies first build the application in one location, and then later move it elsewhere. 

That's for instance the case on the Heroku platform. e.g. the deploy looks like

```bash
git clone <repo> /tmp/build_xxxx
cd /tmp/build_xxxx
rake assets:precompile ...
mv /tmp/build_xxxx /app
```

Because of this, all the ISeq cached by bootsnap when the code was in `/tmp/build_xxx` have to be invalidated as soon as the source is moved to `/app`, rendering ISeq caching ineffective, and even detrimental as it causes extra writes to disk without bringing any benefits.

### Solution

I believe there are two changes that would be needed.

First I think that `load_iseq_eval` should set the `fname` as the top stack location. Either by copying the ISeq instance and change its `pathobj`, or by having a way to pass an optional path to `vm_set_top_stack` that would take precedence.

I experimented with [a quick hack that changes the ISeq `pathobj` in place](https://github.com/Shopify/ruby/commit/192f5b477f924243e3f6621383e7a6ad02fbd63d), and it does solve most of the problem.

However even with that quick hack another problem remain, the `__FILE__` still evaluate to the original source location. However `__dir__` works as expected, because it is a method that returns the top_stack location. I think `__FILE__` could be changed to be a method as well.



-- 
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