[ruby-core:113416] [Ruby master Misc#19597] Process.argv0 returns the same mutable String

Issue #19597 has been updated by nobu (Nobuyoshi Nakada).


I found `$0` had been frozen till 1.9.

```shell-session
$ ruby1.9 -e 'p $0.upcase!; p $0; system("ps #$$")'
-e:1:in `upcase!': can't modify frozen String (RuntimeError)
	from -e:1:in `<main>'
```

I'm not sure why/when it changed, yet.

----------------------------------------
Misc #19597: Process.argv0 returns the same mutable String
https://bugs.ruby-lang.org/issues/19597#change-102990

* Author: andrykonchin (Andrew Konchin)
* Status: Open
* Priority: Normal
----------------------------------------
`Process.argv0` returns a name of the script being executed. But it seems it's always the same object, that can be modified and this change will be visible to the consequent `Process.argv0` calls.

It makes sense to return a frozen String that cannot be modified.

`$0` initial value equals a value returned by `Process.argv0` so `$0` by default also could be a frozen String.

Example:

```ruby
puts "Object id:"
p Process.argv0.object_id
p Process.argv0.object_id

puts "Value before modification:"
p Process.argv0
Process.argv0.upcase!

puts "Value after modification:"
p Process.argv0

puts "$0 after modification:"
p $0
```

It will output:

```
Object id:
60
60
Value before modification:
"argv0.rb"
Value after modification:
"ARGV0.RB"
$0 after modification:
"ARGV0.RB"
```



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