[ruby-core:118500] [Ruby master Bug#20585] Size of memory allocated by String.new(:capacity) is different from the specified value

Issue #20585 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED to 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONE

ruby_3_3 commit:d1ffd5ecfa62a049b7c508f30b6912a890de1b32.

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Bug #20585: Size of memory allocated by String.new(:capacity) is different from the specified value
https://bugs.ruby-lang.org/issues/20585#change-109022

* Author: os (Shigeki OHARA)
* Status: Closed
* ruby -v: ruby 3.3.2 (2024-05-30 revision e5a195edf6) [x86_64-freebsd14.0]
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONE
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IMHO, if :capacity is specified in String.new, capa will be its value.

In fact, Ruby 3.2 seems to allocate the size as specified.

```
% cat string_capacity.rb
unless /\A3\.[23]\./ =~ RUBY_VERSION
  raise NotImplementedError, 'Not Supported Ruby Version'
end

require 'inline'

class String
  def super_inspect
    self.class.superclass.instance_method(:inspect).bind(self).call
  end
  inline do |builder|
    builder.include '<stdio.h>'
    builder.add_compile_flags '-Wall'
    builder.c_raw <<~CODE
      VALUE capacity(int argc, VALUE *argv, VALUE self) {
        struct RString *rstring = RSTRING(self);

        if (! (RBASIC(self)->flags & RSTRING_NOEMBED)) {
          return rb_to_symbol(rb_str_new_cstr("EMBED"));
        } else {
          if (RBASIC(self)->flags & ELTS_SHARED) {
            return rb_to_symbol(rb_str_new_cstr("SHARED"));
          } else {
            return LONG2NUM(rstring->as.heap.aux.capa);
          }
        }
        return Qnil; /* NOTREACHED */
      }
    CODE
  end
end
```

```
% irb -I. -rstring_capacity
irb(main):001:0> [RUBY_PLATFORM, RUBY_VERSION]
=> ["x86_64-freebsd14.0", "3.2.4"]
irb(main):002:0> String.new('', capacity: 1024).capacity
=> 1024
irb(main):003:0> String.new('*'*1024, capacity: 1024).capacity
=> 1024
irb(main):004:0>
```

This is what I expect.

However, Ruby 3.3 seems to behave differently.

```
% irb -I. -rstring_capacity
irb(main):001> [RUBY_PLATFORM, RUBY_VERSION]
=> ["x86_64-freebsd14.0", "3.3.2"]
irb(main):002> String.new('', capacity: 1024).capacity
=> 1023
irb(main):003> String.new('*'*1024, capacity: 1024).capacity
=> 2047
irb(main):004>
```

* If only :capacity is specified, one byte less is allocated.
* If the initial string and its bytesize are specified, about twice the size is allocated.

Is this intentional?




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