[ruby-core:114071] [Ruby master Bug#19753] IO::Buffer#get_string can't handle negative offset

Issue #19753 has been updated by noteflakes (Sharon Rosner).


PR here: https://github.com/ruby/ruby/pull/8015

----------------------------------------
Bug #19753: IO::Buffer#get_string can't handle negative offset
https://bugs.ruby-lang.org/issues/19753#change-103743

* Author: noteflakes (Sharon Rosner)
* Status: Open
* Priority: Normal
* ruby -v: 3.2
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
```ruby
irb(main):001:0> b = IO::Buffer.for('abc')
=> 
#<IO::Buffer 0x00007f858f5450c0+3 EXTERNAL READONLY SLICE>
...
irb(main):002:0> b.get_string(-1)
=> "\x00abc"
irb(main):003:0> b.get_string(-1000, 3)
(irb):3:in `get_string': Specified offset+length exceeds data size! (ArgumentError)
	from (irb):3:in `<main>'
	from /home/sharon/.rbenv/versions/3.2.0/lib/ruby/gems/3.2.0/gems/irb-1.7.1/exe/irb:9:in `<top (required)>'
	from /home/sharon/.rbenv/versions/3.2.0/bin/irb:25:in `load'
	from /home/sharon/.rbenv/versions/3.2.0/bin/irb:25:in `<main>'
```

Using a negative offset returns garbage in the string but it also might segfault:

```ruby
irb(main):003:0> b = IO::Buffer.map(File.open('sgt-nodes.sql', 'r+'))
=> #<IO::Buffer 0x00007f189de14000+2008858 EXTERNAL MAPPED SHARED>
irb(main):004:0> b.get_string(-1000)
(irb):4: [BUG] Segmentation fault at 0x00007f189de13c18
ruby 3.2.0 (2022-12-25 revision a528908271) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0021 p:---- s:0109 e:000108 CFUNC  :get_string
...
```

## Expected behaviour

I think it might be nice to have `#get_string` behave like other methods taking an offset, like `String#[]`, so a negative offset means counting backwards from the end of the buffer. For example:

```ruby
irb(main):001:0> b = IO::Buffer.for('abc')
=> 
#<IO::Buffer 0x00007f858f5450c0+3 EXTERNAL READONLY SLICE>
...
irb(main):002:0> b.get_string(-1)
=> "c"
irb(main):003:0> b.get_string(-2)
=> "bc"
irb(main):003:0> b.get_string(-1000)
=> "abc"
irb(main):003:0> b.get_string(-1000, 2)
=> "ab"
```




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