From: "eightbitraptor (Matt V-H) via ruby-core" <ruby-core@...>
Date: 2025-04-01T21:56:27+00:00
Subject: [ruby-core:121503] [Ruby Bug#21210] IO::Buffer gets invalidated on GC compaction

Issue #21210 has been updated by eightbitraptor (Matt V-H).


I checked with the following `test.rb` that this patch does fixes the Buffer validity after compaction.

```
str = +"hello"

str_buf = IO::Buffer.for(str)

buf = IO::Buffer.new(128)
slice = buf.slice(8, 32)

GC.verify_compaction_references(expand_heap: true, toward: :empty)

p str_buf.valid?
p buf.valid?
p slice.valid?
```

Instead of pinning the source string, did you consider allowing the string to move by calculating the `base` offset, then moving the `source` string and updating the `base` pointer using `rb_gc_location(buffer->source)` and the calculated offset? Would this work?

----------------------------------------
Bug #21210: IO::Buffer gets invalidated on GC compaction
https://bugs.ruby-lang.org/issues/21210#change-112524

* Author: hanazuki (Kasumi Hanazuki)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-04-01T16:11:01Z master 30e5e7c005) +PRISM [x86_64-linux]
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED
----------------------------------------
commit:6012145299cfa4ab561360c78710c7f2941a7e9d implemented compaction for `IO::Buffer`.
It looks like this doesn't work well with an `IO::Buffer` that shares memory region with a String object.
I think the problem is that an `IO::Buffer` holds the raw pointer to the String content, and now the content can be moved by GC when the String is embedded.

```ruby
str = +"hello"
buf = IO::Buffer.for(str)
p buf.valid?

GC.verify_compaction_references(expand_heap: true, toward: :empty)

p buf.valid?  #=> should be true
```

This example should print two trues. Actually:
```
% ./ruby -v --disable-gems test.rb
ruby 3.5.0dev (2025-04-01T16:11:01Z master 30e5e7c005) +PRISM [x86_64-linux]
true
false
```



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