[ruby-core:121505] [Ruby Bug#21210] IO::Buffer gets invalidated on GC compaction

Issue #21210 has been updated by hanazuki (Kasumi Hanazuki).


eightbitraptor (Matt V-H) wrote in #note-4:
> 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?

I think that approach can be possible if we check that the buffer is not locked before allowing the String to move. The base pointer may have been passed to an asynchronous syscall or C library call, in which case we cannot move it. Extensions using the raw pointer guard the section with `rb_io_buffer_lock` and `rb_io_buffer_unlock`. So we can check `RB_IO_BUFFER_LOCKED` flag to see if such a syscall is running.

Besides, there is another (unreported?) problem to be fixed. In the current implementation, the `RB_IO_BUFFER_LOCKED` flag of an IO::Buffer is not correctly synced among its slices, which share the same memory region.
```ruby
str = +"hello"
buf = IO::Buffer.for(str)
slice = buf.slice

slice.locked do
  p buf.locked?  #=> false
  # Moving or freeing `buf` here may cause something bad, as we expect the base pointer of `slice` is pinned.
end
```
I will file this as another ticket.

----------------------------------------
Bug #21210: IO::Buffer gets invalidated on GC compaction
https://bugs.ruby-lang.org/issues/21210#change-112526

* 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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