Issue #21210 has been updated by hanazuki (Kasumi Hanazuki). The `source` field in a `struct rb_io_buffer` can have a String or an IO::Buffer if not nil. When `source` is a String, we need to pin the object (`rb_str_locktmp` does not keep the embedded String content from being moved by GC). When `source` is an IO::Buffer, I believe it can be moved, because the `base` pointer refers to a malloced or mmapped memory region, which does not get compacted by GC. ---------------------------------------- Bug #21210: IO::Buffer gets invalidated on GC compaction https://bugs.ruby-lang.org/issues/21210#change-112521 * 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 ``` -- https://bugs.ruby-lang.org/