Bug #22195: IO::Buffer read after free silently returns empty data instead of raising

• Status: Open
• Assignee: -
• Target version: -
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[himura467 (Akito Shitara)]

Reading from an IO::Buffer after calling #free does not raise. It behaves like an empty buffer:

buffer = IO::Buffer.for("Hello World")
buffer.free
buffer.get_string # => "" (no error)

Why this looks like an unintended regression

Originally (https://github.com/ruby/ruby/commit/e30920354f, Ruby 3.1–3.3), accessing a freed buffer raised IO::Buffer::AllocationError ("The buffer is not allocated!").

https://github.com/ruby/ruby/commit/c5cf4d4e12 made zero-length buffer operations succeed instead of raising, for [Bug #19542] and [Bug #18805]. Since a freed buffer also has base == NULL, size == 0, it was caught in the same code path and stopped raising too. Neither ticket mentions freed buffers, so this appears to be an accidental side effect rather than a decision.
Note that the rdoc of #free still promises that access after free raises.

Proposal

How about marking a buffer internally as freed when its memory is released, so that:

• Any byte access raises IO::Buffer::AllocationError again, restoring the pre-3.4 documented behavior.
• #resize re-allocates and clears the mark, as #free's rdoc promises ("You can resize a freed buffer to re-allocate it").
• The zero-length buffer semantics from https://github.com/ruby/ruby/pull/9532 are preserved.

[himura467 (Akito Shitara)]

• Subject changed from `IO::Buffer` read after free silently returns empty data instead of raising to IO::Buffer read after free silently returns empty data instead of raising

[himura467 (Akito Shitara)]

• Description updated (diff)

[himura467 (Akito Shitara)]

PR: https://github.com/ruby/ruby/pull/17862