Issue #21131 has been reported by chucke (Tiago Cardoso). ---------------------------------------- Bug #21131: IO.copy_stream: yielded string changes value when duped https://bugs.ruby-lang.org/issues/21131 * Author: chucke (Tiago Cardoso) * Status: Open * ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-darwin23] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- I found an odd situation, when using IO.copy_stream with a File writer quack class, where the data passed to #write somehow ends overwritten, despite the instance being duped. class ProcIO def initialize(block) @block = block end # Implementation the IO write protocol, which yield the given chunk to +@block+. def write(data) @block.call(data.dup) data.bytesize end end rng = Random.new(42) body = Tempfile.new("ruby-bug", binmode: true) body.write(rng.bytes(16_385)) body.rewind payload = [] block = ->(data){ payload << data.dup } IO.copy_stream(body, ProcIO.new(block)) body.rewind if payload.join != body.read puts "it's a bug" end if you use the debugger, you'll see that the first yielded chunk has the correct bytes when yielded the first time, but when the second 1 byte chunk is yielded (IO.copy_stream reads in chunks of 16384 bytes), the first chunk string value suddenly changes. This should not happen, as the first yielded chunk was a string duped from the string yielded by IO.copy_stream (which is expected to be a buffer). -- https://bugs.ruby-lang.org/