From: "k0kubun (Takashi Kokubun) via ruby-core" <ruby-core@...>
Date: 2025-03-03T23:07:47+00:00
Subject: [ruby-core:121228] [Ruby master Bug#21131] IO.copy_stream: yielded string changes value when duped

Issue #21131 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED to 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED, 3.4: DONE

ruby_3_4 commit:eb8ee7c189d051e656f781ce2775ede1157edd6a.

----------------------------------------
Bug #21131: IO.copy_stream: yielded string changes value when duped
https://bugs.ruby-lang.org/issues/21131#change-112176

* Author: chucke (Tiago Cardoso)
* Status: Closed
* ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-darwin23]
* Backport: 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED, 3.4: DONE
----------------------------------------
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).



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