[ruby-core:85755] [Ruby trunk Bug#14542] syswrite spuriously empties the given string

Issue #14542 has been reported by sylvain.joyeux (Sylvain Joyeux).

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Bug #14542: syswrite spuriously empties the given string
https://bugs.ruby-lang.org/issues/14542

* Author: sylvain.joyeux (Sylvain Joyeux)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.3.6, 2.4.3
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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I'm doing a bit of buffer management to write on a socket. The main method looks like this:

~~~ ruby
@write_buffer.concat(new_bytes) if new_bytes
written_bytes = io.syswrite(@write_buffer)
@write_buffer = @write_buffer[written_bytes..-1]
!@write_buffer.empty?
~~~

However, in some cases, I get a nil `@write_buffer`. I've added some debugging info:

~~~ ruby
@write_buffer.concat(new_bytes) if new_bytes
size_before_syswrite = @write_buffer.size
written_bytes = io.syswrite(@write_buffer)
size_after_syswrite = @write_buffer.size
updated_write_buffer = @write_buffer[written_bytes..-1]
if !updated_write_buffer
  STDERR.puts "NIL WRITE BUFFER, size_before_syswrite=#{size_before_syswrite} size_after_syswrite=#{size_after_syswrite} written_bytes=#{written_bytes}"
end
@write_buffer = updated_write_buffer
!@write_buffer.empty?
~~~

in all cases, I got the following output:

~~~
NIL WRITE BUFFER, size_before_syswrite=3106 size_after_syswrite=0 written_bytes=3106
~~~

However, I could not reproduce the problem with toy setups. syswrite's string handling is beyond the little I know of MRI's internals ;-)



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