Issue #13085 has been updated by Nobuyoshi Nakada.

Description updated

Another thread may make a substring of the buffer string during writing.
Then the temporarily frozen string becomes a shared root of the new substring.
Later the root gets unfrozen and modified, now the substring has an invalid pointer.

> If another thread calls String#freeze on the string we are
> currently writing; we will blindly unfreeze it during
> fwrite_unfreeze from ensure.  However, I do not expect this to
> be a real-world case.

That is `test_threaded_flush` in test_io.rb,
freezing the same string simultaneously in `io_fwrite`.

----------------------------------------
Bug #13085: io.c io_fwrite creates garbage
https://bugs.ruby-lang.org/issues/13085#change-62396

* Author: Eric Wong
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Relying on rb_str_new_frozen for unconverted strings does not
save memory because copy-on-write is always triggered in
read-write I/O loops were subsequent IO#read calls will
clobber the given write buffer.

  ```ruby
  buf = ''.b
  while input.read(16384, buf)
    output.write(buf)
  end
  ```

This generates a lot of garbage starting with Ruby 2.2 (r44471).
For my use case, even `IO.copy_stream` generates garbage, since
I wrap "write" to do Digest calculation in a single pass.

I tried using rb_str_replace and reusing the string as a hidden
`(klass == 0)` thread-local, but `rb_str_replace` attempts CoW
optimization by creating new frozen objects, too:

  https://80x24.org/spew/20161229004417.12304-1-e / 80x24.org/raw


So, I'm not sure what to do, temporal locking seems wrong for
writing strings (I guess it's for reading?).  I get
`test_threaded_flush` failures with the following:

  https://80x24.org/spew/20161229005701.9712-1-e / 80x24.org/raw


`IO#syswrite` has the same problem with garbage.  I can use
`IO#write_nonblock` on fast filesystems while holding GVL,
I guess...


---Files--------------------------------
0001-io.c-io_fwrite-temporarily-freeze-string-when-writin.patch (2.6 KB)


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