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) -- https://bugs.ruby-lang.org/ Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe> <http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>