[ruby-core:119748] [Ruby master Bug#20869] IO buffer handling is inconsistent when seeking

Issue #20869 has been updated by byroot (Jean Boussier).


I just looked into this a bit, I'm not quite familiar enough with the code to really propose a fix, but I get what is happening:

ungetbyte just shift the buffer offset, but the FD offset in unchanged.

```c
static void
io_ungetbyte(VALUE str, rb_io_t *fptr)
{
    // snip...
    // ungetbyte just shift the buffer offset, but the FD offset in unchanged
    fptr->rbuf.off-=(int)len;
    fptr->rbuf.len+=(int)len;
    MEMMOVE(fptr->rbuf.ptr+fptr->rbuf.off, RSTRING_PTR(str), char, len);
}


`fptr->rbuf.len == 1`, but real FD offset is 0
So we're doing `lseek(-1)` which fail with `EINVAL`

```c
static void
io_unread(rb_io_t *fptr)
{
    rb_off_t r;
    rb_io_check_closed(fptr);
    if (fptr->rbuf.len == 0 || fptr->mode & FMODE_DUPLEX)
        return;
    /* xxx: target position may be negative if buffer is filled by ungetc */
    errno = 0;
    // fptr->rbuf.len == 1, but real FD offset is 0
    // So we're doing lseek(-1) which fail with EINVAL
    r = lseek(fptr->fd, -fptr->rbuf.len, SEEK_CUR);
    if (r < 0 && errno) {
        if (errno == ESPIPE)
            fptr->mode |= FMODE_DUPLEX;
        return;
    }
    fptr->rbuf.off = 0;
    fptr->rbuf.len = 0;
    return;
}
```

So I suppose some more tracking info is needed to know that the real FD position and the buffer offset are desynced.

----------------------------------------
Bug #20869: IO buffer handling is inconsistent when seeking
https://bugs.ruby-lang.org/issues/20869#change-110411

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* ruby -v: ruby 3.3.4 (2024-07-09 revision be1089c8ec) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
When performing any of the seek based operations on IO (IO#seek, IO#pos=, or IO#rewind), the read buffer is inconsistently cleared:

```ruby
require 'tempfile'

Tempfile.open do |f|
  f.write('0123456789')
  f.rewind

  # Calling #ungetbyte as the first read buffer
  # operation uses a buffer that is preserved during
  # seek operations
  f.ungetbyte(97)
  # Byte buffer will not be cleared
  f.seek(2, :SET)

  f.getbyte       # => 97
end

Tempfile.open do |f|
  f.write('0123456789')
  f.rewind

  # Calling #getbyte before #ungetbyte uses a
  # buffer that is not preserved when seeking
  f.getbyte
  f.ungetbyte(97)
  # Byte buffer will be cleared
  f.seek(2, :SET)

  f.getbyte       # => 50
end
```

Similar behavior happens when reading characters:
```ruby
require 'tempfile'

Tempfile.open do |f|
  f.write('0123456789')
  f.rewind

  # Calling #ungetc as the first read buffer
  # operation uses a buffer that is preserved during
  # seek operations
  f.ungetc('a')
  # Character buffer will not be cleared
  f.seek(2, :SET)

  f.getc       # => 'a'
end

Tempfile.open do |f|
  f.write('0123456789')
  f.rewind

  # Calling #getc before #ungetc uses a
  # buffer that is not preserved when seeking
  f.getc
  f.ungetc('a')
  # Character buffer will be cleared
  f.seek(2, :SET)

  f.getc       # => '2'
end
```

When transcoding, however, the character buffer is never cleared when seeking:
```ruby
require 'tempfile'

Tempfile.open(encoding: 'utf-8:utf-16le') do |f|
  f.write('0123456789')
  f.rewind

  f.ungetc('a'.encode('utf-16le'))
  # Character buffer will not be cleared
  f.seek(2, :SET)

  f.getc       # => 'a'.encode('utf-16le')
end

Tempfile.open(encoding: 'utf-8:utf-16le') do |f|
  f.write('0123456789')
  f.rewind

  f.getc
  f.ungetc('a'.encode('utf-16le'))
  # Character buffer will not be cleared
  f.seek(2, :SET)

  f.getc       # => 'a'.encode('utf-16le')
end
```

I would expect the buffers to be cleared in all cases except possibly when the seek operation doesn't actually move the file pointer such as when calling IO#pos or IO#seek(0, :CUR).  The inconsistent behavior demonstrated here is a problem regardless though.



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