Bug #21634: Combining read(1) with eof? causes dropout of results unexpectedly on Windows.

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.5.0dev (2025-10-03T08:59:54Z master 5b2ec0eb1b) +PRISM [x64-mingw-ucrt]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[YO4 (Yoshinao Muramatsu)]

On Windows, when reading a file containing EOF(\x1A), using read(1) with IO#eof? causes unexpected dropout of results.

irb(main):001> IO.binwrite("txt", "abcd\x1A")
=> 5
irb(main):002> open("txt", "r") { p _1.read(1) until _1.eof? }; # works fine
"a"
"b"
"c"
"d"
"\x1A"
irb(main):003> open("txt", "rt") { p _1.read(1) until _1.eof? }; # has failure
"b"
"d"
irb(main):004>

The problem disappeared when I commented out one of the following lines (though this will break other things).

• previous_mode = set_binary_mode_with_seek_cur(fptr); in io_read()
• flush_before_seek(fptr, false); in set_binary_mode_with_seek_cur(()
• io_unread(fptr, discard_rbuf); in flush_before_seek()

Within io_unread(), rbuf.len should have changed as 5, 4, 3,... but instead changed as 4, 2,(end).
Since inconsistencies already exist at this point, the problem appears to originate elsewhere.

I found this in ruby master but the same issue was found at least in ruby-1.9.3-p551.