[msys2-runtime] fhandler_console drops one character at each 32 KiB conversion boundary

[FlyaiF]

Existing issues matching what you're seeing

• I was not able to find an open or closed issue matching what I'm seeing

Git for Windows version

git version 2.55.0.windows.3
cpu: x86_64
built from commit: 52ca1113d651127f89477a8763f86ab20f645e1d
sizeof-long: 4
sizeof-size_t: 8
shell-path: D:/git-sdk-64/usr/bin/sh
rust: disabled
feature: fsmonitor--daemon
gettext: enabled
libcurl: 8.21.0
OpenSSL: OpenSSL 3.5.7 9 Jun 2026
zlib: 1.3.2
SHA-1: SHA1_DC
SHA-256: SHA256_BLK
default-ref-format: files
default-hash: sha1

Windows version

Windows 11

Windows CPU architecture

x86_64 (64-bit)

Additional Windows version information

Microsoft Windows [Version 10.0.22631.7517]

Options set during installation

PS C:\Users\hongxd43274> type "C:\Program Files\Git\etc\install-options.txt"
Editor Option: VIM
Custom Editor Path:
Default Branch Option:
Path Option: Cmd
SSH Option: OpenSSH
Tortoise Option: false
CURL Option: WinSSL
CRLF Option: CRLFAlways
Bash Terminal Option: MinTTY
Git Pull Behavior Option: Merge
Use Credential Manager: Enabled
Performance Tweaks FSCache: Enabled
Enable Symlinks: Disabled
Enable FSMonitor: Disabled

Other interesting things

Windows Terminal package: 1.24.11911.0 (x64)
MSYS runtime: 3.6.9-b4195d69.x86_64, built 2026-06-06 17:49 UTC
/usr/bin/ssh.exe: OpenSSH_10.3p1, OpenSSL 3.5.7 9 Jun 2026

Terminal/shell

Git Bash (C:\Program Files\Git\bin\bash.exe -l) hosted by Windows Terminal/ConPTY. Stdout must remain attached directly to the terminal.

Commands that trigger the issue

LC_ALL=C awk 'BEGIN {
    pattern = "123456789abcdefghijklmnopqrstuvwxyz123456789abcdefghijklmnopqrstuvwxyz"
    for (line = 1; line <= 5000; line++)
        printf "%08d %s\n", line, pattern
}' >/tmp/msys-console-repro.txt

wc -c /tmp/msys-console-repro.txt
sha256sum /tmp/msys-console-repro.txt

dd if=/tmp/msys-console-repro.txt bs=32768 status=none

Expected behaviour

all output line should be "123456789abcdefghijklmnopqrstuvwxyz123456789abcdefghijklmnopqrstuvwxyz"

Actual behaviour

With the unmodified runtime, one character is discarded at each 32768-byte conversion boundary. In the 400,000-byte reproduction, 12 numbered lines are malformed; when a discarded character is a newline, two numbered lines appear merged.

The loss occurs before Windows Terminal receives or renders the output. It is also observable through the Windows console screen buffer, and the two-part runtime patch eliminates it.

Repository

CONVERT_LIMIT is currently NT_MAX_PATH (32768). For an ASCII span of exactly 32768 bytes, fhandler_console::write_normal() advances over all 32768 input bytes and calls _sys_mbstowcs() with a destination length of 32768.

_sys_mbstowcs() converts 32768 characters, then reserves space for its trailing NUL by clamping the returned count to dlen - 1 and storing L'\0' at that position:

count = (count < dlen) ? count : dlen - 1;
dst[count] = L'\0';

The last converted character is therefore overwritten and only 32767 characters are passed to WriteConsoleW(), while write_normal() has already consumed 32768 input bytes. The missing input byte is never retried. With ASCII output, this repeats at each 32768-byte conversion boundary.