POSIX MSYS2 runtime strips {} from arguments when launching processes with CreateProcess

[ikappaki]

When a program built with the MSYS2 POSIX runtime is launched via CreateProcess, any arguments containing {} are stripped before reaching the child process. Programs built with MSYS2/MINGW64 runtime preserve the arguments as expected.

A minimal repro is included in https://github.com/ikappaki/issue-msys2-createprocess.

• wrapper.c — launches a target executable and forwards arguments.
• echo2.c — prints received arguments.

Steps to reproduce:

Follow README.md instructions:

1. Build wrapper in MINGW64.
2. Build echo2 in both MINGW64 and MSYS2 POSIX.
3. Run the wrapper with echo2 and arguments containing {}.

Running the programs directly preserves {}:

$ ./echo2.x86_64.exe 1 {2} 3
1 {2} 3
$ ./echo2.msys2.exe 1 {2} 3
1 {2} 3

Launching via wrapper triggers the POSIX runtime issue:

# OK (MINGW64 target)
./wrapper.x86_64.exe ./echo2.x86_64.exe 1 {2} 3
# Executing: "./echo2.x86_64.exe" 1 {2} 3
# 1 {2} 3

# ISSUE (MSYS2 POSIX target)
./wrapper.x86_64.exe ./echo2.msys2.exe 1 {2} 3
# Executing: "./echo2.msys2.exe" 1 {2} 3
# 1 2 3

The second argument loses the {} when using the MSYS2 POSIX runtime.

Same behaviour occurs with /usr/bin/echo

$ ./wrapper.x86_64.exe /usr/bin/echo 1 {2} 3
# Executing: "D:/msys64/usr/bin/echo" 1 {2} 3
# 1 2 3

Expected behavior:
All arguments, including {}, are passed unchanged.

Actual behavior (POSIX runtime):
Curly braces are stripped from the arguments.

Notes:

• Only occurs with MSYS2 POSIX executables.
• Likely related to argument conversion in the POSIX layer before CreateProcess.
• Impacts CLI tools or scripts that rely on {} in arguments, such as invoking git from a process like git --git-dir C:\abc rev-parse 12345678abcde^{commit}.

Thanks