[PATCH v4 03/10] mingw: special-case `open(symlink, O_CREAT | O_EXCL)`

From: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>
Date: Dec 17, 2025, 14:18 UTC
Message-ID: <f42a2f14bce6920200e4ee993b76b37d4a762441.1765981126.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.2009.v4.git.1765981126.gitgitgadget@gmail.com>

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The `_wopen()` function would gladly follow a symbolic link to a non-existent file and create it when given above-mentioned flags.

Git expects the `open()` call to fail, though. So let's add yet another work-around to pretend that Windows behaves according to POSIX, see: https://pubs.opengroup.org/onlinepubs/007904875/functions/open.html#:~:text=If%20O_CREAT%20and%20O_EXCL%20are,set%2C%20the%20result%20is%20undefined.

This is required to let t4115.8(--reject removes .rej symlink if it exists) pass on Windows when enabling the MSYS2 runtime's symbolic link support.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/compat/mingw.c b/compat/mingw.c
index 736a07a028..9fbf12a3d3 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -627,6 +627,7 @@ int mingw_open (const char *filename, int oflags, ...)
 	int fd, create = (oflags & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL);
 	wchar_t wfilename[MAX_PATH];
 	open_fn_t open_fn;
+	WIN32_FILE_ATTRIBUTE_DATA fdata;
 
 	DECLARE_PROC_ADDR(ntdll.dll, NTSTATUS, NTAPI, RtlGetLastNtStatus, void);
 
@@ -651,6 +652,19 @@ int mingw_open (const char *filename, int oflags, ...)
 	else if (xutftowcs_path(wfilename, filename) < 0)
 		return -1;
 
+	/*
+	 * When `symlink` exists and is a symbolic link pointing to a
+	 * non-existing file, `_wopen(symlink, O_CREAT | O_EXCL)` would
+	 * create that file. Not what we want: Linux would say `EEXIST`
+	 * in that instance, which is therefore what Git expects.
+	 */
+	if (create &&
+	    GetFileAttributesExW(wfilename, GetFileExInfoStandard, &fdata) &&
+	    (fdata.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT)) {
+		errno = EEXIST;
+		return -1;
+	}
+
 	fd = open_fn(wfilename, oflags, mode);
 
 	/*

-- 
gitgitgadget