mingw: optionally use legacy (non-POSIX) delete semantics

At some point between Windows 10 Build 17134.1304 and Build 18363.657,
the default behavior of `DeleteFileW()` was changed to use POSIX
semantics (https://stackoverflow.com/a/60512798). Under those semantics,
a file can be deleted even when another process holds an active
`MapViewOfFile` view on it: the directory entry is removed immediately,
but the underlying data persists until the last handle is closed.

On older Windows versions (and Windows 10 builds before that change),
`DeleteFileW()` uses legacy semantics where deletion fails outright if
any process holds a file mapping.

To allow testing code paths that depend on the legacy behavior, introduce
a `GIT_TEST_LEGACY_DELETE` environment variable. When set, `mingw_unlink()`
uses `SetFileInformationByHandle()` with `FileDispositionInfo` (the
non-POSIX variant) instead of `DeleteFileW()`, forcing legacy delete
semantics regardless of the Windows version.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -449,20 +449,63 @@ static wchar_t *normalize_ntpath(wchar_t *wbuf)
 	return wbuf;
 }
 
+/*
+ * Use SetFileInformationByHandle(FileDispositionInfo) to force legacy
+ * (non-POSIX) delete semantics. On Windows 11, DeleteFileW() uses POSIX
+ * delete semantics internally, allowing deletion even with active
+ * MapViewOfFile views. This helper simulates Windows 10 behavior where
+ * deletion fails if a file mapping exists.
+ *
+ * Returns nonzero on success (like DeleteFileW), 0 on failure.
+ */
+static int legacy_delete_file(const wchar_t *wpathname)
+{
+	FILE_DISPOSITION_INFO fdi = { TRUE };
+	DWORD gle;
+	HANDLE h = CreateFileW(wpathname, DELETE,
+			       FILE_SHARE_READ | FILE_SHARE_WRITE |
+			       FILE_SHARE_DELETE,
+			       NULL, OPEN_EXISTING,
+			       FILE_FLAG_OPEN_REPARSE_POINT, NULL);
+	if (h == INVALID_HANDLE_VALUE)
+		return 0;
+
+	if (SetFileInformationByHandle(h, FileDispositionInfo,
+				       &fdi, sizeof(fdi))) {
+		CloseHandle(h);
+		return 1;
+	}
+	gle = GetLastError();
+	CloseHandle(h);
+	SetLastError(gle);
+	return 0;
+}
+
+static int try_delete_file(const wchar_t *wpathname, int use_legacy)
+{
+	if (use_legacy)
+		return legacy_delete_file(wpathname);
+	return DeleteFileW(wpathname);
+}
+
 int mingw_unlink(const char *pathname, int handle_in_use_error)
 {
+	static int use_legacy_delete = -1;
 	int tries = 0;
 	wchar_t wpathname[MAX_PATH];
 	if (xutftowcs_path(wpathname, pathname) < 0)
 		return -1;
 
-	if (DeleteFileW(wpathname))
+	if (use_legacy_delete < 0)
+		use_legacy_delete = git_env_bool("GIT_TEST_LEGACY_DELETE", 0);
+
+	if (try_delete_file(wpathname, use_legacy_delete))
 		return 0;
 
 	do {
 		/* read-only files cannot be removed */
 		_wchmod(wpathname, 0666);
-		if (!_wunlink(wpathname))
+		if (try_delete_file(wpathname, use_legacy_delete))
 			return 0;
 		if (!is_file_in_use_error(GetLastError()))
 			break;