fixup! mingw: implement a platform-specific strbuf_realpath()

Let's be careful to make this function work near the drive root: to
resolve the drive root path itself, we _need_ a trailing backslash:
if a file handle to the path `C:` is created, it does not actually refer
to the drive root. Instead, it refers to this very Windows-only concept
of a "per-drive current directory".

This also requires the code that wants to re-append the last component
to be more careful and only append a slash _if necessary_.

This commit fixes the problem with `scalar unregister C:/foo` that was
reported at #4200.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1410,7 +1410,15 @@ char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 
 		if (p != wpath && (last_component = find_last_dir_sep(path))) {
 			append = xstrdup(last_component + 1); /* skip directory separator */
-			*p = L'\0';
+			/*
+			 * Do not strip the trailing slash at the drive root, otherwise
+			 * the path would be e.g. `C:` (which resolves to the
+			 * _current_ directory on that drive).
+			 */
+			if (p[-1] == L':')
+				p[1] = L'\0';
+			else
+				*p = L'\0';
 			h = CreateFileW(wpath, 0, FILE_SHARE_READ |
 					FILE_SHARE_WRITE | FILE_SHARE_DELETE,
 					NULL, OPEN_EXISTING,
@@ -1438,7 +1446,7 @@ char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 
 	if (append) {
 		/* Use forward-slash, like `normalize_ntpath()` */
-		strbuf_addch(resolved, '/');
+		strbuf_complete(resolved, '/');
 		strbuf_addstr(resolved, append);
 		FREE_AND_NULL(append);
 	}

t/t0060-path-utils.sh

@@ -237,6 +237,12 @@ test_expect_success SYMLINKS 'real path works on symlinks' '
 	test "$sym" = "$(test-tool path-utils real_path "$dir2/syml")"
 '
 
+test_expect_success MINGW 'real path works near drive root' '
+	# we need a non-existing path at the drive root; simply skip if C:/xyz exists
+	test -e C:/xyz ||
+	test C:/xyz = $(test-tool path-utils real_path C:/xyz)
+'
+
 test_expect_success SYMLINKS 'prefix_path works with absolute paths to work tree symlinks' '
 	ln -s target symlink &&
 	test "$(test-tool path-utils prefix_path prefix "$(pwd)/symlink")" = "symlink"