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

It is possible (and supported) that `strbuf_realpath()` can work
in-place, i.e. the second parameter passed to the function can refer to
the `buf` field of the first parameter.

Our code was ill-prepared for that, though: we first store a pointer to
the last component (that does not necessarily need to exist) before
calling the `GetFinalPathNameByHandleW()` function. Said function is
totally allowed to produce a longer path than the original one (e.g. by
prefixing it with `\\?\`, a relatively unintuitive Windows-only
concept). Which would overwrite the last component part.

Address that by making a full string copy of the last component.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

compat/mingw.c

@@ -1386,6 +1386,7 @@ char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 	DWORD ret;
 	int len;
 	const char *last_component = NULL;
+	char *append = NULL;
 
 	if (xutftowcs_path(wpath, path) < 0)
 		return NULL;
@@ -1408,7 +1409,7 @@ char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 				break; /* found start of last component */
 
 		if (p != wpath && (last_component = find_last_dir_sep(path))) {
-			last_component++; /* skip directory separator */
+			append = xstrdup(last_component + 1); /* skip directory separator */
 			*p = L'\0';
 			h = CreateFileW(wpath, 0, FILE_SHARE_READ |
 					FILE_SHARE_WRITE | FILE_SHARE_DELETE,
@@ -1417,25 +1418,29 @@ char *mingw_strbuf_realpath(struct strbuf *resolved, const char *path)
 		}
 	}
 
-	if (h == INVALID_HANDLE_VALUE)
+	if (h == INVALID_HANDLE_VALUE) {
+realpath_failed:
+		FREE_AND_NULL(append);
 		return NULL;
+	}
 
 	ret = GetFinalPathNameByHandleW(h, wpath, ARRAY_SIZE(wpath), 0);
 	CloseHandle(h);
 	if (!ret || ret >= ARRAY_SIZE(wpath))
-		return NULL;
+		goto realpath_failed;
 
 	len = wcslen(wpath) * 3;
 	strbuf_grow(resolved, len);
 	len = xwcstoutf(resolved->buf, normalize_ntpath(wpath), len);
 	if (len < 0)
-		return NULL;
+		goto realpath_failed;
 	resolved->len = len;
 
-	if (last_component) {
+	if (append) {
 		/* Use forward-slash, like `normalize_ntpath()` */
 		strbuf_addch(resolved, '/');
-		strbuf_addstr(resolved, last_component);
+		strbuf_addstr(resolved, append);
+		FREE_AND_NULL(append);
 	}
 
 	return resolved->buf;