win32(long path support): leave drive-less absolute paths intact

When trying to ensure that long paths are handled correctly, we
first normalize absolute paths as we encounter them.

However, if the path is a so-called "drive-less" absolute path, i.e. if
it is relative to the current drive but _does_ start with a directory
separator, we would want the normalized path to be such a drive-less
absolute path, too.

Let's do that, being careful to still include the drive prefix when we
need to go through the `\\?\` dance (because there, the drive prefix is
absolutely required).

This fixes https://github.com/git-for-windows/git/issues/4586.

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

compat/mingw.c

@@ -304,6 +304,11 @@ int mingw_core_config(const char *var, const char *value,
 	return 0;
 }
 
+static inline int is_wdir_sep(wchar_t wchar)
+{
+	return wchar == L'/' || wchar == L'\\';
+}
+
 /* Normalizes NT paths as returned by some low-level APIs. */
 static wchar_t *normalize_ntpath(wchar_t *wbuf)
 {
@@ -3412,7 +3417,12 @@ int handle_long_path(wchar_t *path, int len, int max_path, int expand)
 	 * "cwd + path" doesn't due to '..' components)
 	 */
 	if (result < max_path) {
-		wcscpy(path, buf);
+		/* Be careful not to add a drive prefix if there was none */
+		if (is_wdir_sep(path[0]) &&
+		    !is_wdir_sep(buf[0]) && buf[1] == L':' && is_wdir_sep(buf[2]))
+			wcscpy(path, buf + 2);
+		else
+			wcscpy(path, buf);
 		return result;
 	}
 

t/t2031-checkout-long-paths.sh

@@ -99,4 +99,13 @@ test_expect_success SHORTABSPATH 'clean up path close to MAX_PATH' '
 	test ! -d "$subdir1"
 '
 
+test_expect_success SYMLINKS_WINDOWS 'leave drive-less, short paths intact' '
+	printf "/Program Files" >symlink-target &&
+	symlink_target_oid="$(git hash-object -w --stdin <symlink-target)" &&
+	git update-index --add --cacheinfo 120000,$symlink_target_oid,PF &&
+	git -c core.symlinks=true checkout -- PF &&
+	cmd //c dir >actual &&
+	grep "<SYMLINKD\\?> *PF *\\[\\\\Program Files\\]" actual
+'
+
 test_done