mingw: be more informative when ownership check fails on FAT32

The FAT file system has no concept of ACLs. Therefore, it cannot store
any ownership information anyway, and the `GetNamedSecurityInfoW()` call
pretends that everything is owned "by the world".

Let's special-case that scenario and tell the user what's going on.

This addresses https://github.com/git-for-windows/git/issues/3886

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

diff --git a/compat/mingw.c b/compat/mingw.c
index 2c09c5bffee006f8904c2aa0f9ee9eb60841bfaf..22f960c7e3443f013c06018c1a03ceaeb3dc4581 100644 (file)
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2674,6 +2674,21 @@ static PSID get_current_user_sid(void)
        return result;
 }
 
+static int acls_supported(const char *path)
+{
+       size_t offset = offset_1st_component(path);
+       WCHAR wroot[MAX_PATH];
+       DWORD file_system_flags;
+
+       if (offset &&
+           xutftowcsn(wroot, path, MAX_PATH, offset) > 0 &&
+           GetVolumeInformationW(wroot, NULL, 0, NULL, NULL,
+                                 &file_system_flags, NULL, 0))
+               return !!(file_system_flags & FILE_PERSISTENT_ACLS);
+
+       return 0;
+}
+
 int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
 {
        WCHAR wpath[MAX_PATH];
@@ -2721,7 +2736,15 @@ int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
                    IsValidSid(current_user_sid) &&
                    EqualSid(sid, current_user_sid))
                        result = 1;
-               else if (report) {
+               else if (report &&
+                        IsWellKnownSid(sid, WinWorldSid) &&
+                        !acls_supported(path)) {
+                       /*
+                        * On FAT32 volumes, ownership is not actually recorded.
+                        */
+                       strbuf_addf(report, "'%s' is on a file system that does"
+                                   "not record ownership\n", path);
+               } else if (report) {
                        LPSTR str1, str2, to_free1 = NULL, to_free2 = NULL;
 
                        if (ConvertSidToStringSidA(sid, &str1))