[glib/wip/smcv/statx-no-required-mask: 2/3] glocalfileoutputstream: Don't rely on compat fields in struct statx

[Simon McVittie <smcv src gnome org>]

commit e6355aee0c0e1fb255ee203b2c00ba8833bf2f13
Author: Simon McVittie <smcv collabora com>
Date:   Tue Sep 15 11:06:53 2020 +0100

    glocalfileoutputstream: Don't rely on compat fields in struct statx
    
    If the kernel has told us that certain fields in the struct statx are
    meaningless (a non-representable type, or no meaningful link count,
    or no meaningful uid, or similar) then we shouldn't try to copy their
    placeholder values onto the new file.
    
    (This commit is incomplete.)
    
    Signed-off-by: Simon McVittie <smcv collabora com>

 gio/glocalfileoutputstream.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)
---
diff --git a/gio/glocalfileoutputstream.c b/gio/glocalfileoutputstream.c
index 4a7af6072..a28ac332c 100644
--- a/gio/glocalfileoutputstream.c
+++ b/gio/glocalfileoutputstream.c
@@ -920,9 +920,11 @@ handle_overwrite_open (const char    *filename,
     }
   
   /* not a regular file */
-  if (!S_ISREG (_g_stat_mode (&original_stat)))
+  if (!_g_stat_has_field (&original_stat, G_LOCAL_FILE_STAT_FIELD_TYPE) ||
+      !S_ISREG (_g_stat_mode (&original_stat)))
     {
-      if (S_ISDIR (_g_stat_mode (&original_stat)))
+      if (_g_stat_has_field (&original_stat, G_LOCAL_FILE_STAT_FIELD_TYPE) &&
+          S_ISDIR (_g_stat_mode (&original_stat)))
        g_set_error_literal (error,
                              G_IO_ERROR,
                              G_IO_ERROR_IS_DIRECTORY,
@@ -937,6 +939,8 @@ handle_overwrite_open (const char    *filename,
   
   if (etag != NULL)
     {
+      /* TODO: What if original_stat doesn't have the mtime, or whatever
+       * other information goes into the etag? */
       current_etag = _g_local_file_info_create_etag (&original_stat);
       if (strcmp (etag, current_etag) != 0)
        {
@@ -962,7 +966,8 @@ handle_overwrite_open (const char    *filename,
    */
   
   if ((flags & G_FILE_CREATE_REPLACE_DESTINATION) ||
-      (!(_g_stat_nlink (&original_stat) > 1) && !is_symlink))
+      (!(_g_stat_has_field (&original_stat, G_LOCAL_FILE_STAT_FIELD_NLINK) &&
+         _g_stat_nlink (&original_stat) > 1) && !is_symlink))
     {
       char *dirname, *tmp_filename;
       int tmpfd;
@@ -980,6 +985,7 @@ handle_overwrite_open (const char    *filename,
       
       /* try to keep permissions (unless replacing) */
 
+      /* TODO: Check for G_LOCAL_FILE_STAT_FIELD_UID, _GID, _MODE */
       if ( ! (flags & G_FILE_CREATE_REPLACE_DESTINATION) &&
           (
 #ifdef HAVE_FCHOWN
@@ -1003,6 +1009,7 @@ handle_overwrite_open (const char    *filename,
                                      &tmp_statbuf);
 
          /* Check that we really needed to change something */
+         /* TODO: Check for G_LOCAL_FILE_STAT_FIELD_UID, _GID, _MODE */
          if (tres != 0 ||
              _g_stat_uid (&original_stat) != _g_stat_uid (&tmp_statbuf) ||
              _g_stat_gid (&original_stat) != _g_stat_gid (&tmp_statbuf) ||
@@ -1042,6 +1049,9 @@ handle_overwrite_open (const char    *filename,
          goto err_out;
        }
 
+      /* TODO: Check for G_LOCAL_FILE_STAT_FIELD_MODE? (But if it's
+       * unset, we have to specify *something*: will our fallback be
+       * any better than the kernel's?) */
       bfd = g_open (backup_filename,
                    O_WRONLY | O_CREAT | O_EXCL | O_BINARY,
                    _g_stat_mode (&original_stat) & 0777);
@@ -1073,9 +1083,12 @@ handle_overwrite_open (const char    *filename,
          goto err_out;
        }
       
-      if ((_g_stat_gid (&original_stat) != _g_stat_gid (&tmp_statbuf))  &&
+      if (_g_stat_has_field (&tmp_statbuf, G_LOCAL_FILE_STAT_FIELD_GID) &&
+          _g_stat_has_field (&original_stat, G_LOCAL_FILE_STAT_FIELD_GID) &&
+          (_g_stat_gid (&original_stat) != _g_stat_gid (&tmp_statbuf)) &&
          fchown (bfd, (uid_t) -1, _g_stat_gid (&original_stat)) != 0)
        {
+         /* TODO: Check for G_LOCAL_FILE_STAT_FIELD_MODE? */
          if (fchmod (bfd,
                      (_g_stat_mode (&original_stat) & 0707) |
                      ((_g_stat_mode (&original_stat) & 07) << 3)) != 0)