WM: various changes to file writing behavior

Saving with only a filename (from Python) wasn't being prevented,
while it would successfully write the file to the working-directory,
path remapping and setting relative paths wouldn't work afterwards
as `Main.filepath` would have no directory component.

Disallow this since it's a corner case which only ever occurs
when path names without any directories are used from Python,
the overhead of expanding the working-directory for all data saving
operations isn't worthwhile.

The following changes have been made:

- bpy.ops.wm.save_mainfile() without a filepath argument
  fails & reports and error when the file hasn't been saved.

  Previously it would write to "untitled.blend" and set the
  `G.main->filepath` to this as well.

- bpy.ops.wm.save_mainfile(filepath="untitled.blend")
  fails & reports and error as the filename has no directory component.

- `BLI_path_is_abs_from_cwd` was added to check if the path would
  attempt to expand to the CWD.

1 files changed, 7 insertions, 1 deletions

diff --git a/source/blender/blenlib/BLI_path_util.h b/source/blender/blenlib/BLI_path_util.h
index 9e2970f6013..3cffecae187 100644
--- a/source/blender/blenlib/BLI_path_util.h
+++ b/source/blender/blenlib/BLI_path_util.h
@@ -324,8 +324,14 @@ void BLI_path_frame_strip(char *path, char *ext) ATTR_NONNULL();
  */
 bool BLI_path_frame_check_chars(const char *path) ATTR_NONNULL();
 /**
+ * Checks for a relative path (ignoring Blender's "//") prefix
+ * (unlike `!BLI_path_is_rel(path)`).
+ * When false, #BLI_path_abs_from_cwd would expand the absolute path.
+ */
+bool BLI_path_is_abs_from_cwd(const char *path) ATTR_NONNULL();
+/**
  * Checks for relative path, expanding them relative to the current working directory.
- * Returns true if the expansion was performed.
+ * \returns true if the expansion was performed.
  *
  * \note Should only be called with command line paths.
  * This is _not_ something Blender's internal paths support, instead they use the "//" prefix.