Problem:
The cmdpreview saved undo nodes on cmdpreview_prepare() from ex_getln.c may
become invalid (free) if the preview function makes undo operations, causing
heap-use-after-free errors.
Solution:
Save the buffer undo list on cmdpreview_prepare)_ and start a new empty one. On
cmdpreview_restore_state(), undo all the entries in the new undo list and
restore the original one. With this approach, the preview function will be
allowed to undo only its own changes.
Fix#20036Fix#20248
Allows preview callbacks to modify multiple buffers in order to show the
preview. Previously, if multiple buffers were modified, only the current
buffer would have its state restored. After this change, all buffers
have their state restored after preview.
Closes#19103.
clang: Result of operation is garbage or undefined
clang: Uninitialized argument value
Also check for == 's' instead of != 'n' as it is more straightforward.
Also fix another unrelated PVS warning:
PVS/V1071: Return value of win_comp_pos() is not always used
