[wix-users] Custom actions: Correctly storing rollback data and cleaning up afterwards

[Stephen Oberholtzer]

There are two problems I'm struggling with, both related to temporary
storage required by custom actions.

Rollback data
===========
I've found several articles online explaining how to use rollback (and
sometimes even commit) custom actions, but I've not found *any* trustworthy
examples of how to actually store the data needed to perform a rollback.

The most obvious solution is to use some sort of temporary storage, most
likely a file. But I haven't found *any* documentation on where to find
acceptable temporary storage for rollback data.  The MSDN article for
Rollback Custom Actions (
https://docs.microsoft.com/en-us/windows/desktop/msi/rollback-custom-actions)
simply says that the installer uses a "hidden system directory" to store
backups for rollback - not where to find that directory, despite the fact
that the very existence of that page is evidence that someone might need it!


Temporary storage for immediate actions
===============================
I found myself in a rather unenviable position: Due to authoring errors in
the MSIs that were published for older versions, the RemoveExistingProducts
step could cause certain files to deleted when they should not be -- either
due to a malfunctioning custom action run at uninstall, or due to a
component that should have been marked as Permanent, but wasn't.
The solution to that seems obvious -- save the file off somewhere before
RemoveExistingProducts, and restore it afterwards.  This situation carries
with it *two* problems, however:
1. The same problem as rollback data -- where do we save the file to?
2. How can I ensure that any temporary storage is cleaned up if the
installation is aborted (e.g. user hits Cancel?)




-- 
-- Stevie-O
Real programmers use COPY CON PROGRAM.EXE