Barman (on Linux) fails to back up Windows system with tablespaces to CIFS share on Windows host

[einhverfr]

Situation is that a user has PostgreSQL on Windows with a significant amount of data on a tablespace on the F drive. Barman is running on Ubuntu Focal with /var/lib/barman/[server] mounted as a CIFS share from a Windows file server.

Barman works, and can stream wals to the backup. However creating a backup fails.

The reason is that the tablespace mapping just appends the path to the destination path, providing a path like:
/var/lib/barman/pg/base/20240124T132339/data/F:\postgres\data

This cannot work because the colon (:) has special meaning on NTFS/Windows file paths as a selector for alternate data streams for a given file. If you are not familiar with this feature of NTFS, here's a quick primer including security implications: https://blog.netwrix.com/2022/12/16/alternate_data_stream/

I also assume but am not 100% sure that the changes in slashes do work on Python in this way. I am not sure about pg_basebackup though.

Proposed resolution:

If the barman_home starts with / and the tablespace directory starts with [A-Z]:\ then:
change the mapper to __windows__/[A-Z]/ and drop the colon.
Optionally we may need to also transpose \ to / in order to normalize the paths.

[einhverfr]

another option would be to allow a custom tablespace map to be provided in the configuration.

[einhverfr]

As a note, while this issue only is unavoidable with CIFS shares, the problem will surface in other forms if NFS 4.1 is used against a Windows server as the underlying issue is found int he filesystem. Additionally workarounds for the issue in NFS 3 Server for Windows are brittle and difficult to implement perfectly. This would be very nice to fix and it might make it easier to eventually support Windows hosts for Barman