Backup of files with reserved filenames fails on windows

[MichaelEischer]

based on: https://forum.restic.net/t/scan-and-error-createfile-con-the-parameter-is-incorrect/2710

Output of restic version

restic (version 0.9.6) on Windows 10

How did you run restic exactly?

restic -r D:/ --verbose backup C:/Users/myusername/work
Output:

error: CreateFile \\?\\con: The parameter is incorrect.

What backend/server/service did you use to store the repository?

Not relevant

Expected behavior

restic reports the correct filename of the problematic file and/or properly backups the file.

Actual behavior

Unintelligible error message and the file is not contained in the backup.

Steps to reproduce the behavior

This turned out to be cause by some files named con.m.

Do you have any idea what may have caused this?

CON is a reserved filename on windows. This rule still applies when adding a file extension like .m.
When restic tries to normalize e.g. C:/Users/myusername/work\con.m then the computation of the absolute path yields con (I'm looking at you GetFullPathName). As restic supports paths longer than 256 characters it has to prepend \\?\ which gives us \\?\con. (btw, the correct and non-ambiguous filename for the console device would be \\.\con) I'm not sure where one of the backslashes disappeared in the error message.

Do you have an idea how to solve the issue?

The Definitive Guide on Win32 to NT Path Conversion shows that prepending \\?\ to an already absolute path seems to get rid of most weird behavior during filepath normalization. I'm just not sure whether the Go functions to process filenames can handle this.

restic should probably convert relative paths to absolute ones as early as possible and then always add \\?\ before passing the path to the windows API. fixpath in internal/fs/file_windows.go should only call filepath.Abs for relative paths.

[greatroar]

fixpath in internal/fs/file_windows.go should only call filepath.Abs for relative paths.

#2614 does that, specifically commit 052aff5.