logging.handler fails when attempting to rotate a fifo (with potential fix)

[rubiksdot]

Bug report

Bug description:

With patch e9b4ec6 (which was a fix to issues #105623 and #105887) the test for whether or not the logfile is a file comes too late for fifos.

When the rotation check in shouldRollover() happens it does a self.stream.tell() which fails if the stream is a fifo.

Since doing a filetype check on NFS is expensive (as per those bugs) I'd like to suggest either:

try:
    pos = self.stream.tell()
except io.UnsupportedOperation:
    # It's a FIFO or other non-seekable stream
    pos = None

or

pos = self.stream.tell() if self.stream.seekable() else None

Alas, I'm not using NFS anywhere so can't quite test which would still be performant here.

I also haven't checked if this kind of fix should be done elsewhere but if the fix is fine I'm happy to put in the effort and see what I can spot. :)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-143237: Fix support of named pipes in the rotating logging handlers #143259

[serhiy-storchaka]

How does rotation supposed to work with FIFO files?

[rubiksdot]

It's not hence this patch.

I hit this with certbot where I like to redirect its logging to syslog. Before this got fixed for NFS I was happily telling it to log to a fifo and handling that. Now it bombs out because logging.handler wants to seek on the fifo.