Subject: Avoiding race condition between partprobe and getting PARTUUID using lsblk?
From: Anton Hvornum <[email-masked]>
To: [email-masked]
Date: Wed, 15 Dec 2021 13:12:43 +0000
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi.

So I've been struggling a bit with optimizing applications calling
partprobe right after lsblk to retrieve PARTUUID of newly generated
partitions using parted.

Assuming the following loop:
* partprobe
* lsblk
* if !result: sleep + loop

It feels as if every partprobe (indirectly or directly) flushes a
cache/struct of information somewhere, which makes lsblk "fail" to
retrieve certain information if executed too quickly in succession. A
normal approach to solving this is adding a delay between partprobe
and lsblk, something I wanted to avoid by putting the sleep at the
very end of the loop, and instead do many quick loops until the
information is available. I could also put partprobe before the loop,
and continue looping until the information becomes available. But that
poses an issue if the disk-operation (parted) completes after
partprobe performs it's tasks. Leaving the loop in a dead state until
a new partprobe is called (or that seems to be the issue at least).

My question boils down to:
 1. Is there a way to get an indication of when the partprobe (or
kernel) has completed its task of populating the partition
information?
 2. Is there a way to tell partprobe/kernel to not clear the old
struct before the new information is available (leaving "old
information" intact until new exists, rather than wiping it and then
populating)

If none of the two above is possible, I would like to consider those
being a feature as it would greatly help to verify when disk
operations are 100% complete. I don't mind optionally hanging
applications/scripts until parted is complete, as I would like to
avoid continuing based on parted exit code if the exit code is not a
guarantee of the process being completed in this case.

Best wishes:
Anton Hvornum