nbd-server hangs after fork

Yes, that's exactly correct, and the issue will be fixed with the upload of nbd 3.25 to Debian, which will happen in a few months (after Debian releases). However, that won't fix Lunar.

In order to fix this, cherry-picking commits 3cb679f4a97e1a54a7285ee1f22e2888c9d3b900 and a10d07b43fe5169cd295604632faf6a08c52f75b from git HEAD should suffice.

Thank you for the report.

I can confirm this problem. Here are the steps to reproduce the bug:

$ lxc launch ubuntu:lunar nbd-bug --vm
$ lxc shell nbd-bug
# apt update && apt install -y nbd-server nbd-client
# modprobe nbd
# dd if=/dev/zero of=/srv/test-nbd bs=1024 count=10000
# mke2fs /srv/test-nbd
# chown nbd:nbd /srv/test-nbd
# nbd-server 9000 /srv/test-nbd
# nbd-client 127.0.0.1 9000 /dev/nbd0
# fdisk -l /dev/nbd0

The fdisk command should successfully return, but on Lunar and Mantic it hangs there.

Here's a debdiff that should work.

NOTE: I have no ubuntu lunar system available, so I couldn't test these packages. The patches have been tested and confirmed upstream, and I am 99% sure that this is sufficient, but please confirm by building locally before uploading ;-)

Previous attachment contained some autoreconf noise, this one should be more readable.

Justification for lunar update:

[Impact]
Glib 2.76.0 reworked the implementation of GThreadPool, which nbd uses, in ways that it now no longer works if the treadpool is initialized before a fork() call, but used afterwards (which nbd used to do). A result of this bug in nbd is that, when run against glib 2.76.0 or above, it will accept() a connection from a client but then just sit there and never reply to any message from the client.

[Test plan]
The build contains a test which calls nbd-tester-client against the newly-built nbd-server. If the bug is still present, the test suite will fail. If the bug is not present, the test suite will complete successfully.

If wanted, the test plan in comment 2 can also be run.

(Note that the NBD build-time test suite will fail if an nbd-server is running)

This bug was fixed in the package nbd - 1:3.24-1.1ubuntu1

---------------
nbd (1:3.24-1.1ubuntu1) mantic; urgency=medium

  * cherry-pick patches 3cb679f4a97e1a54a7285ee1f22e2888c9d3b900 and
    a10d07b43fe5169cd295604632faf6a08c52f75b from upstream git HEAD to
    fix operation with glib >= 2.76.0. LP: #2019852.

 -- Wouter Verhelst <email address hidden> Fri, 19 May 2023 10:38:13 +0200

Hello Marco, or anyone else affected,

Accepted nbd into lunar-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/nbd/1:3.24-1.1ubuntu0.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-lunar to verification-done-lunar. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-lunar. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Performing the verification for Lunar.

First, checking that the issue can be reproduced.

# apt policy nbd-server nbd-client
nbd-server:
  Installed: 1:3.24-1.1
  Candidate: 1:3.24-1.1
  Version table:
 *** 1:3.24-1.1 500
        500 http://archive.ubuntu.com/ubuntu lunar/main amd64 Packages
        100 /var/lib/dpkg/status
nbd-client:
  Installed: 1:3.24-1.1
  Candidate: 1:3.24-1.1
  Version table:
 *** 1:3.24-1.1 500
        500 http://archive.ubuntu.com/ubuntu lunar/universe amd64 Packages
        100 /var/lib/dpkg/status
# fdisk -l /dev/nbd0
<hangs>

As expected, fdisk hangs with the problematic nbd. Now, upgrading the package and making sure that the new version fixes the issue.

# apt policy nbd-server nbd-client
nbd-server:
  Installed: 1:3.24-1.1ubuntu0.1
  Candidate: 1:3.24-1.1ubuntu0.1
  Version table:
 *** 1:3.24-1.1ubuntu0.1 100
        100 http://archive.ubuntu.com/ubuntu lunar-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     1:3.24-1.1 500
        500 http://archive.ubuntu.com/ubuntu lunar/main amd64 Packages
nbd-client:
  Installed: 1:3.24-1.1ubuntu0.1
  Candidate: 1:3.24-1.1ubuntu0.1
  Version table:
 *** 1:3.24-1.1ubuntu0.1 100
        100 http://archive.ubuntu.com/ubuntu lunar-proposed/universe amd64 Packages
        100 /var/lib/dpkg/status
     1:3.24-1.1 500
        500 http://archive.ubuntu.com/ubuntu lunar/universe amd64 Packages
# fdisk -l /dev/nbd0
Disk /dev/nbd0: 9.77 MiB, 10240000 bytes, 20000 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

As can be seen, fdisk worked correctly after upgrading nbd.

This concludes the verification for Lunar.

Can confirm this fixed my original issue.
Great work :)

This bug was fixed in the package nbd - 1:3.24-1.1ubuntu0.1

---------------
nbd (1:3.24-1.1ubuntu0.1) lunar; urgency=medium

  * cherry-pick patches 3cb679f4a97e1a54a7285ee1f22e2888c9d3b900 and
    a10d07b43fe5169cd295604632faf6a08c52f75b from upstream git HEAD to
    fix operation with glib >= 2.76.0. LP: #2019852.

 -- Wouter Verhelst <email address hidden> Fri, 19 May 2023 10:38:13 +0200

The verification of the Stable Release Update for nbd has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.