Dear Maintainer,

matrix-synapse doesn't start when python3-eventlet is installed

Hello,

Can you give me more details please?
I don’t see any mentions of this anywhere in the source.

renders matrix-synapse inoperable:

$ sudo apt-get install python3-eventlet && sudo systemctl restart
matrix-synapse.service
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Solving dependencies... Done
The following packages were automatically installed and are no longer
required:
   linux-doc-6.16 linux-doc-6.17 linux-image-6.16.12+deb14+1-rt-amd64
linux-image-6.17.11+deb14-rt-amd64 linux-image-6.17.12+deb14-rt-amd64
   linux-image-6.17.13+deb14-rt-amd64 linux-image-6.17.8+deb14-rt-amd64
Use 'sudo apt autoremove' to remove them.
Suggested packages:
   python-eventlet-doc
The following NEW packages will be installed:
   python3-eventlet
0 upgraded, 1 newly installed, 0 to remove and 4 not upgraded.
Need to get 311 kB of archives.
After this operation, 987 kB of additional disk space will be used.
Get:1 http://deb.debian.org/debian testing/main all python3-eventlet all
0.40.3-2 [311 kB]
Fetched 311 kB in 0s (1,948 kB/s)
Selecting previously unselected package python3-eventlet.
(Reading database… 1820926 files and directories currently installed.)
Preparing to unpack …/python3-eventlet_0.40.3-2_all.deb…
Unpacking python3-eventlet (0.40.3-2)…
Setting up python3-eventlet (0.40.3-2)…
Scanning processes...

Scanning candidates...

Scanning processor microcode...

Scanning linux images...


The processor microcode seems to be up-to-date.

Restarting services...
Service restarts being deferred:
  /etc/needrestart/restart.d/dbus.service
  systemctl restart nodm.service
  systemctl restart unattended-upgrades.service

No containers need to be restarted.

User sessions running outdated binaries:
  user @ session #2: bash[969641,2416463,2918488],
chromium[2918522,2918602], crashhelper[3589], firefox[3360],
forkserver[4282],
   glycin-svg[984474,984532], obsidian[3334,3482,3926], sh[2946],
urxvtd[3114,3115], x-session-manag[2982]
  user @ user manager: chromium[2918499], (sd-pam)[2625]
  user @ user service: at-spi-dbus-bus.service[3161],
blueman-applet.service[969792,969802], dbus.service[2947,4381,2070721],
   dirmngr.service[4002], dunst.service[3801],
filter-chain.service[3018], gvfs-udisks2-volume-monitor.service[3736],
   pipewire-pulse.service[3019], pipewire.service[2953],
wireplumber.service[3015], xdg-desktop-portal-gtk.service[3594],
   xdg-desktop-portal.service[3574]

No VM guests are running outdated hypervisor (qemu) binaries on this host.
Job for matrix-synapse.service failed because the control process exited
with error code.
See "systemctl status matrix-synapse.service" and "journalctl -xeu
matrix-synapse.service" for details.

$ journalctl -xeu matrix-synapse
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/sentry_sdk/utils.py", line 1346, in
_get_contextvars
Feb 20 16:59:17 nexus python3[2979902]:     if not _is_contextvars_broken():
Feb 20 16:59:17 nexus python3[2979902]:            ~~~~~~~~~~~~~~~~~~~~~~^^
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/sentry_sdk/utils.py", line 1287, in
_is_contextvars_broken
Feb 20 16:59:17 nexus python3[2979902]:     from eventlet.patcher import
is_monkey_patched  # type: ignore
Feb 20 16:59:17 nexus python3[2979902]:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/__init__.py", line 6, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     from eventlet import convenience
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/convenience.py", line 7, in
<module>
Feb 20 16:59:17 nexus python3[2979902]:     from eventlet.green import
socket
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/green/socket.py", line 21, in
<module>
Feb 20 16:59:17 nexus python3[2979902]:     from eventlet.support import
greendns
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/support/greendns.py", line 78,
in <module>
Feb 20 16:59:17 nexus python3[2979902]:     setattr(dns, pkg,
import_patched('dns.' + pkg))
Feb 20 16:59:17 nexus python3[2979902]:
~~~~~~~~~~~~~~^^^^^^^^^^^^^^
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/support/greendns.py", line 60,
in import_patched
Feb 20 16:59:17 nexus python3[2979902]:     return
patcher.import_patched(module_name, **modules)
Feb 20 16:59:17 nexus python3[2979902]:
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/patcher.py", line 136, in
import_patched
Feb 20 16:59:17 nexus python3[2979902]:     return inject(
Feb 20 16:59:17 nexus python3[2979902]:         module_name, None,
*additional_modules + tuple(kw_additional_modules.items())
Feb 20 16:59:17 nexus python3[2979902]:     )
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/eventlet/patcher.py", line 113, in inject
Feb 20 16:59:17 nexus python3[2979902]:     module =
__import__(module_name, {}, {}, module_name.split(".")[:-1])
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/dns/asyncquery.py", line 34, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     import dns.quic
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/dns/quic/__init__.py", line 39, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     import trio
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/trio/__init__.py", line 25, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     from . import abc,
from_thread, lowlevel, socket, to_thread
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/trio/socket.py", line 16, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     from . import _socket
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/trio/_socket.py", line 546, in <module>
Feb 20 16:59:17 nexus python3[2979902]:     class SocketType:
Feb 20 16:59:17 nexus python3[2979902]:     ...<221 lines>...
Feb 20 16:59:17 nexus python3[2979902]:                 raise
NotImplementedError
Feb 20 16:59:17 nexus python3[2979902]:   File
"/usr/lib/python3/dist-packages/trio/_socket.py", line 759, in SocketType
Feb 20 16:59:17 nexus python3[2979902]:
@_wraps(_stdlib_socket.socket.sendmsg, assigned=(), updated=())
Feb 20 16:59:17 nexus python3[2979902]:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Feb 20 16:59:17 nexus python3[2979902]: AttributeError: type object
'GreenSocket' has no attribute 'sendmsg'
Feb 20 16:59:17 nexus systemd[1]: matrix-synapse.service: Control
process exited, code=exited, status=1/FAILURE
░░ Subject: Unit process exited
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ An ExecStartPre= process belonging to unit matrix-synapse.service has
exited.
░░
░░ The process' exit code is 'exited' and its exit status is 1.
Feb 20 16:59:17 nexus systemd[1]: matrix-synapse.service: Failed with
result 'exit-code'.
░░ Subject: Unit failed
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ The unit matrix-synapse.service has entered the 'failed' state with
result 'exit-code'.
Feb 20 16:59:17 nexus systemd[1]: Failed to start matrix-synapse.service
- Synapse Matrix homeserver.
░░ Subject: A start job for unit matrix-synapse.service has failed
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ A start job for unit matrix-synapse.service has finished with a failure.
░░
░░ The job identifier is 230940 and the job result is failed.
Feb 20 16:59:17 nexus systemd[1]: matrix-synapse.service: Consumed
1.643s CPU time over 4.812s wall clock time, 69.8M memory peak.
░░ Subject: Resources consumed by unit runtime
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ The unit matrix-synapse.service completed and consumed the indicated
resources.
Feb 20 16:59:20 nexus systemd[1]: matrix-synapse.service: Scheduled
restart job, restart counter is at 10.
░░ Subject: Automatic restarting of a unit has been scheduled
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ Automatic restarting of the unit matrix-synapse.service has been
scheduled, as the result for
░░ the configured Restart= setting for the unit.
Feb 20 16:59:20 nexus systemd[1]: Starting matrix-synapse.service -
Synapse Matrix homeserver...
░░ Subject: A start job for unit matrix-synapse.service has begun execution
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░
░░ A start job for unit matrix-synapse.service has begun execution.
░░
░░ The job identifier is 231070.

removing python3-eventlet makes matrix-synapse work again:

$ sudo apt-get purge python3-eventlet && sudo systemctl restart
matrix-synapse.service
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Solving dependencies... Done
The following packages were automatically installed and are no longer
required:
   linux-doc-6.16 linux-doc-6.17 linux-image-6.16.12+deb14+1-rt-amd64
linux-image-6.17.11+deb14-rt-amd64 linux-image-6.17.12+deb14-rt-amd64
   linux-image-6.17.13+deb14-rt-amd64 linux-image-6.17.8+deb14-rt-amd64
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
   python3-eventlet*
0 upgraded, 0 newly installed, 1 to remove and 4 not upgraded.
After this operation, 987 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database… 1821035 files and directories currently installed.)
Removing python3-eventlet (0.40.3-2)…

$ sudo systemctl status matrix-synapse.service
● matrix-synapse.service - Synapse Matrix homeserver
      Loaded: loaded (/etc/systemd/system/matrix-synapse.service;
enabled; preset: enabled)
      Active: active (running) since Fri 2026-02-20 17:14:01 CET; 24s ago
  Invocation: b60ad88a34d04dc180f7609a665f5e0b
        Docs: file:/usr/share/doc/matrix-synapse/README.rst
     Process: 2985474 ExecStartPre=/usr/bin/python3 -m
synapse.app.homeserver --config-path=/etc/matrix-synapse/homeserver.yaml
--config-path=>
    Main PID: 2985525 (python3)
       Tasks: 12 (limit: 76824)
      Memory: 124.4M (peak: 125.5M)
         CPU: 6.230s
      CGroup: /system.slice/matrix-synapse.service
              └─2985525 /usr/bin/python3 -m synapse.app.homeserver
--config-path=/etc/matrix-synapse/homeserver.yaml --config-path=/etc/matrix>

Feb 20 17:13:55 nexus systemd[1]: Starting matrix-synapse.service -
Synapse Matrix homeserver...
Feb 20 17:14:00 nexus synapse[2985525]: synapse.config.logger: [main]
***** STARTING SERVER *****
Feb 20 17:14:00 nexus synapse[2985525]: synapse.config.logger: [main]
Server /usr/lib/python3/dist-packages/synapse/app/homeserver.py version>
Feb 20 17:14:00 nexus synapse[2985525]: synapse.config.logger: [main]
Copyright (c) 2023 New Vector, Inc
Feb 20 17:14:00 nexus synapse[2985525]: synapse.config.logger: [main]
Licensed under the AGPL 3.0 license. Website: https://github.com/elemen>
Feb 20 17:14:01 nexus systemd[1]: Started matrix-synapse.service -
Synapse Matrix homeserver.

Hello,

Looks like something in the sentry SDK (which I think you should have mentioned you enabled).

I do have python3-sentry-sdk installed because it's a dependency of an
unrelated package but I have no idea what that is or what I did to
"enable" it. Removing python3-sentry-sdk makes matrix-synapse work again
even with python3-eventlet installed so I guess both have to be
installed to trigger it.

Hello,

I recently uploaded some new versions of sentry-python, can you please test this bug again?

For reference, I tested this bug very quickly here and couldn't reproduce it.

Sentry is only used in matrix-synapse if it is enabled in the config. Otherwise, it is not imported
at all: https://github.com/element-hq/synapse/blob/d7e9a3ff8321fa820f38c17a4484f0437233cebb/synapse/app/_base.py#L830

To "enable" sentry in matrix-synapse you need to have something like this in your
config (/etc/matrix-synapse/homeserver.yaml):

  sentry:
      environment: production
      dsn: '...'


AFAICT, the default configuration in Debian does NOT set the sentry configuration,
so by default really you shouldn't be hitting this path.

Can you check if you setup sentry at all in the synapse configuration?

I guess we should add python3-sentry-sdk as a runtime-dep to matrix-synapse as well, since it can be enabled at runtime.


Cheers!

Chris

I don't have something like that in my configuration. Having both
python3-sentry-sdk (2.60.0-3) and python3-eventlet (0.40.4-1) installed
still renders matrix-synapse inoperable:

Control: reassign -1 python3-eventlet
Control: affects -1 matrix-synapse python3-sentry-sdk
Control: found -1 0.40.4-1

Thanks for the traceback - I can now reproduce this after installing python3-trio.

A minimal reproducer appears to be:

    apt install python3-sentry-sdk python3-eventlet python3-trio python3-dnspython python3-httpcore
    python3 -c 'import sentry_sdk'

which fails with:

    AttributeError: type object 'GreenSocket' has no attribute 'sendmsg'

The Synapse failure does not require Sentry to be configured. Synapse imports sentry_sdk during startup which causes the traceback.

Setting EVENTLET_NO_GREENDNS=yes avoids the crash:

    EVENTLET_NO_GREENDNS=yes python3 -c 'import sentry_sdk'

I get the same crash when just importing eventlet:

    python3 -c 'import eventlet'
    AttributeError: type object 'GreenSocket' has no attribute 'sendmsg'


This looks very similar to #1042466 but #1042466 the missing method is eventlet.green.select.epoll.
Here the missing method is eventlet.green.socket.GreenSocket.sendmsg so I won't merge the bugs yet.

Reassigning to python3-eventlet. @Thomas, could you take a look at this one?

Cheers!

Chris

Hi,

The issue is that eventlet didn't implement:
GreenSocket.sendmsg.

Likely, recvmsg as well.

What's happening is this import stack:
eventlet → greendns → dnspython → httpcore → trio

Adding EVENTLET_NO_GREENDNS=yes as an env var avoids this.

So what's happening is Eventlet leaking a monkey-patched socket into a
dependency graph that now performs import-time API introspection.

 From my perspective, the only thing I could do would be adding a
Breaks: python3-trio in Eventlet, and call it a day. Especially because:

# python3 -c 'import eventlet'
<string>:1: EventletDeprecationWarning:
Eventlet is deprecated. It is currently being maintained in bugfix mode, and
we strongly recommend against using it for new projects.

If you are already using Eventlet, we recommend migrating to a different
framework.  For more detail see
https://eventlet.readthedocs.io/en/latest/asyncio/migration.html

FYI, the only reason Eventlet is still in Debian, is because OpenStack
hasn't finished migrating away from it.

I'll add the Breaks: and close this bug. Anyone willing to fix this bug
better must implement sendmsg / recvmsg in GreenSocket, but I'm not even
sure some new functionality will be accepted in upstream Eventlet, which
is a dying project.

Cheers,

Thomas Goirand (zigo)

Hello,

Bug #1128500 in python-eventlet reported by you has been fixed in the
Git repository and is awaiting an upload. You can see the commit
message below and you can check the diff of the fix at:

https://salsa.debian.org/openstack-team/third-party/python-eventlet/-/commit/52fd2d72d11b28cc710783d7782f7e3ecd14b177

(this message was generated automatically)
-- 
Greetings

https://bugs.debian.org/1128500

We believe that the bug you reported is fixed in the latest version of
python-eventlet, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1128500@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Thomas Goirand <zigo@debian.org> (supplier of updated python-eventlet package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Mon, 22 Jun 2026 09:09:21 +0200
Source: python-eventlet
Binary: python-eventlet-doc python3-eventlet
Architecture: source all
Version: 0.40.4-2
Distribution: unstable
Urgency: medium
Maintainer: Debian OpenStack <team+openstack@tracker.debian.org>
Changed-By: Thomas Goirand <zigo@debian.org>
Description:
 python-eventlet-doc - concurrent networking library - doc
 python3-eventlet - concurrent networking library
Closes: 1128500
Changes:
 python-eventlet (0.40.4-2) unstable; urgency=medium
 .
   * Add Breaks: python3-trio (Closes: #1128500).
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