journalctl --sync should guarantee all currently in flight logs are written to the journal

[DaanDeMeyer]

Component

No response

Is your feature request related to a problem? Please describe

We currently have a bunch of racy code in our integration tests to check if a message was written to the journal. This is because there's is no command in journalctl to wait until all currently in flight logs have been written to the journal. We could implement this by iterating over all open log streams, checking the number of bytes in each stream and only reading up to that amount from the socket. This will guarantee all currently in flight logs have been written to the journal.

Describe the solution you'd like

No response

Describe alternatives you've considered

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The systemd version you checked that didn't have the feature you are asking for

No response

[poettering]

hmm what's wrong with the existing journalctl --sync? what am i missing?

[DaanDeMeyer]

@poettering AFAIK that doesn't seem to do anything with logs that haven't been read from the service's file descriptor(s) yet? If I call journalctl --sync before journald reads the logs from the unit's file descriptor (or /run/systemd/journal/socket), then journalctl --sync doesn't actually guarantee those logs will be written to the journal?

If I currently do something like this

systemctl start oneshot-unit-that-logs-to-journal
journalctl --sync
journalctl --grep "log-message-from-oneshot-unit-that-logs-to-journal"

The journalctl grep might return an entry, or it might not return any entries at all. What I want is a way to ensure that that call to journalctl --grep will contain the logs from the oneshot service.

[poettering]

hmm, the idea was that the --sync stuff does exactly what you are looking for here, i.e. it only returns once every stream was fully processed.

[yuwata]

Yeah, currently journalctl --sync does not check stream, and there may be some pending data.

[yuwata]

-> #32617.