[systemd-bugs] [Bug 63672] journalctl skips first entry after reboot in certain scenario

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=63672

--- Comment #3 from Marius Vollmer <marius.vollmer at redhat.com> ---
(In reply to comment #2)
> Hmm, so, yeah, you found the issue I think, this is indeed broken the way
> you describe. That said, it was actually supposed to be a good thing to
> cache the last positions in all files, to make simple iteration faster.

You mean, in the case where multiple files overlap in time?  I didn't think of
that.  Isn't my fix broken for that case?  Is that actually supported?

(E.g, when iterating, you get entry 1 from file A, then entries 2 and 3 from
file B, then entry 2 from file A, etc.  I think with my fix we would go back to
entry 1 from file A after entries 2 and 3 from file B.)

My first idea was to reset current_offset when a file is exhausted (when
next_beyond_location returns 0 in real_journal_next), but that would break the
'guarantee' that you are still on the same position after sd_journal_next has
returned 0, and can get a cursor for that position, for example.  Is there such
a guarantee, actually?  If not, that should be documented.  If there is such a
guarantee, we can do the resetting of exhausted files only if they are not the
current_file, at the end of real_journal_next (I think).

In any case, current_offset should not be reset to 0 when a file is exhausted,
but to a value that indicates that we are one step beyond the beginning _or_
end, as appropriate.

> Anyway, I'll commit your fix for now, given that O(log(n)) isn't so much
> worse than O(1), and I want to get a new release out, but I will leave this
> bug open, to look into this, add a comment about this and add a TODO item,
> so I really won't forget...

Please consider whether this breaks the overlapping files scenario...  I'll try
to make another patch as outlined above.

> BTW, I greatly appreciate the patches from you! It's awesome getting bug
> reports plus the right patches all the time. That's awesome! Thanks a ton!

My pleasure!  It's fun to hack on some real algorithms and data structures for
a change. :)

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