Anatomy of a bug

DNF not downloading zchunk metadata in Fedora

I’m not sure how many have noticed, but DNF hasn’t been downloading zchunk metadata in Fedora since the beginning of this month due to a bug in… well, it’s complicated.

Let’s start with the good news. The fact that almost nobody has noticed means that the fallback to non-zchunk metadata is working perfectly. Fedora is still generating zchunk metadata. When we get the fix built for Fedora, DNF will automatically go back to downloading zchunk metadata. And, when that happens, almost nobody will notice (but their metadata downloads will be greatly reduced in size again)

So, what happened?

Back in July, a bug was found in RHEL 8 that caused a crash if zchunk-enabled repos were found. The bug turned out to be in librepo, where LRO_SUPPORTS_CACHEDIR was defined in all recent versions. This define is used by libdnf to detect whether to ask libsolv and librepo to attempt to use zchunk (libdnf doesn’t actually use zchunk itself). The problem is that RHEL 8’s librepo had this defined, but didn’t actually have zchunk enabled, which caused Bad Things To Happen™.

The DNF developers quickly settled on an easy fix, wrapping the define in an #ifdef WITH_ZCHUNK so it would only be defined if zchunk was enabled in librepo. At least, that’s how it was supposed to work.

This fix made it into Fedora and had no noticeable effect until libdnf was rebuilt early this month. It turns out that the header file with the define is included as is in libdnf. libdnf wasn’t being build with WITH_ZCHUNK enabled, so LRO_SUPPORTS_CACHEDIR wasn’t defined, so libdnf never passed librepo the base cache directory, so librepo can’t find the old zchunk files, so librepo downloads the non-zchunk metadata. Oops.

How do we fix it?

There are a couple of ways to fix this. The method we’ve gone with is to define WITH_ZCHUNK in libdnf so LRO_SUPPORTS_CACHEDIR is defined once more. This fix is currently in review, and once it’s pushed upstream, we’ll try to get libdnf builds done in Fedora with it included.

Once a libdnf build is pushed out with LRO_SUPPORTS_CACHEDIR defined, DNF will once more start downloading zchunk metadata.


David Anderson
Monday, Sep 2, 2019

I’m not sure how many have noticed, but DNF hasn’t been downloading zchunk metadata in Fedora since the beginning of this month due to a bug in… well, it’s complicated.

I’d noticed… speaking personally, there’s enough other parts of the updates system that are unfriendly in low-bandwidth or connections-drop situations, and have been known to be for a long time (e.g. resumption of large RPMs that gets timed out always re-starts at byte 0 (I’ve had the experience of downloading 1.5GB of data during a morning but no RPMs because once it reaches the large ones they go round and round when the HTTP connection drops after 50MB or whatever…); drpms apparently get wiped from the updates server after a day), that one more thing in that sort of territory didn’t make a big difference.


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