In a Hydra instance I saw:
possibly transient failure building ‘/nix/store/X.drv’ on ‘localhost’:
dependency '/nix/store/Y' of '/nix/store/Y.drv' does not exist,
and substitution is disabled
This is confusing because the Hydra in question does have substitution enabled.
This instance uses:
keep-outputs = true
keep-derivations = true
and an S3 binary cache which is not configured as a substituter in the nix.conf.
It appears this instance encountered a situation where store path Y was built
and present in the binary cache, and Y.drv was GC rooted on the instance,
however Y was not on the host.
When Hydra would try to build this path locally, it would look in the binary
cache to see if it was cached:
(nix)
439 bool valid = isValidPathUncached(storePath);
440
441 if (diskCache && !valid)
442 // FIXME: handle valid = true case.
443 diskCache->upsertNarInfo(getUri(), hashPart, 0);
444
445 return valid;
Since it was cached, the store path was considered Valid.
The queue monitor would then not put this input in for substitution, because
the path is valid:
(hydra)
470 if (!destStore->isValidPath(*i.second.path(*localStore, step->drv->name, i.first))) {
471 valid = false;
472 missing.insert_or_assign(i.first, i.second);
473 }
Hydra appears to correctly handle the case of missing paths that need
to be substituted from the binary cache already, but since most
Hydra instances use `keep-outputs` *and* all paths in the binary cache
originate from that machine, it is not common for a path to be cached
and not GC rooted locally.
I'll run Hydra with this patch for a while and see if we run in to the
problem again.
A big thanks to John Ericson who helped debug this particular issue.
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