ndb: unexpected NDB blob Magic for pkg error

[djoreilly]

On openSUSE Tumbleweed with the ndb backend. When zypper up runs and updates many packages there can be a lots of changes to the packages file. Attempting to use the library at the same time can fail with unexpected NDB blob Magic for pkg ...

go-rpmdb/pkg/ndb/ndb.go

Line 175 in ac49267

Err: xerrors.Errorf("unexpected NDB blob Magic for pkg %d: %x", slot.PkgIndex, blobHeaderBuff.BlobMagic),

I think the rpm binary is using an exclusive file lock when updating the file
https://github.com/rpm-software-management/rpm/blob/rpm-4.17.0-release/lib/backend/ndb/rpmpkg.c

Adding a shared lock seems to fix the problem. I'll post a PR for review.

[djoreilly]

Here are steps to reproduce:

1. Run https://github.com/knqyf263/go-rpmdb/blob/master/cmd/rpmdb/main.go continuously until an error happens printing only stderr:

bash -e -c "cd /var/lib/rpm; while true; do ~/go-rpmdb/main >/dev/null; done"

2. Run zypper up in another terminal. The more packages to be updated the more likely the problem will happen.