Invalid packet/keyring. How to find out what's responsible?

[Kevin Gallagher]

I've been seeing this error lately both with one of my local GPG
keyrings, and with apt.

    gpg: [don't know]: invalid packet (ctb=2d)
    gpg: keydb_get_keyblock failed: Value not found
    gpg: [don't know]: invalid packet (ctb=2d)
    gpg: /tmp/tmp.rObzKgJEj5/pubring.gpg: copy to
    '/tmp/tmp.rObzKgJEj5/pubring.gpg.tmp' failed: Invalid packet
    gpg: error writing keyring '/tmp/tmp.rObzKgJEj5/pubring.gpg':
    Invalid packet
    gpg: [don't know]: invalid packet (ctb=2d)
    gpg: error reading '-': Invalid packet
    gpg: import from '-' failed: Invalid packet

In the latter case, I solved it by exporting all my keys and importing
them back again. But that doesn't work this time:

apt-key exportall says: gpg: key export failed: Invalid keyring

How can I figure out which specific key is corrupted or responsible for
this, so I can repair my keyring?
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