Consistency check keeps failing when starting from an empty Rekor log

[fghanmi]

Description

We're encountering an issue when starting rekor-monitor against a fresh Rekor instance with an empty log.

Steps to Reproduce

1. Start with a fresh Rekor server (empty log).
2. Run the monitor:

./rekor_monitor --file=checkpoint_log.txt --once=false --url=https://<rekor-server-url>

# output:
error running consistency check: failed to verify previous checkpoint: failed to verify log consistency: consistency proofs can not be computed starting from an empty log

3. Push a single item to the Rekor log.
4. Re-run the monitor:

./rekor_monitor --file=checkpoint_log.txt --once=false --url=https://<rekor-server-url>

# output again:
error running consistency check: failed to verify previous checkpoint: failed to verify log consistency: consistency proofs can not be computed starting from an empty log

Expected Behavior
I am wondering if the expected behavior is as follows:
Once a single entry is present, rekor-monitor should recognize it as a valid checkpoint and produce output similar to:

Root hash consistency verified - Current Size: 1 Root Hash: 385adfc2cb022834e1b9208e4da6f3820715794edd74c19acb5f1c2518e0106a - Previous Size: 1 Root Hash: 385adfc2cb022834e1b9208e4da6f3820715794edd74c19acb5f1c2518e0106a

In this case, it appears the tool currently doesn't handle the transition from an empty log (size 0) to size 1 gracefully.

Version

latest

[fghanmi]

Hello @haydentherapper, is this relevant as a bug ?

[haydentherapper]

You can't compute a consistency proof from an empty log to any size, so that is working as intended. However it looks like something is being persisted when the size is 0, so future attempts fail. The correct behavior would be that for a size of 0, the monitor should fail, and for the size of >0, the monitor should initialize without computing a proof.