Error in CheckBlockHeader() when doing full checkblocks on master #4246

When I run with -checkblocks=0 -checklevel=2 on master, I get the following error:

2014-05-29 06:53:38 ERROR: CheckBlockHeader() : block with timestamp before last checkpoint
2014-05-29 06:53:38 ERROR: VerifyDB() : *** found bad block at 278999, hash=0000000000000001ed51db67f98e83ac76209fe00dabc2a42c80cbc9a4ef6c45

We've discussed on IRC a bit and @gmaxwell was able to reproduce it, so it's not local corruption. Seemingly it picks 279000 as last checkpoint instead of 250000 for block 278999.

This problem does not exist on the 0.9.2 branch, so it must have been introduced in one of the recent changes in main.cpp/h.

I believe the problem is that when processing the block, the header is already processed. This means that 290000's header is already in mapBlockIndex when its block data is processed. As that invokes CheckBlock, which on its turn invokes CheckBlockHeader again, which calls GetLastCheckPoint, which returns the block being verified itself, and then compares its parent against that, resulting in time seemingly going backward.

Two possible solutions:

• Make GetLastCheckpoint take the verification level of the mapBlockIndex passed into account, and ignore not-fully-validated ones (easier).
• Pass chainActive to GetLastCheckpoint instead of mapBlockIndex, which will never contain a block itself while it is still being verified (cleaner, imho).

I'm probably wrong.