ChainUpdates: model bad peer behavior #492
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The BFT protocol uses block numbers as its `SelectView`, so the induced order on blocks is not total. When we generate a rollback, we usually end up with the same chain length, which is then not improving upon the previous chain.
Right now, the peer disconnection logic relies on the interplay of the BlockFetch and the ChainSync client to catch invalid behavior, so this commit adds an actual ChainSync client for every peer. Concretely, consider the case when a peer wants to extend an invalid block. In that case, the ChainSync client will disconnect, either when the extending header is received, or via the invalid block rejector in a background thread. In contrast, when we simply add a block (together with a punishment) to the ChainDB, this punishment will *not* be enacted, as the block is not validated as it is not reachable via any (valid) block in the VolDB.
The `serverUpdates` only depend on the `securityParam`, and the `clientUpdates` additionally depend on `serverUpdates` due to `removeLateClientUpdates`.
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Closes CAD-4314
Previously, we did not generate any ChainUpdate behavior modeling a malicious or ill-configured peer. This meant that existing tests could only check that our disconnection routines are not overly restrictive, but not that they are strict enough.
This PR adds this possibility, namely by randomly toggling some blocks to be invalid. As the result is not necessarily modeling bad behavior, we add a function to "classify" a sequence of chain updates after the fact.
Concerning the BlockFetch client test: as certain invalid behavior is only caught by the ChainSync client, we now also run it in the background.
Historical remark: This PR has been migrated from IntersectMBO/ouroboros-network#3856.