clarify epoch sync handling (effectively a comment update only)

why:
  autorisation list verification is performed in the main module along
  with other header verifications

nimbus/p2p/clique/clique_defs.nim

@@ -250,7 +250,7 @@ type
     errCliqueGasRepriceFork
     errCliqueSealSigFn
 
-    errCliqueStopped = "Process was interrupted"
+    errCliqueStopped = "process was interrupted"
     errCliqueExpectedBaseFee = "header is missing baseFee"
     errCliqueGasLimitTooLow = "gas limit is too low"
     errCliqueGasLimitTooHigh = "gas limit is too high"

nimbus/p2p/clique/clique_poll.nim

@@ -110,6 +110,11 @@ proc delVote*(t: var CliquePoll; signer, address: EthAddress) {.
         t.votes[address].signers.del(signer)
 
 
+proc flushVotes*(t: var CliquePoll) =
+  ## Reset/flush pending votes, authorised signers remain the same.
+  t.votes.clear
+
+
 # clique/snapshot.go(141): func (s *Snapshot) validVote(address [..]
 proc validVote*(t: var CliquePoll; address: EthAddress; authorize: bool): bool =
   ## Check whether voting would have an effect in `addVote()`

nimbus/p2p/clique/snapshot.nim

@@ -30,7 +30,6 @@ import
   ./clique_cfg,
   ./clique_defs,
   ./clique_poll,
-  ./clique_utils,
   ./ec_recover,
   chronicles,
   eth/[common, rlp, trie/db]
@@ -82,7 +81,7 @@ proc pp(s: var Snapshot; h: var AddressHistory): string =
 
 proc pp(s: var Snapshot; v: Vote): string =
   proc authorized(b: bool): string =
-    if b: "auhorize" else: "de-authorize"
+    if b: "authorise" else: "de-authorise"
   ppExceptionWrap:
     "(" & &"address={s.pp(v.address)}" &
           &",signer={s.pp(v.signer)}" &
@@ -239,24 +238,15 @@ proc applySnapshot*(s: var Snapshot;
 
     # Remove any votes on checkpoint blocks
     if (number mod s.cfg.epoch) == 0:
-      # clique/snapshot.go(210): snap.Votes = nil
-      s.say "applySnapshot epoch => reset, state=", s.pp(41)
-
-      # This part differs from the go implementation in that the `signer`
-      # list is re-assigned. The original implementation silently assumes
-      # that the `signer` list is the same as the previous one but this is
-      # not enforced.
+      # Note that the correctness of the authorised accounts list is verified in
+      #   clique/clique.verifyCascadingFields(),
+      #   see clique/clique.go(355): if number%c.config.Epoch == 0 {
+      # This means, the account list passed with the epoch header is verified
+      # to be the same as the one we already have.
       #
-      # The eip225 discussion has it as: [..] where every epoch transition
-      # flushes all pending votes. Furthermore, these epoch transitions can
-      # also act as stateless checkpoints containing the list of current
-      # authorized signers within the header extra-data. This permits clients
-      # to sync up based only on a checkpoint hash without having to replay
-      # all the voting that was done on the chain up to that point. It also
-      # allows the genesis header to fully define the chain, containing the
-      # list of initial signers.
-      s.data.ballot.initCliquePoll(header.extraData.extraDataAddresses)
-      s.data.ballot.setDebug(s.data.debug)
+      # clique/snapshot.go(210): snap.Votes = nil
+      s.data.ballot.flushVotes
+      s.say "applySnapshot epoch => reset, state=", s.pp(41)
 
     # Delete the oldest signer from the recent list to allow it signing again
     block:

tests/test_clique.nim

@@ -36,7 +36,7 @@ proc cliqueMain*(noisy = defined(debug)) =
     # clique/snapshot_test.go(379): for i, tt := range tests {
     for tt in voterSamples.filterIt(it.id in testSet):
 
-      test &"Snapshots {tt.id: 2}: {tt.info.substr(0,50)}...":
+      test &"Snapshots {tt.id:2}: {tt.info.substr(0,50)}...":
         pool.say "\n"
 
         if tt.id in skipSet: