mining: No error log on expected head reorg errors.

This changes the log for head reorg failures to a debug log instead of
an error log since it is actually expected to fail reorgs from time to
time due to the fact mining is a race to find blocks.

It also cleans up the code slightly while here and adds an additional
check to avoid additional work in the case reorgs to other tips fail and
the current tip then becomes the best candidate among the eligible
parents.

internal/mining/mining.go

@@ -1082,32 +1082,32 @@ func (g *BlkTmplGenerator) NewBlockTemplate(payToAddress dcrutil.Address) (*Bloc
 			"block on, proceeding to create a new block template",
 			eligibleParents[0])
 
-		// Force a reorganization to the parent with the most votes if we need
-		// to.
-		if eligibleParents[0] != prevHash {
-			for i := range eligibleParents {
-				newHead := &eligibleParents[i]
-				err := g.cfg.ForceHeadReorganization(prevHash, *newHead)
-				if err != nil {
-					log.Errorf("failed to reorganize to new parent: %v", err)
-					continue
-				}
-
-				// Check to make sure we actually have the transactions
-				// (votes) we need in the mempool.
-				voteHashes := g.cfg.TxSource.VoteHashesForBlock(newHead)
-				if len(voteHashes) == 0 {
-					return nil, fmt.Errorf("no vote metadata for block %v",
-						newHead)
-				}
-
-				if exist := g.cfg.TxSource.HaveAllTransactions(voteHashes); !exist {
-					continue
-				} else {
-					prevHash = *newHead
-					break
-				}
+		// Force a reorganization to the parent with the most votes if needed.
+		for i := range eligibleParents {
+			newHead := &eligibleParents[i]
+			if *newHead == prevHash {
+				break
 			}
+
+			err := g.cfg.ForceHeadReorganization(prevHash, *newHead)
+			if err != nil {
+				log.Debugf("failed to reorganize to new parent: %v", err)
+				continue
+			}
+
+			// Ensure the needed votes are actually in the mempool.
+			voteHashes := g.cfg.TxSource.VoteHashesForBlock(newHead)
+			if len(voteHashes) == 0 {
+				return nil, fmt.Errorf("no vote metadata for block %v",
+					newHead)
+			}
+			haveAllVotes := g.cfg.TxSource.HaveAllTransactions(voteHashes)
+			if !haveAllVotes {
+				continue
+			}
+
+			prevHash = *newHead
+			break
 		}
 
 		// Obtain the maximum allowed treasury expenditure.