mining: Add ticket exhaustion check.

This modifies the mining template creation logic to make use of the new
ability to check if the template would result in an unrecoverable chain
due to inevitable ticket exhaustion along with an associated new mining
rule error.

internal/mining/error.go

@@ -33,6 +33,11 @@ const (
 	// to a msgblock.
 	ErrTransactionAppend
 
+	// ErrTicketExhaustion indicates that there will not be enough mature
+	// tickets by the end of the next ticket maturity period to progress the
+	// chain.
+	ErrTicketExhaustion
+
 	// ErrCheckConnectBlock indicates that a newly created block template
 	// failed blockchain.CheckConnectBlock.
 	ErrCheckConnectBlock
@@ -57,6 +62,7 @@ var miningErrorCodeStrings = map[MiningErrorCode]string{
 	ErrGetTopBlock:           "ErrGetTopBlock",
 	ErrGettingDifficulty:     "ErrGettingDifficulty",
 	ErrTransactionAppend:     "ErrTransactionAppend",
+	ErrTicketExhaustion:      "ErrTicketExhaustion",
 	ErrCheckConnectBlock:     "ErrCheckConnectBlock",
 	ErrFraudProofIndex:       "ErrFraudProofIndex",
 	ErrFetchTxStore:          "ErrFetchTxStore",

internal/mining/mining.go

@@ -1893,6 +1893,14 @@ mempoolLoop:
 		}
 	}
 
+	// Ensure that mining the block would not cause the chain to become
+	// unrecoverable due to ticket exhaustion.
+	err = g.chain.CheckTicketExhaustion(&best.Hash, uint8(freshStake))
+	if err != nil {
+		log.Debug(err)
+		return nil, miningRuleError(ErrTicketExhaustion, err.Error())
+	}
+
 	// Get the ticket revocations (SSRtx tx) and store them and their
 	// number.
 	revocations := 0