multi: add resolver reports to Checkpoint

To allow us to write the outcome of our resolver to disk, we add
optional resolver reports to the CheckPoint function. Variadic params
are used because some checkpoints may have no reports (when the resolver
is not yet complete) and some may have two (in the case of a two stage
resolution).

contractcourt/briefcase.go

@@ -62,8 +62,10 @@ type ArbitratorLog interface {
 
 	// InsertUnresolvedContracts inserts a set of unresolved contracts into
 	// the log. The log will then persistently store each contract until
-	// they've been swapped out, or resolved.
-	InsertUnresolvedContracts(...ContractResolver) error
+	// they've been swapped out, or resolved. It takes a set of report which
+	// should be written to disk if as well if it is non-nil.
+	InsertUnresolvedContracts(reports []*channeldb.ResolverReport,
+		resolvers ...ContractResolver) error
 
 	// FetchUnresolvedContracts returns all unresolved contracts that have
 	// been previously written to the log.
@@ -533,7 +535,9 @@ func (b *boltArbitratorLog) FetchUnresolvedContracts() ([]ContractResolver, erro
 // swapped out, or resolved.
 //
 // NOTE: Part of the ContractResolver interface.
-func (b *boltArbitratorLog) InsertUnresolvedContracts(resolvers ...ContractResolver) error {
+func (b *boltArbitratorLog) InsertUnresolvedContracts(reports []*channeldb.ResolverReport,
+	resolvers ...ContractResolver) error {
+
 	return kvdb.Batch(b.db, func(tx kvdb.RwTx) error {
 		contractBucket, err := fetchContractWriteBucket(tx, b.scopeKey[:])
 		if err != nil {
@@ -547,6 +551,14 @@ func (b *boltArbitratorLog) InsertUnresolvedContracts(resolvers ...ContractResol
 			}
 		}
 
+		// Persist any reports that are present.
+		for _, report := range reports {
+			err := b.cfg.PutResolverReport(tx, report)
+			if err != nil {
+				return err
+			}
+		}
+
 		return nil
 	})
 }
@@ -908,15 +920,28 @@ func (b *boltArbitratorLog) WipeHistory() error {
 
 // checkpointContract is a private method that will be fed into
 // ContractResolver instances to checkpoint their state once they reach
-// milestones during contract resolution.
-func (b *boltArbitratorLog) checkpointContract(c ContractResolver) error {
+// milestones during contract resolution. If the report provided is non-nil,
+// it should also be recorded.
+func (b *boltArbitratorLog) checkpointContract(c ContractResolver,
+	reports ...*channeldb.ResolverReport) error {
+
 	return kvdb.Update(b.db, func(tx kvdb.RwTx) error {
 		contractBucket, err := fetchContractWriteBucket(tx, b.scopeKey[:])
 		if err != nil {
 			return err
 		}
 
-		return b.writeResolver(contractBucket, c)
+		if err := b.writeResolver(contractBucket, c); err != nil {
+			return err
+		}
+
+		for _, report := range reports {
+			if err := b.cfg.PutResolverReport(tx, report); err != nil {
+				return err
+			}
+		}
+
+		return nil
 	})
 }