multi: Add CalcPaymentStats to DB and RPC server

This adds a CalcPaymentStats call to the db, RPC server and dcrlncli.
This allows querying the database for a report on the total number of
payments tracked.

An upstream lnd PR (number 5635) will include a migration that goes
through every payment. While the PR has been tested upstream to not be a
problem even for nodes with large numbers of payments, there is no
actual way to know whether a node _would_ be a problem (due to no
existing way to query the total number of payments in the DB).

This PR prepares for an upstream port of an lnd version that includes
the aforementioned PR by exposing the total counts of payments, which
would be migrated.

This will allow node operators to evaluate whether the payments should
be cleared from the database before performing the migration.

channeldb/payments_stats.go

@@ -0,0 +1,94 @@
+package channeldb
+
+import (
+	"github.com/decred/dcrlnd/kvdb"
+)
+
+// PaymentCountStats returns stats about the payments stored in the DB.
+type PaymentCountStats struct {
+	Total     uint64
+	Failed    uint64
+	Succeeded uint64
+
+	HTLCAttempts uint64
+	HTLCFailed   uint64
+	HTLCSettled  uint64
+
+	OldDupePayments uint64
+}
+
+// CalcPaymentStats goes through the DB, counting up the payment stats.
+func (d *DB) CalcPaymentStats() (*PaymentCountStats, error) {
+	res := &PaymentCountStats{}
+	err := kvdb.View(d, func(tx kvdb.RTx) error {
+		paymentsBucket := tx.ReadBucket(paymentsRootBucket)
+		if paymentsBucket == nil {
+			return nil
+		}
+
+		// Standard payments.
+		return paymentsBucket.ForEach(func(k, v []byte) error {
+			payBucket := paymentsBucket.NestedReadBucket(k)
+			if payBucket == nil {
+				return nil
+			}
+
+			res.Total += 1
+
+			htlcsBucket := payBucket.NestedReadBucket(paymentHtlcsBucket)
+			if htlcsBucket != nil {
+				var inflight, settled bool
+				err := htlcsBucket.ForEach(func(k, _ []byte) error {
+					res.HTLCAttempts += 1
+					htlcBucket := htlcsBucket.NestedReadBucket(k)
+					hasFail := htlcBucket.Get(htlcFailInfoKey) != nil
+					hasSettle := htlcBucket.Get(htlcSettleInfoKey) != nil
+					if hasFail {
+						res.HTLCFailed += 1
+					} else if hasSettle {
+						res.HTLCSettled += 1
+						settled = true
+					} else {
+						inflight = true
+					}
+					return nil
+				})
+				if err != nil {
+					return err
+				}
+
+				hasFailureReason := payBucket.Get(paymentFailInfoKey) != nil
+
+				switch {
+				// If any of the the HTLCs did succeed and there are no HTLCs in
+				// flight, the payment succeeded.
+				case !inflight && settled:
+					res.Succeeded += 1
+
+				// If we have no in-flight HTLCs, and the payment failure is set, the
+				// payment is considered failed.
+				case !inflight && hasFailureReason:
+					res.Failed += 1
+				}
+
+			}
+
+			// Old duplicate payments.
+			dupBucket := payBucket.NestedReadBucket(duplicatePaymentsBucket)
+			if dupBucket == nil {
+				return nil
+			}
+
+			return dupBucket.ForEach(func(k, v []byte) error {
+				subBucket := dupBucket.NestedReadBucket(k)
+				if subBucket == nil {
+					return nil
+				}
+				res.OldDupePayments += 1
+				return nil
+			})
+		})
+	}, func() {})
+
+	return res, err
+}

cmd/dcrlncli/cmd_dcrlnd.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"github.com/decred/dcrlnd/lnrpc"
+	"github.com/urfave/cli"
+)
+
+var calcPayStatsCommand = cli.Command{
+	Name:      "calcpaystats",
+	Usage:     "Scans the db and generates a report on total payment counts.",
+	ArgsUsage: "",
+	Category:  "Payments",
+	Description: `
+	Goes through the DB and generates a report on total number of payments
+	made, settled and failed.
+
+	NOTE: This requires a scan through the entire set of payments in the DB,
+	so it may be slow on nodes that have a large number of payments.
+	`,
+	Action: actionDecorator(calcPayStats),
+}
+
+func calcPayStats(ctx *cli.Context) error {
+	ctxc := getContext()
+
+	client, cleanUp := getClient(ctx)
+	defer cleanUp()
+
+	req := &lnrpc.CalcPaymentStatsRequest{}
+	resp, err := client.CalcPaymentStats(ctxc, req)
+	if err != nil {
+		return err
+	}
+
+	printRespJSON(resp)
+
+	return nil
+}

cmd/dcrlncli/main.go

@@ -394,6 +394,7 @@ func main() {
 		queryRoutesCommand,
 		getNetworkInfoCommand,
 		debugLevelCommand,
+		calcPayStatsCommand,
 		decodePayReqCommand,
 		listChainTxnsCommand,
 		stopCommand,