cmd/stdiscosrv: Sort addresses before replication (fixes #6093) (#6094)

This makes sure addresses are sorted when coming in from the API. The
database merge operation still checks for correct ordering (which is
quick) and sorts if it isn't correct (legacy database record or
replication peer), but then does a copy first.

Tested with -race in production...

cmd/stdiscosrv/database.go

@@ -10,6 +10,7 @@
 package main
 
 import (
+	"log"
 	"sort"
 	"time"
 
@@ -263,12 +264,15 @@ func (s *levelDBStore) Stop() {
 // chosen for any duplicates.
 func merge(a, b DatabaseRecord) DatabaseRecord {
 	// Both lists must be sorted for this to work.
-	sort.Slice(a.Addresses, func(i, j int) bool {
-		return a.Addresses[i].Address < a.Addresses[j].Address
-	})
-	sort.Slice(b.Addresses, func(i, j int) bool {
-		return b.Addresses[i].Address < b.Addresses[j].Address
-	})
+	if !sort.IsSorted(databaseAddressOrder(a.Addresses)) {
+		log.Println("Warning: bug: addresses not correctly sorted in merge")
+		a.Addresses = sortedAddressCopy(a.Addresses)
+	}
+	if !sort.IsSorted(databaseAddressOrder(b.Addresses)) {
+		// no warning because this is the side we read from disk and it may
+		// legitimately predate correct sorting.
+		b.Addresses = sortedAddressCopy(b.Addresses)
+	}
 
 	res := DatabaseRecord{
 		Addresses: make([]DatabaseAddress, 0, len(a.Addresses)+len(b.Addresses)),
@@ -352,3 +356,24 @@ func expire(addrs []DatabaseAddress, now int64) []DatabaseAddress {
 	}
 	return addrs
 }
+
+func sortedAddressCopy(addrs []DatabaseAddress) []DatabaseAddress {
+	sorted := make([]DatabaseAddress, len(addrs))
+	copy(sorted, addrs)
+	sort.Sort(databaseAddressOrder(sorted))
+	return sorted
+}
+
+type databaseAddressOrder []DatabaseAddress
+
+func (s databaseAddressOrder) Less(a, b int) bool {
+	return s[a].Address < s[b].Address
+}
+
+func (s databaseAddressOrder) Swap(a, b int) {
+	s[a], s[b] = s[b], s[a]
+}
+
+func (s databaseAddressOrder) Len() int {
+	return len(s)
+}