all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

cmd/stdiscosrv/database.go

@@ -15,6 +15,7 @@ import (
 	"sort"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/sliceutil"
 	"github.com/syndtr/goleveldb/leveldb"
 	"github.com/syndtr/goleveldb/leveldb/storage"
 	"github.com/syndtr/goleveldb/leveldb/util"
@@ -352,14 +353,7 @@ func expire(addrs []DatabaseAddress, now int64) []DatabaseAddress {
 	i := 0
 	for i < len(addrs) {
 		if addrs[i].Expires < now {
-			// This item is expired. Replace it with the last in the list
-			// (noop if we are at the last item).
-			addrs[i] = addrs[len(addrs)-1]
-			// Wipe the last item of the list to release references to
-			// strings and stuff.
-			addrs[len(addrs)-1] = DatabaseAddress{}
-			// Shorten the slice.
-			addrs = addrs[:len(addrs)-1]
+			addrs = sliceutil.RemoveAndZero(addrs, i)
 			continue
 		}
 		i++

cmd/stdiscosrv/database_test.go

@@ -185,7 +185,7 @@ func TestFilter(t *testing.T) {
 		},
 		{
 			a: []DatabaseAddress{{Address: "a", Expires: 5}, {Address: "b", Expires: 15}, {Address: "c", Expires: 5}, {Address: "d", Expires: 15}, {Address: "e", Expires: 5}},
-			b: []DatabaseAddress{{Address: "d", Expires: 15}, {Address: "b", Expires: 15}}, // gets reordered
+			b: []DatabaseAddress{{Address: "b", Expires: 15}, {Address: "d", Expires: 15}},
 		},
 	}