lib/config, lib/connections: Add optional connection limits (fixes #7176) (#7223)

This adds two new configuration options:

    // The number of connections at which we stop trying to connect to more
    // devices, zero meaning no limit. Does not affect incoming connections.
    ConnectionLimitEnough int

    // The maximum number of connections which we will allow in total, zero
    // meaning no limit. Affects incoming connections and prevents
    // attempting outgoing connections.
    ConnectionLimitMax int

These can be used to limit the number of concurrent connections in
various ways.

lib/connections/dialqueue_test.go

@@ -0,0 +1,119 @@
+// Copyright (C) 2021 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package connections
+
+import (
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+func TestDialQueueSort(t *testing.T) {
+	t.Parallel()
+
+	t.Run("ByLastSeen", func(t *testing.T) {
+		t.Parallel()
+
+		// Devices seen within the last week or so should be sorted stricly in order.
+		now := time.Now()
+		queue := dialQueue{
+			{id: device1, lastSeen: now.Add(-5 * time.Hour)},  // 1
+			{id: device2, lastSeen: now.Add(-50 * time.Hour)}, // 3
+			{id: device3, lastSeen: now.Add(-25 * time.Hour)}, // 2
+			{id: device4, lastSeen: now.Add(-2 * time.Hour)},  // 0
+		}
+		expected := []protocol.ShortID{device4.Short(), device1.Short(), device3.Short(), device2.Short()}
+
+		queue.Sort()
+
+		if !reflect.DeepEqual(shortDevices(queue), expected) {
+			t.Error("expected different order")
+		}
+	})
+
+	t.Run("OldConnections", func(t *testing.T) {
+		t.Parallel()
+
+		// Devices seen long ago should be randomized.
+		now := time.Now()
+		queue := dialQueue{
+			{id: device1, lastSeen: now.Add(-5 * time.Hour)},       // 1
+			{id: device2, lastSeen: now.Add(-50 * 24 * time.Hour)}, // 2, 3
+			{id: device3, lastSeen: now.Add(-25 * 24 * time.Hour)}, // 2, 3
+			{id: device4, lastSeen: now.Add(-2 * time.Hour)},       // 0
+		}
+
+		expected1 := []protocol.ShortID{device4.Short(), device1.Short(), device3.Short(), device2.Short()}
+		expected2 := []protocol.ShortID{device4.Short(), device1.Short(), device2.Short(), device3.Short()}
+
+		var seen1, seen2 int
+
+		for i := 0; i < 100; i++ {
+			queue.Sort()
+			res := shortDevices(queue)
+			if reflect.DeepEqual(res, expected1) {
+				seen1++
+				continue
+			}
+			if reflect.DeepEqual(res, expected2) {
+				seen2++
+				continue
+			}
+			t.Fatal("expected different order")
+		}
+
+		if seen1 < 10 || seen2 < 10 {
+			t.Error("expected more even distribution", seen1, seen2)
+		}
+	})
+
+	t.Run("ShortLivedConnections", func(t *testing.T) {
+		t.Parallel()
+
+		// Short lived connections should be sorted as if they were long ago
+		now := time.Now()
+		queue := dialQueue{
+			{id: device1, lastSeen: now.Add(-5 * time.Hour)},                   // 1
+			{id: device2, lastSeen: now.Add(-3 * time.Hour)},                   // 0
+			{id: device3, lastSeen: now.Add(-25 * 24 * time.Hour)},             // 2, 3
+			{id: device4, lastSeen: now.Add(-2 * time.Hour), shortLived: true}, // 2, 3
+		}
+
+		expected1 := []protocol.ShortID{device2.Short(), device1.Short(), device3.Short(), device4.Short()}
+		expected2 := []protocol.ShortID{device2.Short(), device1.Short(), device4.Short(), device3.Short()}
+
+		var seen1, seen2 int
+
+		for i := 0; i < 100; i++ {
+			queue.Sort()
+			res := shortDevices(queue)
+			if reflect.DeepEqual(res, expected1) {
+				seen1++
+				continue
+			}
+			if reflect.DeepEqual(res, expected2) {
+				seen2++
+				continue
+			}
+			t.Fatal("expected different order")
+		}
+
+		if seen1 < 10 || seen2 < 10 {
+			t.Error("expected more even distribution", seen1, seen2)
+		}
+	})
+}
+
+func shortDevices(queue dialQueue) []protocol.ShortID {
+	res := make([]protocol.ShortID, len(queue))
+	for i, qe := range queue {
+		res[i] = qe.id.Short()
+	}
+	return res
+}