lib/ignore: Centralize handling of temporary filenames (fixes #3899)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3901
LGTM: calmh, AudriusButkevicius

lib/ignore/tempname_test.go

@@ -0,0 +1,26 @@
+// Copyright (C) 2015 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 http://mozilla.org/MPL/2.0/.
+
+package ignore
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLongTempFilename(t *testing.T) {
+	filename := ""
+	for i := 0; i < 300; i++ {
+		filename += "l"
+	}
+	tFile := TempName(filename)
+	if len(tFile) < 10 || len(tFile) > 200 {
+		t.Fatal("Invalid long filename")
+	}
+	if !strings.HasSuffix(TempName("short"), "short.tmp") {
+		t.Fatal("Invalid short filename", TempName("short"))
+	}
+}