lib/ignore: Fast reload of unchanged ignores (fixes #3394)

This changes the "seen" map that we're anyway keeping around to track
the modtimes of loaded files instead. When doing a Load() we check that
1) the file we are loading is in the modtime set, and 2) that none of
the files in the modtime set have changed modtimes. If that's the case
we do a quick return without parsing anything or clearing the cache.

This required adding two one seconds sleeps in the tests to make sure
the modtimes were updated when we expect cache reloads, because I'm on a
crappy filesystem with one second timestamp granularity. That also
proves it works...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3754

lib/ignore/ignore_test.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"testing"
+	"time"
 )
 
 func TestIgnore(t *testing.T) {
@@ -276,9 +277,12 @@ func TestCaching(t *testing.T) {
 		t.Fatal("Expected 4 cached results")
 	}
 
-	// Modify the include file, expect empty cache
+	// Modify the include file, expect empty cache. Ensure the timestamp on
+	// the file changes.
 
+	time.Sleep(time.Second)
 	fd2.WriteString("/z/\n")
+	fd2.Sync()
 
 	err = pats.Load(fd1.Name())
 	if err != nil {
@@ -307,7 +311,9 @@ func TestCaching(t *testing.T) {
 
 	// Modify the root file, expect cache to be invalidated
 
+	time.Sleep(time.Second)
 	fd1.WriteString("/a/\n")
+	fd1.Sync()
 
 	err = pats.Load(fd1.Name())
 	if err != nil {
@@ -472,6 +478,8 @@ func TestCacheReload(t *testing.T) {
 
 	// Rewrite file to match f1 and f3
 
+	time.Sleep(time.Second)
+
 	err = fd.Truncate(0)
 	if err != nil {
 		t.Fatal(err)