lib/ignore, lib/model: Use an interface to detect file changes, improving tests

This solves the erratic test failures on model.TestIgnores by ensuring
that the ignore patterns are reloaded even in the face of unchanged
timestamps.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4208

lib/ignore/cache.go

@@ -8,6 +8,12 @@ package ignore
 
 import "time"
 
+type nower interface {
+	Now() time.Time
+}
+
+var clock = nower(defaultClock{})
+
 type cache struct {
 	patterns []Pattern
 	entries  map[string]cacheEntry
@@ -27,7 +33,7 @@ func newCache(patterns []Pattern) *cache {
 
 func (c *cache) clean(d time.Duration) {
 	for k, v := range c.entries {
-		if time.Since(v.access) > d {
+		if clock.Now().Sub(v.access) > d {
 			delete(c.entries, k)
 		}
 	}
@@ -36,7 +42,7 @@ func (c *cache) clean(d time.Duration) {
 func (c *cache) get(key string) (Result, bool) {
 	entry, ok := c.entries[key]
 	if ok {
-		entry.access = time.Now()
+		entry.access = clock.Now()
 		c.entries[key] = entry
 	}
 	return entry.result, ok
@@ -50,3 +56,9 @@ func (c *cache) len() int {
 	l := len(c.entries)
 	return l
 }
+
+type defaultClock struct{}
+
+func (defaultClock) Now() time.Time {
+	return time.Now()
+}