blockchain: Mock time.Now for utxo cache tests.

This updates the utxo cache flush tests to use a mock function for
getting the current time.  Without this the flush tests will fail if
the flush happens fast enough for time.Now to not have elapsed any time.

blockchain/utxocache.go

@@ -121,6 +121,11 @@ type UtxoCache struct {
 	// are used to measure the overall cache hit ratio.
 	hits   uint64
 	misses uint64
+
+	// timeNow defines the function to use to get the current local time.  It
+	// defaults to time.Now but an alternative function can be provided for
+	// testing purposes.
+	timeNow func() time.Time
 }
 
 // UtxoCacheConfig is a descriptor which specifies the utxo cache instance
@@ -151,6 +156,7 @@ func NewUtxoCache(config *UtxoCacheConfig) *UtxoCache {
 		maxSize:       config.MaxSize,
 		entries:       make(map[wire.OutPoint]*UtxoEntry, uint64(maxEntries)),
 		lastFlushTime: time.Now(),
+		timeNow:       time.Now,
 	}
 }
 
@@ -556,7 +562,7 @@ func (c *UtxoCache) flush(bestHash *chainhash.Hash, bestHeight uint32, logFlush
 	// Update the last flush on the cache instance now that the flush has been
 	// completed.
 	c.lastFlushHash = *bestHash
-	c.lastFlushTime = time.Now()
+	c.lastFlushTime = c.timeNow()
 
 	// Update the last eviction height on the cache instance if we evicted just
 	// now.

blockchain/utxocache_test.go

@@ -975,7 +975,14 @@ func TestMaybeFlush(t *testing.T) {
 		utxoCache.maxSize = test.maxSize
 		utxoCache.lastEvictionHeight = test.lastEvictionHeight
 		utxoCache.lastFlushHash = *test.lastFlushHash
-		origLastFlushTime := utxoCache.lastFlushTime
+
+		// Mock the current time as 1 minute after the last flush time.  This allows
+		// for validating that the last flush time is correctly updated to the
+		// current time when a flush occurs.
+		mockedCurrentTime := utxoCache.lastFlushTime.Add(time.Minute)
+		utxoCache.timeNow = func() time.Time {
+			return mockedCurrentTime
+		}
 
 		// Add entries specified by the test to the test database.
 		err := db.Update(func(dbTx database.Tx) error {
@@ -1036,7 +1043,7 @@ func TestMaybeFlush(t *testing.T) {
 			t.Fatalf("%q: unexpected last flush hash -- got %x, want %x", test.name,
 				utxoCache.lastFlushHash, *test.wantLastFlushHash)
 		}
-		updatedLastFlushTime := utxoCache.lastFlushTime != origLastFlushTime
+		updatedLastFlushTime := utxoCache.lastFlushTime == mockedCurrentTime
 		if updatedLastFlushTime != test.wantUpdatedLastFlushTime {
 			t.Fatalf("%q: unexpected updated last flush time -- got %v, want %v",
 				test.name, updatedLastFlushTime, test.wantUpdatedLastFlushTime)