Fix descending cache cursor

tsdb/engine/tsm1/engine.go

@@ -434,26 +434,36 @@ type devCursor struct {
 // SeekTo positions the cursor at the timestamp specified by seek and returns the
 // timestamp and value.
 func (c *devCursor) SeekTo(seek int64) (int64, interface{}) {
+	// Seek to position in cache.
+	c.cacheKeyBuf, c.cacheValueBuf = func() (int64, interface{}) {
 		// Seek to position in cache index.
 		c.cachePos = sort.Search(len(c.cache), func(i int) bool {
-		if c.ascending {
 			return c.cache[i].Time().UnixNano() >= seek
-		}
-		return c.cache[i].Time().UnixNano() <= seek
 		})
 
-	if len(c.cache) == 0 {
-		c.cacheKeyBuf = tsdb.EOF
-	}
-
 		if c.cachePos < len(c.cache) {
-		c.cacheKeyBuf = c.cache[c.cachePos].Time().UnixNano()
+			v := c.cache[c.cachePos]
-		c.cacheValueBuf = c.cache[c.cachePos].Value()
+			if v.UnixNano() == seek || c.ascending {
-	} else {
+				// Exact seek found or, if ascending, next one is good.
-		c.cacheKeyBuf = tsdb.EOF
+				return v.UnixNano(), v.Value()
+			}
+			// Nothing available if descending.
+			return tsdb.EOF, nil
 		}
 
-	// TODO: Get the first block from tsm files for the given 'seek'
+		// Ascending cursor, no match in the cache.
+		if c.ascending {
+			return tsdb.EOF, nil
+		}
+
+		// Descending cursor, go to previous value in cache, and return if it exists.
+		c.cachePos--
+		if c.cachePos < 0 {
+			return tsdb.EOF, nil
+		}
+		return c.cache[c.cachePos].UnixNano(), c.cache[c.cachePos].Value()
+	}()
+
 	// Seek to position to tsm block.
 	if c.ascending {
 		c.tsmValues, _ = c.tsm.Scan(SeriesFieldKey(c.series, c.fields[0]), time.Unix(0, seek-1), c.ascending)
@@ -525,11 +535,19 @@ func (c *devCursor) read() (int64, interface{}) {
 
 // nextCache returns the next value from the cache.
 func (c *devCursor) nextCache() (int64, interface{}) {
+	if c.ascending {
 		c.cachePos++
 		if c.cachePos >= len(c.cache) {
 			return tsdb.EOF, nil
 		}
 		return c.cache[c.cachePos].UnixNano(), c.cache[c.cachePos].Value()
+	} else {
+		c.cachePos--
+		if c.cachePos < 0 {
+			return tsdb.EOF, nil
+		}
+		return c.cache[c.cachePos].UnixNano(), c.cache[c.cachePos].Value()
+	}
 }
 
 // nextTSM returns the next value from the TSM files.

tsdb/engine/tsm1/engine_test.go

@@ -131,7 +131,6 @@ func TestDevEngine_QueryTSM_Ascending(t *testing.T) {
 
 // Ensure an engine containing cached values responds correctly to queries.
 func TestDevEngine_QueryCache_Descending(t *testing.T) {
-	t.Skip("fixme")
 	// Generate temporary file.
 	f, _ := ioutil.TempFile("", "tsm1dev")
 	f.Close()