Squashed commit of the PR cortexproject#2022:

commit df8b95f
Author: Bryan Boreham <bryan@weave.works>
Date:   Wed Jan 22 13:52:44 2020 +0000

    Pass the correct flush reason to closeHead()

    Pass the reason from `shouldFlushChunk()` to `closeHead()`;
    sending `reasonImmediate` was a bug.

    We don't need to check `len(chunks)` is non-zero, since that would
    return `noFlush` from `shouldFlushSeries()`.

    Signed-off-by: Bryan Boreham <bryan@weave.works>

commit 17c0c21
Author: Bryan Boreham <bryan@weave.works>
Date:   Wed Jan 22 13:45:22 2020 +0000

    Make sure we return noFlush on an empty series

    Signed-off-by: Bryan Boreham <bryan@weave.works>

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

Author: Ganesh Vernekar

pkg/ingester/flush.go

@@ -193,6 +193,9 @@ func (i *Ingester) sweepSeries(userID string, fp model.Fingerprint, series *memo
 }
 
 func (i *Ingester) shouldFlushSeries(series *memorySeries, fp model.Fingerprint, immediate bool) flushReason {
+	if len(series.chunkDescs) == 0 {
+		return noFlush
+	}
 	if immediate {
 		return reasonImmediate
 	}
@@ -203,12 +206,9 @@ func (i *Ingester) shouldFlushSeries(series *memorySeries, fp model.Fingerprint,
 			return series.chunkDescs[0].flushReason
 		}
 		return reasonMultipleChunksInSeries
-	} else if len(series.chunkDescs) > 0 {
-		// Otherwise look in more detail at the first chunk
-		return i.shouldFlushChunk(series.chunkDescs[0], fp, series.isStale())
 	}
-
-	return noFlush
+	// Otherwise look in more detail at the first chunk
+	return i.shouldFlushChunk(series.chunkDescs[0], fp, series.isStale())
 }
 
 func (i *Ingester) shouldFlushChunk(c *desc, fp model.Fingerprint, lastValueIsStale bool) flushReason {
@@ -290,11 +290,14 @@ func (i *Ingester) flushUserSeries(flushQueueIndex int, userID string, fp model.
 		return nil
 	}
 
-	// Assume we're going to flush everything, and maybe don't flush the head chunk if it doesn't need it.
+	// shouldFlushSeries() has told us we have at least one chunk
 	chunks := series.chunkDescs
-	if immediate || (len(chunks) > 0 && i.shouldFlushChunk(series.head(), fp, series.isStale()) != noFlush) {
+	if immediate {
 		series.closeHead(reasonImmediate)
+	} else if chunkReason := i.shouldFlushChunk(series.head(), fp, series.isStale()); chunkReason != noFlush {
+		series.closeHead(chunkReason)
 	} else {
+		// The head chunk doesn't need flushing; step back by one.
 		chunks = chunks[:len(chunks)-1]
 	}