Merge pull request #2413 from prometheus/beorn7/storage

storage: Fix offset returned by dropAndPersistChunks

storage/local/persistence.go

@@ -845,12 +845,14 @@ func (p *persistence) loadSeriesMapAndHeads() (sm *seriesMap, chunksToPersist in
 // dropAndPersistChunks deletes all chunks from a series file whose last sample
 // time is before beforeTime, and then appends the provided chunks, leaving out
 // those whose last sample time is before beforeTime. It returns the timestamp
-// of the first sample in the oldest chunk _not_ dropped, the offset within the
-// series file of the first chunk persisted (out of the provided chunks), the
-// number of deleted chunks, and true if all chunks of the series have been
-// deleted (in which case the returned timestamp will be 0 and must be ignored).
-// It is the caller's responsibility to make sure nothing is persisted or loaded
-// for the same fingerprint concurrently.
+// of the first sample in the oldest chunk _not_ dropped, the chunk offset
+// within the series file of the first chunk persisted (out of the provided
+// chunks, or - if no chunks were provided - the chunk offset where chunks would
+// have been persisted, i.e. the end of the file), the number of deleted chunks,
+// and true if all chunks of the series have been deleted (in which case the
+// returned timestamp will be 0 and must be ignored).  It is the caller's
+// responsibility to make sure nothing is persisted or loaded for the same
+// fingerprint concurrently.
 //
 // Returning an error signals problems with the series file. In this case, the
 // caller should quarantine the series.
@@ -966,13 +968,16 @@ func (p *persistence) dropAndPersistChunks(
 	if err != nil {
 		return
 	}
-	totalChunks := int(fi.Size())/chunkLenWithHeader + len(chunks)
+	chunksInFile := int(fi.Size()) / chunkLenWithHeader
+	totalChunks := chunksInFile + len(chunks)
 	if numDropped == 0 || float64(numDropped)/float64(totalChunks) < p.minShrinkRatio {
 		// Nothing to drop. Just adjust the return values and append the chunks (if any).
 		numDropped = 0
 		firstTimeNotDropped = firstTimeInFile
 		if len(chunks) > 0 {
 			offset, err = p.persistChunks(fp, chunks)
+		} else {
+			offset = chunksInFile
 		}
 		return
 	}

storage/local/persistence_test.go

@@ -173,14 +173,16 @@ func testPersistLoadDropChunks(t *testing.T, encoding chunk.Encoding) {
 
 		}
 	}
-	// Try to drop one chunk, which must be prevented by the shrink ratio.
+	// Try to drop one chunk, which must be prevented by the shrink
+	// ratio. Since we do not pass in any chunks to persist, the offset
+	// should be the number of chunks in the file.
 	for fp, _ := range fpToChunks {
 		firstTime, offset, numDropped, allDropped, err := p.dropAndPersistChunks(fp, 1, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
-		if offset != 0 {
-			t.Errorf("want offset 0, got %d", offset)
+		if offset != 10 {
+			t.Errorf("want offset 10, got %d", offset)
 		}
 		if firstTime != 0 {
 			t.Errorf("want first time 0, got %d", firstTime)
@@ -422,14 +424,14 @@ func testPersistLoadDropChunks(t *testing.T, encoding chunk.Encoding) {
 			t.Error("all chunks dropped")
 		}
 	}
-	// Drop only the first two chunks should not happen, either.
+	// Drop only the first two chunks should not happen, either. Chunks in file is now 9.
 	for fp := range fpToChunks {
 		firstTime, offset, numDropped, allDropped, err := p.dropAndPersistChunks(fp, 2, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
-		if offset != 0 {
-			t.Errorf("want offset 0, got %d", offset)
+		if offset != 9 {
+			t.Errorf("want offset 9, got %d", offset)
 		}
 		if firstTime != 0 {
 			t.Errorf("want first time 0, got %d", firstTime)