storage: Fix chunkIndexToStartSeek calculation

With a high enough shrink ratio and enough chunks to persist, the
cutoff point could be _outside_ of the file, which wreaks havoc in the
storage.

storage/local/persistence.go

@@ -932,8 +932,9 @@ func (p *persistence) dropAndPersistChunks(
 	chunkIndexToStartSeek := 0
 	if p.minShrinkRatio < 1 {
 		chunkIndexToStartSeek = int(math.Floor(float64(totalChunks) * p.minShrinkRatio))
-	} else {
+	}
-		chunkIndexToStartSeek = totalChunks - 1
+	if chunkIndexToStartSeek >= chunksInFile {
+		chunkIndexToStartSeek = chunksInFile - 1
 	}
 	numDropped = chunkIndexToStartSeek
 

storage/local/persistence_test.go

@@ -462,6 +462,81 @@ func testPersistLoadDropChunks(t *testing.T, encoding chunk.Encoding) {
 			t.Error("all chunks dropped")
 		}
 	}
+	// Drop all the chunks again.
+	for fp := range fpToChunks {
+		firstTime, offset, numDropped, allDropped, err := p.dropAndPersistChunks(fp, 100, nil)
+		if firstTime != 0 {
+			t.Errorf("want first time 0, got %d", firstTime)
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		if offset != 0 {
+			t.Errorf("want offset 0, got %d", offset)
+		}
+		if numDropped != 7 {
+			t.Errorf("want 7 dropped chunks, got %v", numDropped)
+		}
+		if !allDropped {
+			t.Error("not all chunks dropped")
+		}
+	}
+	// Re-add first two of the chunks again.
+	for fp, chunks := range fpToChunks {
+		firstTimeNotDropped, offset, numDropped, allDropped, err :=
+			p.dropAndPersistChunks(fp, model.Earliest, chunks[:2])
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got, want := firstTimeNotDropped, model.Time(0); got != want {
+			t.Errorf("Want firstTimeNotDropped %v, got %v.", got, want)
+		}
+		if got, want := offset, 0; got != want {
+			t.Errorf("Want offset %v, got %v.", got, want)
+		}
+		if got, want := numDropped, 0; got != want {
+			t.Errorf("Want numDropped %v, got %v.", got, want)
+		}
+		if allDropped {
+			t.Error("All dropped.")
+		}
+	}
+	// Try to drop the first of the chunks while adding eight more. The drop
+	// should not happen because of the shrink ratio. Also, this time the
+	// minimum cut-off point is within the added chunks and not in the file
+	// anymore.
+	for fp, chunks := range fpToChunks {
+		firstTime, offset, numDropped, allDropped, err := p.dropAndPersistChunks(fp, 1, chunks[2:])
+		if err != nil {
+			t.Fatal(err)
+		}
+		if offset != 2 {
+			t.Errorf("want offset 2, got %d", offset)
+		}
+		if firstTime != 0 {
+			t.Errorf("want first time 0, got %d", firstTime)
+		}
+		if numDropped != 0 {
+			t.Errorf("want 0 dropped chunk, got %v", numDropped)
+		}
+		if allDropped {
+			t.Error("all chunks dropped")
+		}
+		wantChunks := chunks
+		indexes := make([]int, len(wantChunks))
+		for i := range indexes {
+			indexes[i] = i
+		}
+		gotChunks, err := p.loadChunks(fp, indexes, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for i, wantChunk := range wantChunks {
+			if !chunksEqual(wantChunk, gotChunks[i]) {
+				t.Errorf("%d. Chunks not equal.", i)
+			}
+		}
+	}
 }
 
 func TestPersistLoadDropChunksType0(t *testing.T) {