simplify if-else,test before the tombstone failure, more comments

Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com>

db.go

@@ -861,13 +861,10 @@ func (db *DB) CleanTombstones() (err error) {
 
 	deletable := []string{}
 	for _, b := range blocks {
-		uid, er := b.CleanTombstones(db.Dir(), db.compactor)
-		if er != nil {
+		if uid, er := b.CleanTombstones(db.Dir(), db.compactor); er != nil {
 			err = errors.Wrapf(er, "clean tombstones: %s", b.Dir())
 			return err
-		}
-
-		if uid != nil { // New block was created.
+		} else if uid != nil { // New block was created.
 			deletable = append(deletable, b.Dir())
 			newUIDs = append(newUIDs, *uid)
 		}

db_test.go

@@ -783,6 +783,9 @@ func TestTombstoneClean(t *testing.T) {
 	}
 }
 
+// TestTombstoneCleanFail tests that a failing TombstoneClean doesn't leave any blocks behind.
+// When TombstoneClean errors the original block that should be rebuilt doesn't get deleted so
+// if TombstoneClean leaves any blocks behind these will overlap.
 func TestTombstoneCleanFail(t *testing.T) {
 
 	db, close := openTestDB(t, nil)
@@ -790,8 +793,10 @@ func TestTombstoneCleanFail(t *testing.T) {
 
 	var expectedBlockDirs []string
 
-	// Create 2 empty blocks with some fake tombstones to have something to work with.
-	for i := 0; i < 2; i++ {
+	// Create some empty blocks pending for compaction.
+	// totalBlocks should be >=2 so we have enough blocks to trigger compaction failure.
+	totalBlocks := 2
+	for i := 0; i < totalBlocks; i++ {
 		entropy := rand.New(rand.NewSource(time.Now().UnixNano()))
 		uid := ulid.MustNew(ulid.Now(), entropy)
 		meta := &BlockMeta{
@@ -800,6 +805,8 @@ func TestTombstoneCleanFail(t *testing.T) {
 		}
 		blockDir := filepath.Join(db.Dir(), uid.String())
 		block := createEmptyBlock(t, blockDir, meta)
+
+		// Add some some fake tombstones to trigger the compaction.
 		tomb := memTombstones{}
 		tomb[0] = Intervals{{0, 1}}
 		block.tombstones = tomb
@@ -807,29 +814,39 @@ func TestTombstoneCleanFail(t *testing.T) {
 		db.blocks = append(db.blocks, block)
 		expectedBlockDirs = append(expectedBlockDirs, blockDir)
 	}
-	db.compactor = &mockCompactorFailing{t: t}
 
-	// This should fail as we are using the failing compactor!
+	// Initialize the mockCompactorFailing with a room for a single compaction iteration.
+	// mockCompactorFailing will fail on the second iteration so we can check if the cleanup works as expected.
+	db.compactor = &mockCompactorFailing{
+		t:      t,
+		blocks: db.blocks,
+		max:    totalBlocks + 1,
+	}
+
+	// The compactor should trigger a failure here.
 	testutil.NotOk(t, db.CleanTombstones())
 
+	// Now check that the CleanTombstones didn't leave any blocks behind after a failure.
 	actualBlockDirs, err := blockDirs(db.dir)
 	testutil.Ok(t, err)
 	testutil.Equals(t, expectedBlockDirs, actualBlockDirs)
 }
 
+// mockCompactorFailing creates a new empty block on every write and fails when reached the max allowed total.
 type mockCompactorFailing struct {
 	t      *testing.T
 	blocks []*Block
+	max    int
 }
 
 func (*mockCompactorFailing) Plan(dir string) ([]string, error) {
-
 	return nil, nil
 }
 func (c *mockCompactorFailing) Write(dest string, b BlockReader, mint, maxt int64) (ulid.ULID, error) {
-	if len(c.blocks) > 0 {
-		return ulid.ULID{}, fmt.Errorf("the compactor already did one block so forcefuly failing")
+	if len(c.blocks) >= c.max {
+		return ulid.ULID{}, fmt.Errorf("the compactor already did the maximum allowed blocks so it is time to fail")
 	}
+
 	entropy := rand.New(rand.NewSource(time.Now().UnixNano()))
 	uid := ulid.MustNew(ulid.Now(), entropy)
 	meta := &BlockMeta{
@@ -837,9 +854,20 @@ func (c *mockCompactorFailing) Write(dest string, b BlockReader, mint, maxt int6
 		ULID:    uid,
 	}
 
-	fmt.Println(dest)
 	block := createEmptyBlock(c.t, filepath.Join(dest, meta.ULID.String()), meta)
 	c.blocks = append(c.blocks, block)
+
+	// Now check that all expected blocks are actually persisted on disk.
+	// This way we make sure that the we have some blocks that are supposed to be removed.
+	var expectedBlocks []string
+	for _, b := range c.blocks {
+		expectedBlocks = append(expectedBlocks, filepath.Join(dest, b.Meta().ULID.String()))
+	}
+	actualBlockDirs, err := blockDirs(dest)
+	testutil.Ok(c.t, err)
+
+	testutil.Equals(c.t, expectedBlocks, actualBlockDirs)
+
 	return block.Meta().ULID, nil
 }