Add Replace function

Signed-off-by: Fabian Reinartz <freinartz@google.com>

checkpoint.go

@@ -37,7 +37,7 @@ type CheckpointStats struct {
 	DroppedTombstones int
 	TotalSeries       int // Processed series including dropped ones.
 	TotalSamples      int // Processed samples inlcuding dropped ones.
-	TotalTombstones   int // Processed tombstones including droppes ones.
+	TotalTombstones   int // Processed tombstones including dropped ones.
 }
 
 // LastCheckpoint returns the directory name of the most recent checkpoint.
@@ -260,8 +260,8 @@ func Checkpoint(logger log.Logger, w *wal.WAL, m, n int, keep func(id uint64) bo
 	if err := cp.Close(); err != nil {
 		return nil, errors.Wrap(err, "close checkpoint")
 	}
-	if err := fileutil.Rename(cpdirtmp, cpdir); err != nil {
-		return nil, errors.Wrap(err, "rename checkpoint file")
+	if err := fileutil.Replace(cpdirtmp, cpdir); err != nil {
+		return nil, errors.Wrap(err, "rename checkpoint directory")
 	}
 	if err := w.Truncate(n + 1); err != nil {
 		// If truncating fails, we'll just try again at the next checkpoint.

docs/format/wal.md

@@ -7,7 +7,7 @@ may be partial. A WAL record is an opaque byte slice that gets split up into sub
 should it exceed the remaining space of the current page. Records are never split across
 segment boundaries. If a single record exceeds the default segment size, a segment with
 a larger size will be created.
-The encoding of pages is largely borrowed from [LevelDB's/RocksDB's write ahead log.][1]
+The encoding of pages is largely borrowed from [LevelDB's/RocksDB's write ahead log.](https://github.com/facebook/rocksdb/wiki/Write-Ahead-Log-File-Format)
 
 Notable deviations are that the record fragment is encoded as:
 
@@ -84,5 +84,3 @@ and specify an interval for which samples of a series got deleted.
 │                        . . .                        │
 └─────────────────────────────────────────────────────┘
 ```
-
-[1][https://github.com/facebook/rocksdb/wiki/Write-Ahead-Log-File-Format]

fileutil/fileutil.go

@@ -43,3 +43,26 @@ func Rename(from, to string) error {
 	}
 	return pdir.Close()
 }
+
+// Replace moves a file or directory to a new location and deletes any previous data.
+// It is not atomic.
+func Replace(from, to string) error {
+	if err := os.RemoveAll(to); err != nil {
+		return nil
+	}
+	if err := os.Rename(from, to); err != nil {
+		return err
+	}
+
+	// Directory was renamed; sync parent dir to persist rename.
+	pdir, err := OpenDir(filepath.Dir(to))
+	if err != nil {
+		return err
+	}
+
+	if err = Fsync(pdir); err != nil {
+		pdir.Close()
+		return err
+	}
+	return pdir.Close()
+}

wal/wal.go

@@ -393,11 +393,11 @@ func (w *WAL) flushPage(clear bool) error {
 type recType uint8
 
 const (
-	recPageTerm recType = 0 // rest of page is empty
-	recFull     recType = 1 // full record
-	recFirst    recType = 2 // first fragment of a record
-	recMiddle   recType = 3 // middle fragments of a record
-	recLast     recType = 4 // final fragment of a record
+	recPageTerm recType = 0 // Rest of page is empty.
+	recFull     recType = 1 // Full record.
+	recFirst    recType = 2 // First fragment of a record.
+	recMiddle   recType = 3 // Middle fragments of a record.
+	recLast     recType = 4 // Final fragment of a record.
 )
 
 func (t recType) String() string {
@@ -442,8 +442,8 @@ func (w *WAL) log(rec []byte, final bool) error {
 	// If the record is too big to fit within pages in the current
 	// segment, terminate the active segment and advance to the next one.
 	// This ensures that records do not cross segment boundaries.
-	left := w.page.remaining() - recordHeaderSize                                   // active page
-	left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // free pages
+	left := w.page.remaining() - recordHeaderSize                                   // Active pages.
+	left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages.
 
 	if len(rec) > left {
 		if err := w.nextSegment(); err != nil {
@@ -716,7 +716,7 @@ func (r *Reader) next() (err error) {
 			// It's not strictly necessary but may catch sketchy state early.
 			k := pageSize - (r.total % pageSize)
 			if k == pageSize {
-				continue // initial 0 byte was last page byte
+				continue // Initial 0 byte was last page byte.
 			}
 			n, err := io.ReadFull(r.rdr, buf[:k])
 			if err != nil {