From 7a763ff61e048b04321b6f373b4d4af8dff65edf Mon Sep 17 00:00:00 2001
From: Ganesh Vernekar <cs15btech11018@iith.ac.in>
Date: Tue, 14 Jul 2020 14:59:28 +0530
Subject: [PATCH] Avoid empty mmap files by using .tmp files to write headers

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>
---
 tsdb/chunks/chunks.go     | 29 +++++++++++++++++++++++------
 tsdb/fileutil/fileutil.go | 16 +---------------
 2 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/tsdb/chunks/chunks.go b/tsdb/chunks/chunks.go
index acdcc4a17..ffd3e5302 100644
--- a/tsdb/chunks/chunks.go
+++ b/tsdb/chunks/chunks.go
@@ -220,19 +220,20 @@ func (w *Writer) cut() error {
 func cutSegmentFile(dirFile *os.File, magicNumber uint32, chunksFormat byte, allocSize int64) (headerSize int, newFile *os.File, seq int, err error) {
 	p, seq, err := nextSequenceFile(dirFile.Name())
 	if err != nil {
-		return 0, nil, 0, err
+		return 0, nil, 0, errors.Wrap(err, "next sequence file")
 	}
-	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE, 0666)
+	ptmp := p + ".tmp"
+	f, err := os.OpenFile(ptmp, os.O_WRONLY|os.O_CREATE, 0666)
 	if err != nil {
-		return 0, nil, 0, err
+		return 0, nil, 0, errors.Wrap(err, "open temp file")
 	}
 	if allocSize > 0 {
 		if err = fileutil.Preallocate(f, allocSize, true); err != nil {
-			return 0, nil, 0, err
+			return 0, nil, 0, errors.Wrap(err, "preallocate")
 		}
 	}
 	if err = dirFile.Sync(); err != nil {
-		return 0, nil, 0, err
+		return 0, nil, 0, errors.Wrap(err, "sync directory")
 	}
 
 	// Write header metadata for new file.
@@ -242,7 +243,23 @@ func cutSegmentFile(dirFile *os.File, magicNumber uint32, chunksFormat byte, all
 
 	n, err := f.Write(metab)
 	if err != nil {
-		return 0, nil, 0, err
+		return 0, nil, 0, errors.Wrap(err, "write header")
+	}
+	if err := f.Close(); err != nil {
+		return 0, nil, 0, errors.Wrap(err, "close temp file")
+	}
+
+	if err := fileutil.Rename(ptmp, p); err != nil {
+		return 0, nil, 0, errors.Wrap(err, "replace file")
+	}
+
+	f, err = os.OpenFile(p, os.O_WRONLY, 0666)
+	if err != nil {
+		return 0, nil, 0, errors.Wrap(err, "open final file")
+	}
+	// Skip the header for the further writes.
+	if _, err := f.Seek(int64(n), 0); err != nil {
+		return 0, nil, 0, errors.Wrap(err, "seek in final file")
 	}
 	return n, f, seq, nil
 }
diff --git a/tsdb/fileutil/fileutil.go b/tsdb/fileutil/fileutil.go
index be9c6f226..00bf63950 100644
--- a/tsdb/fileutil/fileutil.go
+++ b/tsdb/fileutil/fileutil.go
@@ -125,19 +125,5 @@ func Replace(from, to string) error {
 		}
 	}
 
-	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 = pdir.Sync(); err != nil {
-		pdir.Close()
-		return err
-	}
-	return pdir.Close()
+	return Rename(from, to)
 }