From 56888d83dd50a10bad56eaa512ce0b6a2f2d41ed Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Sun, 6 Dec 2015 15:05:52 +0100
Subject: fallocate the space needed for the file header beforehand

This makes sure writing to a file fails early if the underlying
filesystem does not support fallocate. It also prevents partial header
write due to ENOSPC.
---
 cryptfs/names_diriv.go  |  2 ++
 pathfs_frontend/file.go | 13 ++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/cryptfs/names_diriv.go b/cryptfs/names_diriv.go
index 3e24a73..035eac1 100644
--- a/cryptfs/names_diriv.go
+++ b/cryptfs/names_diriv.go
@@ -22,6 +22,7 @@ type DirIVCache struct {
 	lock sync.RWMutex
 }
 
+// lookup - fetch entry for "dir" from the cache
 func (c *DirIVCache) lookup(dir string) (bool, []byte, string) {
 	c.lock.RLock()
 	defer c.lock.RUnlock()
@@ -31,6 +32,7 @@ func (c *DirIVCache) lookup(dir string) (bool, []byte, string) {
 	return false, nil, ""
 }
 
+// store - write entry for "dir" into the caches
 func (c *DirIVCache) store(dir string, iv []byte, translatedDir string) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
diff --git a/pathfs_frontend/file.go b/pathfs_frontend/file.go
index 5c76fb1..16d83c1 100644
--- a/pathfs_frontend/file.go
+++ b/pathfs_frontend/file.go
@@ -93,7 +93,18 @@ func (f *file) readHeader() error {
 func (f *file) createHeader() error {
 	h := cryptfs.RandomHeader()
 	buf := h.Pack()
-	_, err := f.fd.WriteAt(buf, 0)
+
+	// Prevent partially written (=corrupt) header by preallocating the space beforehand
+	f.fdLock.Lock()
+	defer f.fdLock.Unlock()
+	err := syscall.Fallocate(int(f.fd.Fd()), FALLOC_FL_KEEP_SIZE, 0, cryptfs.HEADER_LEN)
+	if err != nil {
+			cryptfs.Warn.Printf("createHeader: Fallocate failed: %s\n", err.Error())
+			return err
+	}
+
+	// Actually write header
+	_, err = f.fd.WriteAt(buf, 0)
 	if err != nil {
 		return err
 	}
-- 
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