From a3834e558cf6ae04826b44e65a02ee22286f7952 Mon Sep 17 00:00:00 2001
From: Joey Hess <joeyh@joeyh.name>
Date: Thu, 11 Aug 2016 20:44:07 -0400
Subject: write via temp file

avoids short reads, and also if a backup program came along while the write
was happening, avoids short backups
---
 Storage/LocalFiles.hs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

(limited to 'Storage')

diff --git a/Storage/LocalFiles.hs b/Storage/LocalFiles.hs
index 083a74e..b81ecbe 100644
--- a/Storage/LocalFiles.hs
+++ b/Storage/LocalFiles.hs
@@ -34,11 +34,14 @@ store :: StorableObjectIdent -> Shard -> IO StoreResult
 store i s = onError (StoreFailure . show) $ do
 	dir <- shardDir
 	createDirectoryIfMissing True dir
-	fd <- openFd (dir </> shardFile i) WriteOnly (Just 0o666)
+	let dest = dir </> shardFile i
+	let tmp = dest ++ ".tmp"
+	fd <- openFd tmp WriteOnly (Just 0o666)
 		(defaultFileFlags { exclusive = True } )
 	h <- fdToHandle fd
 	B.hPut h (toByteString s)
 	hClose h
+	renameFile tmp dest
 	return StoreSuccess
 
 retrieve :: ShardNum -> StorableObjectIdent -> IO RetrieveResult
@@ -56,6 +59,9 @@ retrieve n i = onError (RetrieveFailure . show) $ do
 -- There is no way to set the ctime to the epoch, but setting the other
 -- times does at least set it to the current time, which makes all
 -- currently stored files look alike.
+--
+-- Note that the contents of shards is never changed, so it's ok to set the
+-- mtime to the epoch; backup programs won't be confused.
 obscure :: IO ObscureResult
 obscure = onError (ObscureFailure . show) $ do
 	dir <- shardDir
-- 
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