deal with FAT on Linux timestamp issue

Deal with FAT's low resolution timestamps, which in combination with
Linux's caching of higher res timestamps while a FAT is mounted, caused
direct mode repositories on FAT to seem to have modified files after they
were unmounted and remounted.

This commit was sponsored by Fabrice Rossi.

Utility/InodeCache.hs

@@ -28,10 +28,17 @@ compareStrong (InodeCache x) (InodeCache y) = x == y
 
 {- Weak comparison of the inode caches, comparing the size and mtime,
  - but not the actual inode.  Useful when inodes have changed, perhaps
- - due to some filesystems being remounted. -}
+ - due to some filesystems being remounted.
+ -
+ - The weak mtime comparison treats any mtimes that are within 2 seconds
+ - of one-anther as the same. This is because FAT has only a 2 second
+ - resolution. When a FAT filesystem is used on Linux, higher resolution
+ - timestamps are cached and used by Linux, but this is lost on unmount,
+ - so after a remount, the timestamp can appear to have changed.
+ -}
 compareWeak :: InodeCache -> InodeCache -> Bool
 compareWeak (InodeCache (InodeCachePrim _ size1 mtime1)) (InodeCache (InodeCachePrim _ size2 mtime2)) =
-	size1 == size2 && mtime1 == mtime2
+	size1 == size2 && (abs (mtime1 - mtime2) < 2)
 
 compareBy :: InodeComparisonType -> InodeCache -> InodeCache -> Bool
 compareBy Strongly = compareStrong

debian/changelog

@@ -7,6 +7,10 @@ git-annex (5.20140607) UNRELEASED; urgency=medium
   * Windows: Fix opening webapp when repository is in a directory with
     spaces in the path.
   * Fix build with wai 0.3.0.
+  * Deal with FAT's low resolution timestamps, which in combination with
+    Linux's caching of higher res timestamps while a FAT is mounted, caused
+    direct mode repositories on FAT to seem to have modified files after
+    they were unmounted and remounted.
 
  -- Joey Hess <joeyh@debian.org>  Mon, 09 Jun 2014 14:44:09 -0400
 

doc/bugs/FAT:_Date_resolution_for_mtime_2s--__62___implications.mdwn

@@ -135,3 +135,4 @@ Here's an konsole session to show this behaviour:
     +Modifiziert: 2014-05-03 19:44:38.000000000 +0200
 
 
+> fixed [[done]] --[[Joey]]