Significantly sped up processing of large numbers of directories passed to a single git-annex command. (try 2)

New approach is to do it the expensive way for the first 100 paths
on the command line, but then assume the user doesn't care about order too
much and fall back to the cheap way that does not preserve order.

Utility/Path.hs

@@ -170,17 +170,26 @@ prop_relPathDirToFile_regressionTest = same_dir_shortcurcuits_at_difference
 				== joinPath ["..", "..", "..", "..", ".git", "annex", "objects", "18", "gk", "SHA256-foo", "SHA256-foo"]
 
 {- Given an original list of paths, and an expanded list derived from it,
- - generates a list of lists, where each sublist corresponds to one of the
+ - which may be arbitrarily reordered, generates a list of lists, where
- - original paths. When the original path is a directory, any items
+ - each sublist corresponds to one of the original paths.
- - in the expanded list that are contained in that directory will appear in
+ -
- - its segment.
+ - When the original path is a directory, any items in the expanded list
+ - that are contained in that directory will appear in its segment.
+ -
+ - The order of the original list of paths is attempted to be preserved in
+ - the order of the returned segments. However, doing so has a O^NM
+ - growth factor. So, if the original list has more than 100 paths on it,
+ - we stop preserving ordering at that point. Presumably a user passing
+ - that many paths in doesn't care too much about order of the later ones.
  -}
 segmentPaths :: [FilePath] -> [FilePath] -> [[FilePath]]
 segmentPaths [] new = [new]
 segmentPaths [_] new = [new] -- optimisation
 segmentPaths (l:ls) new = found : segmentPaths ls rest
   where
-	(found, rest)=partition (l `dirContains`) new
+	(found, rest) = if length ls < 100
+		then partition (l `dirContains`) new
+		else break (\p -> not (l `dirContains` p)) new
 
 {- This assumes that it's cheaper to call segmentPaths on the result,
  - than it would be to run the action separately with each path. In

debian/changelog

@@ -23,6 +23,8 @@ git-annex (5.20150328) UNRELEASED; urgency=medium
   * fsck: Added --distributed and --expire options,
     for distributed fsck.
   * Fix truncation of parameters that could occur when using xargs git-annex.
+  * Significantly sped up processing of large numbers of directories
+    passed to a single git-annex command.
 
  -- Joey Hess <id@joeyh.name>  Fri, 27 Mar 2015 16:04:43 -0400
 

doc/bugs/feeding_git_annex_with_xargs_can_fail.mdwn

@@ -11,3 +11,5 @@ Feeding git-annex a long list off directories, eg with xargs can have
   git-ls-files results. There is probably an exponential blowup in the time
   relative to the number of parameters. Some of the stuff being done to
   preserve original ordering etc is likely at fault.
+
+  > [[fixed|done]] --[[Joey]]