cat-file resource pool

Avoid running a large number of git cat-file child processes when run with
a large -J value.

This implementation takes care to avoid adding any overhead to git-annex
when run without -J. When run with -J, there is a small bit of added
overhead, to manipulate the resource pool. That optimisation added a
fair bit of complexity.

Types/CatFileHandles.hs

@@ -0,0 +1,30 @@
+{- git-cat file handles pools
+ -
+ - Copyright 2020 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.CatFileHandles (
+	CatFileHandles(..),
+	catFileHandlesNonConcurrent,
+	catFileHandlesPool,
+) where
+
+import Control.Concurrent.STM
+import qualified Data.Map as M
+
+import Utility.ResourcePool
+import Git.CatFile (CatFileHandle)
+
+data CatFileHandles
+	= CatFileHandlesNonConcurrent CatMap
+	| CatFileHandlesPool (TMVar CatMap)
+
+type CatMap = M.Map FilePath (ResourcePool CatFileHandle)
+
+catFileHandlesNonConcurrent :: CatFileHandles
+catFileHandlesNonConcurrent = CatFileHandlesNonConcurrent M.empty
+
+catFileHandlesPool :: IO CatFileHandles
+catFileHandlesPool = CatFileHandlesPool <$> newTMVarIO M.empty