fix false positive in export conflict detection

Like the earlier fixed one in Command.Export, it occurred when the same
tree was exported by multiple clones. Previous fix was incomplete since
several other places looked at the list of exported trees to detect when
there was an export conflict. Added a single unified function to avoid
missing any places it needed to be fixed.

This commit was sponsored by mo on Patreon.

Logs/Export.hs

@@ -1,11 +1,21 @@
 {- git-annex export log
  -
- - Copyright 2017 Joey Hess <id@joeyh.name>
+ - Copyright 2017-2019 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-module Logs.Export where
+module Logs.Export (
+	Exported,
+	mkExported,
+	ExportParticipants,
+	ExportChange(..),
+	getExport,
+	exportedTreeishes,
+	incompleteExportedTreeishes,
+	recordExport,
+	recordExportBeginning,
+) where
 
 import qualified Data.Map as M
 
@@ -23,12 +33,29 @@ import qualified Data.Attoparsec.ByteString.Lazy as A
 import qualified Data.Attoparsec.ByteString.Char8 as A8
 import Data.ByteString.Builder
 
+-- This constuctor is not itself exported to other modules, to enforce
+-- consistent use of exportedTreeishes.
 data Exported = Exported
 	{ exportedTreeish :: Git.Ref
 	, incompleteExportedTreeish :: [Git.Ref]
 	}
 	deriving (Eq, Show)
 
+mkExported :: Git.Ref -> [Git.Ref] -> Exported
+mkExported = Exported
+
+-- | Get the list of exported treeishes.
+--
+-- If the list contains multiple items, there was an export conflict,
+-- and different trees were exported to the same special remote.
+exportedTreeishes :: [Exported] -> [Git.Ref]
+exportedTreeishes = nub . map exportedTreeish
+
+-- | Treeishes that started to be exported, but were not finished.
+incompleteExportedTreeishes :: [Exported] -> [Git.Ref]
+incompleteExportedTreeishes = concatMap incompleteExportedTreeish
+
+
 data ExportParticipants = ExportParticipants
 	{ exportFrom :: UUID
 	, exportTo :: UUID
@@ -41,9 +68,6 @@ data ExportChange = ExportChange
 	}
 
 -- | Get what's been exported to a special remote.
---
--- If the list contains multiple items, there was an export conflict,
--- and different trees were exported to the same special remote.
 getExport :: UUID -> Annex [Exported]
 getExport remoteuuid = nub . mapMaybe get . M.toList . simpleMap 
 	. parseExportLog