lock to avoid more than one export to a remote at a time

This commit was sponsored by Jack Hill on Patreon.

Annex/Locations.hs

@@ -37,6 +37,7 @@ module Annex.Locations (
 	gitAnnexFsckDbLock,
 	gitAnnexFsckResultsLog,
 	gitAnnexExportDbDir,
+	gitAnnexExportLock,
 	gitAnnexScheduleState,
 	gitAnnexTransferDir,
 	gitAnnexCredsDir,
@@ -300,6 +301,10 @@ gitAnnexExportDir u r = gitAnnexDir r </> "export" </> fromUUID u
 gitAnnexExportDbDir :: UUID -> Git.Repo -> FilePath
 gitAnnexExportDbDir u r = gitAnnexExportDir u r </> "db"
 
+{- Lock file for export state for a special remote. -}
+gitAnnexExportLock :: UUID -> Git.Repo -> FilePath
+gitAnnexExportLock u r = gitAnnexExportDir u r ++ ".lck"
+
 {- .git/annex/schedulestate is used to store information about when
  - scheduled jobs were last run. -}
 gitAnnexScheduleState :: Git.Repo -> FilePath

Command/Export.hs

@@ -23,6 +23,7 @@ import Types.Remote
 import Types.Export
 import Annex.Content
 import Annex.CatFile
+import Annex.LockFile
 import Logs.Location
 import Logs.Export
 import Database.Export
@@ -85,7 +86,10 @@ seek o = do
 	r <- getParsed (exportRemote o)
 	unlessM (isExportSupported r) $
 		giveup "That remote does not support exports."
+	withExclusiveLock (gitAnnexExportLock (uuid r)) (seek' o r)
 
+seek' :: ExportOptions -> Remote -> CommandSeek
+seek' o r = do
 	new <- fromMaybe (giveup "unknown tree") <$>
 		-- Dereference the tree pointed to by the branch, commit,
 		-- or tag.

doc/todo/export.mdwn

@@ -26,11 +26,11 @@ Work is in progress. Todo list:
   export database is not populated. So, seems that the export database needs
   to get populated based on the export log in these cases.  
 
-  This needs a (local) record of what treeish the (local) export db
+  This needs a (local) record of what tree the (local) export db
   was last updated for, which is updated at the same time as the export log. 
   One way to record that would be as a git ref.
 
-  When the export log contains a different treeish than the local
+  When the export log contains a different tree than the local
   record, the export was updated in another repository, and so the
   export db needs to be updated.