migrate: Fix failure to migrate from URL keys. (Reversion introduced in version 6.20180926)

CHANGELOG

@@ -51,6 +51,8 @@ git-annex (7.20181025) UNRELEASED; urgency=medium
     was in the response, use it.
   * webapp: Fixed a crash when adding a git remote.
     (Reversion introduced in version 6.20180112)
+  * migrate: Fix failure to migrate from URL keys.
+    (Reversion introduced in version 6.20180926)
 
  -- Joey Hess <id@joeyh.name>  Sat, 13 Oct 2018 00:52:02 -0400
 

Command/Migrate.hs

@@ -82,10 +82,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			next $ Command.ReKey.cleanup file oldkey newkey
 		, giveup "failed creating link from old to new key"
 		)
-	genkey Nothing = return Nothing
+	genkey Nothing = do
-	genkey (Just fm) = fm oldkey newbackend afile >>= \case
-		Just newkey -> return $ Just (newkey, True)
-		Nothing -> do
 		content <- calcRepo $ gitAnnexLocation oldkey
 		let source = KeySource
 			{ keyFilename = file
@@ -96,4 +93,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 		return $ case v of
 			Just (newkey, _) -> Just (newkey, False)
 			_ -> Nothing
+	genkey (Just fm) = fm oldkey newbackend afile >>= \case
+		Just newkey -> return (Just (newkey, True))
+		Nothing -> genkey Nothing
 	afile = AssociatedFile (Just file)

doc/bugs/git-annex-migrate_fails.mdwn

@@ -24,3 +24,5 @@ Migrating from a URL key to an MD5E key (after addurl --fast using an external s
     [2018-10-29 15:17:15.621264] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","ls-files","--cached","-z","--","A4.taxfilt.bam"]
     migrate A4.taxfilt.bam failed
     git-annex: migrate: 1 failed
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/git-annex-migrate_fails/comment_1_4f8fb57ce001e0a2e1636bd7b3b79d43._comment

@@ -0,0 +1,27 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2018-10-29T20:04:48Z"
+ content="""
+To reproduce this bug:
+
+	joey@darkstar:/tmp/t>git annex addurl --fast --raw http://www.google.com/ --file A4.taxfilt.bam
+	addurl http://www.google.com/ (to A4.taxfilt.bam) ok
+	(recording state in git...)
+	joey@darkstar:/tmp/t>git annex get A4.taxfilt.bam
+	get A4.taxfilt.bam (from web...) 
+	
+	ok                             
+	(recording state in git...)
+	joey@darkstar:/tmp/t>git annex migrate --backend=MD5E A4.taxfilt.bam
+	migrate A4.taxfilt.bam failed
+	git-annex: migrate: 1 failed
+
+Seems to be a bug in handling backends with fastMigrate = Nothing, causing
+it to stop immediately without doing anything. Reversion introduced in
+[[!commit 4ecba916a14e02dd62f8ba4257db810fa859f017]].
+
+And calling `stop` in a CommandPerform causes this failure without an
+explanation. I think that would be worth cleaning up at some point;
+[[todo/do_not_allow_using_stop_in_CommandPerform]].
+"""]]

doc/todo/do_not_allow_using_stop_in_CommandPerform.mdwn

@@ -0,0 +1,19 @@
+`stop` does different things depending on the stage of a command that it's
+used in.
+
+In CommandStart, it quietly stops any further action.
+
+In CommandPerform, it causes the command to fail,
+which looks like "commandname actionitem failed".
+If no other output is emitted to say why it failed,
+this is not nice.
+
+(It can't be used in CommandCleanup.)
+
+It would be good to get rid of support for running `stop` in
+CommandPerform. Simply change the type:
+
+	- stop :: Annex (Maybe a)
+	+ stop :: CommandStart
+
+And follow the compile errors.