migrate: New --remove-size option

While intended for converting URL keys added by addurl --fast to be
as if added by addurl --relaxed, it can also be used to remove size
from other types of keys. Although that is not likely to be useful
for checksummed keys, I suppose it could be used for WORM or other
non-checksum keys.

Specifying the --remove-size option does not prevent other migrations
from taking effect if there's a key upgrade to perform, or if the
backend has changed. So --backend=URL needs to be used to prevent
migrating an URL key to the default backend.

Note that it's not possible to use git-annex migrate to convert from a
non-URL key to an URL key, as URL keys cannot be generated, except by
addurl. So while this can get the same effect as --relaxed would have
when addurl --fast was used, when --fast was not used, it won't work, or
if --backend=URL is not used will remove the size but not prevent
checksum verification, which is not useful. Due to this complexity, I
decided not to mention it in the git-annex addurl man page.

Sponsored-by: Jochen Bartl on Patreon

6 files changed, 87 insertions, 13 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 6474b20204..0b9ff1bb6f 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -13,6 +13,7 @@ git-annex (8.20211029) UNRELEASED; urgency=medium
   * uninit: Avoid error message when there is no git-annex branch.
   * git-lfs: Fix interoperability with gitlab's implementation of the
     git-lfs protocol, which requests Content-Encoding chunked.
+  * migrate: New --remove-size option.
 
  -- Joey Hess <id@joeyh.name>  Mon, 01 Nov 2021 13:19:46 -0400
 
diff --git a/Command/Migrate.hs b/Command/Migrate.hs
index 9a0d69f35a..1844f9a63b 100644
--- a/Command/Migrate.hs
+++ b/Command/Migrate.hs
@@ -23,20 +23,33 @@ cmd :: Command
 cmd = withGlobalOptions [annexedMatchingOptions] $
 	command "migrate" SectionUtility 
 		"switch data to different backend"
-		paramPaths (withParams seek)
+		paramPaths (seek <$$> optParser)
 
-seek :: CmdParams -> CommandSeek
-seek = withFilesInGitAnnex ww seeker <=< workTreeItems ww
+data MigrateOptions = MigrateOptions
+	{ migrateThese :: CmdParams
+	, removeSize :: Bool
+	}
+
+optParser :: CmdParamsDesc -> Parser MigrateOptions
+optParser desc = MigrateOptions
+	<$> cmdParams desc
+	<*> switch
+		( long "remove-size"
+		<> help "remove size field from keys"
+		)
+
+seek :: MigrateOptions -> CommandSeek
+seek o = withFilesInGitAnnex ww seeker =<< workTreeItems ww (migrateThese o)
   where
 	ww = WarnUnmatchLsFiles
 	seeker = AnnexedFileSeeker
-		{ startAction = start
+		{ startAction = start o
 		, checkContentPresent = Nothing
 		, usesLocationLog = False
 		}
 
-start :: SeekInput -> RawFilePath -> Key -> CommandStart
-start si file key = do
+start :: MigrateOptions -> SeekInput -> RawFilePath -> Key -> CommandStart
+start o si file key = do
 	forced <- Annex.getState Annex.force
 	v <- Backend.getBackend (fromRawFilePath file) key
 	case v of
@@ -46,9 +59,14 @@ start si file key = do
 			newbackend <- maybe defaultBackend return 
 				=<< chooseBackend file
 			if (newbackend /= oldbackend || upgradableKey oldbackend key || forced) && exists
-				then starting "migrate" (mkActionItem (key, file)) si $
-					perform file key oldbackend newbackend
-				else stop
+				then go False oldbackend newbackend
+				else if removeSize o && exists
+					then go True oldbackend oldbackend
+					else stop
+  where
+	go onlyremovesize oldbackend newbackend =
+		starting "migrate" (mkActionItem (key, file)) si $
+			perform onlyremovesize o file key oldbackend newbackend
 
 {- Checks if a key is upgradable to a newer representation.
  - 
@@ -70,13 +88,14 @@ upgradableKey backend key = isNothing (fromKey keySize key) || backendupgradable
  - data cannot get corrupted after the fsck but before the new key is
  - generated.
  -}
-perform :: RawFilePath -> Key -> Backend -> Backend -> CommandPerform
-perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbackend)
+perform :: Bool -> MigrateOptions -> RawFilePath -> Key -> Backend -> Backend -> CommandPerform
+perform onlyremovesize o file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbackend)
   where
 	go Nothing = stop
 	go (Just (newkey, knowngoodcontent))
-		| knowngoodcontent = finish newkey
-		| otherwise = stopUnless checkcontent $ finish newkey
+		| knowngoodcontent = finish (removesize newkey)
+		| otherwise = stopUnless checkcontent $
+			finish (removesize newkey)
 	checkcontent = Command.Fsck.checkBackend oldbackend oldkey Command.Fsck.KeyPresent afile
 	finish newkey = ifM (Command.ReKey.linkKey file oldkey newkey)
 		( do
@@ -89,6 +108,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 			next $ Command.ReKey.cleanup file newkey
 		, giveup "failed creating link from old to new key"
 		)
+	genkey _ | onlyremovesize = return $ Just (oldkey, False)
 	genkey Nothing = do
 		content <- calcRepo $ gitAnnexLocation oldkey
 		let source = KeySource
@@ -101,4 +121,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 	genkey (Just fm) = fm oldkey newbackend afile >>= \case
 		Just newkey -> return (Just (newkey, True))
 		Nothing -> genkey Nothing
+	removesize k
+		| removeSize o = alterKey k $ \kd -> kd { keySize = Nothing } 
+		| otherwise = k
 	afile = AssociatedFile (Just file)
diff --git a/doc/forum/Dropping_checksum_from_URL_key/comment_1_ce952a9db35d398a9e67adcd7f927d59._comment b/doc/forum/Dropping_checksum_from_URL_key/comment_1_ce952a9db35d398a9e67adcd7f927d59._comment
new file mode 100644
index 0000000000..4e09933a18
--- /dev/null
+++ b/doc/forum/Dropping_checksum_from_URL_key/comment_1_ce952a9db35d398a9e67adcd7f927d59._comment
@@ -0,0 +1,17 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2021-11-12T16:10:01Z"
+ content="""
+Migrating to URL will not do anything since they already are url keys.
+
+This could be scripted using `git-annex examinekey` to 
+convert such a key into one without a size, and then using
+`git-annex rekey`, which lets the new key for a file be specified.
+
+However, that command is a low level plumbing command, and does not copy
+over the url list from the old to the new key as migrate does (nor other
+metadata). So you would also have to use `git-annex addurl file url`
+afterwards to add the url, and use `git-annex metadata` if you have
+metadata. Very unergonomic.
+"""]]
diff --git a/doc/forum/Dropping_checksum_from_URL_key/comment_2_bb458296d464c2025f96502338f34000._comment b/doc/forum/Dropping_checksum_from_URL_key/comment_2_bb458296d464c2025f96502338f34000._comment
new file mode 100644
index 0000000000..65576564f5
--- /dev/null
+++ b/doc/forum/Dropping_checksum_from_URL_key/comment_2_bb458296d464c2025f96502338f34000._comment
@@ -0,0 +1,16 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2021-11-12T16:57:15Z"
+ content="""
+Implemented: `git-annex migrate --remove-size --backend=URL`
+
+Be sure to only run it on files using url keys, since it will also 
+remove sizes from other keys. (Or use `--inbackend=URL` with it.)
+
+Do note that `git-annex migrate` can only migrate files whose content
+is present. If you have never downloaded those urls, and `git-annex get`
+cannot download them now, because their size has changed, you
+won't be able to migrate data you don't have. In this case, re-running
+`git-annex addurl` with `--relaxed` seems like the only option.
+"""]]
diff --git a/doc/git-annex-migrate.mdwn b/doc/git-annex-migrate.mdwn
index 79b3434807..b05991930d 100644
--- a/doc/git-annex-migrate.mdwn
+++ b/doc/git-annex-migrate.mdwn
@@ -39,6 +39,18 @@ it's best to run migrate in all of them.
 
 * Also the [[git-annex-common-options]](1) can be used.
 
+* `--remove-size`
+
+  Keys often include the size of their content, which is generally a useful
+  thing. In fact, this command defaults to adding missing size information
+  to keys. With this option, the size information is removed instead.
+
+  One use of this option is to convert URL keys that were added
+  by `git-annex addurl --fast` to ones that would have been added if
+  that command was run with the `--relaxed` option. Eg:
+
+  	git-annex migrate --remove-size --backend=URL somefile
+
 # SEE ALSO
 
 [[git-annex]](1)
diff --git a/doc/git-annex-rekey.mdwn b/doc/git-annex-rekey.mdwn
index e3171f7000..85458bdc55 100644
--- a/doc/git-annex-rekey.mdwn
+++ b/doc/git-annex-rekey.mdwn
@@ -13,6 +13,9 @@ both the file, and the new key to use for it.
 
 Multiple pairs of file and key can be given in a single command line.
 
+Note that, unlike `git-annex migrate`, this does not copy over metadata,
+urls, and other such information from the old to the new key
+
 # OPTIONS
 
 * `--force`
@@ -37,6 +40,8 @@ Multiple pairs of file and key can be given in a single command line.
 
 [[git-annex]](1)
 
+[[git-annex-migrate]](1)
+
 # AUTHOR
 
 Joey Hess <id@joeyh.name>