prevent dropping required content of other file using same content

When two files have the same content, and a required content expression
matches one but not the other, dropping the latter file will fail as it
would also remove the content of the required file.

This will slow down drop (w/o --auto), dropunused, mirror, and move, by one
keys db lookup per file. But I did include an optimisation to avoid a
double db lookup in the drop --auto / sync --content case. I suspect that
dropunused could also use PreferredContentChecked True, but haven't
entirely thought it through and it's rarely used with enough files for the
optimisation to matter.

Sponsored-by: Dartmouth College's Datalad project

Annex/Drop.hs

@@ -27,8 +27,8 @@ import qualified Data.Set as S
 
 type Reason = String
 
-{- Drop a key from local and/or remote when allowed by the preferred content
- - and numcopies settings.
+{- Drop a key from local and/or remote when allowed by the preferred content,
+ - required content, and numcopies settings.
  -
  - Skips trying to drop from remotes that are appendonly, since those drops
  - would presumably fail. Also skips dropping from exporttree/importtree remotes,
@@ -105,8 +105,9 @@ handleDropsFrom locs rs reason fromhere key afile si preverified runner = do
 
 	checkdrop fs n u a =
 		let afs = map (AssociatedFile . Just) fs
+		    pcc = Command.Drop.PreferredContentChecked True
 		in ifM (wantDrop True u (Just key) afile (Just afs))
-			( dodrop n u a
+			( dodrop n u (a pcc)
 			, return n
 			)
 
@@ -126,12 +127,12 @@ handleDropsFrom locs rs reason fromhere key afile si preverified runner = do
 			, return n
 			)
 
-	dropl fs n = checkdrop fs n Nothing $ \numcopies mincopies ->
+	dropl fs n = checkdrop fs n Nothing $ \pcc numcopies mincopies ->
 		stopUnless (inAnnex key) $
-			Command.Drop.startLocal afile ai si numcopies mincopies key preverified
+			Command.Drop.startLocal pcc afile ai si numcopies mincopies key preverified
 
-	dropr fs r n  = checkdrop fs n (Just $ Remote.uuid r) $ \numcopies mincopies ->
-		Command.Drop.startRemote afile ai si numcopies mincopies key r
+	dropr fs r n  = checkdrop fs n (Just $ Remote.uuid r) $ \pcc numcopies mincopies ->
+		Command.Drop.startRemote pcc afile ai si numcopies mincopies key r
 
 	ai = mkActionItem (key, afile)