avoid storing same filename repeatedly in versioned import from S3

Logically, this should make it need a lot less memory when files have
been changed many times. In my tests, it didn't seem to change memory
use at all. Unsure why, it is working. It's possible the Response is not
getting garbage collected due to pinning. But as far as I can see, all
parts of it that are retained get copied in a way that won't keep the
whole thing pinned in memory.

Remote/S3.hs

@@ -582,7 +582,7 @@ listImportableContentsS3 hv r info c =
 		| versioning info = do
 			rsp <- sendS3Handle h $ 
 				S3.getBucketObjectVersions (bucket info)
-			continuelistversioned 0 h [] rsp
+			continuelistversioned Nothing h [] rsp
 		| otherwise = do
 			rsp <- sendS3Handle h $ 
 				(S3.getBucket (bucket info))
@@ -608,11 +608,13 @@ listImportableContentsS3 hv r info c =
 				Nothing -> nomore
 		| otherwise = nomore
 	  where
-		nomore = return $
-			mkImportableContentsUnversioned
-				(reverse (extractunversioned rsp:l))
+		nomore = do
+			l' <- extractFromResourceT $
+				extractunversioned rsp
+			return $ mkImportableContentsUnversioned
+				(reverse (l':l))
 
-	continuelistversioned n h l rsp
+	continuelistversioned reuse h l rsp
 		| S3.gbovrIsTruncated rsp = do
 			rsp' <- sendS3Handle h $
 				(S3.getBucketObjectVersions (bucket info))
@@ -620,12 +622,14 @@ listImportableContentsS3 hv r info c =
 					, S3.gbovVersionIdMarker = S3.gbovrNextVersionIdMarker rsp
 					, S3.gbovPrefix = fileprefix
 					}
-			l' <- extractFromResourceT $
-				extractversioned rsp
-			continuelistversioned (length l' + n) h (l':l) rsp'
-		| otherwise = return $
-			mkImportableContentsVersioned
-				(reverse (extractversioned rsp:l))
+			(l', reuse') <- extractFromResourceT $
+				extractversioned reuse rsp
+			continuelistversioned reuse' h (l':l) rsp'
+		| otherwise = do
+			(l', _) <- extractFromResourceT $
+				extractversioned reuse rsp
+			return $ mkImportableContentsVersioned
+				(reverse (l':l))
 
 	extractunversioned = mapMaybe extractunversioned' . S3.gbrContents
 	extractunversioned' oi = do
@@ -635,14 +639,24 @@ listImportableContentsS3 hv r info c =
                   let cid = mkS3UnversionedContentIdentifier $ S3.objectETag oi
                   return (loc, (cid, sz))
 	
-	extractversioned = mapMaybe extractversioned' . S3.gbovrContents
-  	extractversioned' ovi@(S3.ObjectVersion {}) = do
+	extractversioned reuse = extractversioned' reuse . S3.gbovrContents
+	extractversioned' reuse [] = ([], reuse)
+	extractversioned' reuse (x:xs) = case extractversioned'' reuse x of
+		Just (v, reuse') -> 
+			let (l, reuse'') = extractversioned' reuse' xs
+			in (v:l, reuse'')
+		Nothing -> extractversioned' reuse xs
+	extractversioned'' reuse ovi@(S3.ObjectVersion {}) = do
 		loc <- bucketImportLocation info $
 			T.unpack $ S3.oviKey ovi
+		-- Avoid storing the same filename in memory repeatedly.
+		let loc' = case reuse of
+			Just reuseloc | reuseloc == loc -> reuseloc
+			_ -> loc
 		let sz  = S3.oviSize ovi
 		let cid = mkS3VersionedContentIdentifier' ovi
-		return ((loc, (cid, sz)), S3.oviLastModified ovi)
-	extractversioned' (S3.DeleteMarker {}) = Nothing
+		return (((loc', (cid, sz)), S3.oviLastModified ovi), Just loc')
+	extractversioned'' _ (S3.DeleteMarker {}) = Nothing
 
 mkImportableContentsUnversioned :: [[(ImportLocation, (ContentIdentifier, ByteSize))]] -> ImportableContents (ContentIdentifier, ByteSize)
 mkImportableContentsUnversioned l = ImportableContents