S3 use last Key when there is no Marker element

Fix infinite loop and memory blowup when importing from an unversioned S3
bucket that is large enough to need pagination.

I don't think there actually ever will be a Marker element, a delimiter is
not set.

Probably this code path was never tested with pagination! Also the aws
library's lack of any docs made it easy to mess up.

Versioned buckets seem to not have the same problem. The API docs for
ListObjectVersions say that NextKeyMarker will always be provided when
paginating.

CHANGELOG

@@ -11,6 +11,8 @@ git-annex (10.20241032) UNRELEASED; urgency=medium
     (Needs aws-0.24.3)
   * S3: Send git-annex or other configured User-Agent.
     (Needs aws-0.24.3)
+  * S3: Fix infinite loop and memory blowup when importing from an
+    unversioned S3 bucket that is large enough to need pagination.
 
  -- Joey Hess <id@joeyh.name>  Mon, 11 Nov 2024 12:26:00 -0400
 

Remote/S3.hs

@@ -589,14 +589,23 @@ listImportableContentsS3 hv r info c =
 			continuelistunversioned h [] rsp
 
 	continuelistunversioned h l rsp
-		| S3.gbrIsTruncated rsp = do
-			rsp' <- sendS3Handle h $
-				(S3.getBucket (bucket info))
-					{ S3.gbMarker = S3.gbrNextMarker rsp
-					, S3.gbPrefix = fileprefix
-					}
-			continuelistunversioned h (rsp:l) rsp'
-		| otherwise = return $
+		| S3.gbrIsTruncated rsp =
+			let marker = 
+				S3.gbrNextMarker rsp
+					<|>
+				(S3.objectKey <$> lastMaybe (S3.gbrContents rsp))
+			in case marker of
+				Just _ -> do
+					rsp' <- sendS3Handle h $
+						(S3.getBucket (bucket info))
+							{ S3.gbMarker = marker
+							, S3.gbPrefix = fileprefix
+							}
+					continuelistunversioned h (rsp:l) rsp'
+				Nothing -> nomore
+		| otherwise = nomore
+	  where
+		nomore = return $
 			mkImportableContentsUnversioned info (reverse (rsp:l))
 	
 	continuelistversioned h l rsp