Fix the SharedFileSetUnregister API.

Commit 808e13b introduced a few APIs to extend the existing Buffile
interface. In SharedFileSetDeleteOnProcExit, it tries to delete the list
element while traversing the list with 'foreach' construct which makes the
behavior of list traversal unpredictable.

Author: Amit Kapila
Reviewed-by: Dilip Kumar
Tested-by: Dilip Kumar and Neha Sharma
Discussion: https://postgr.es/m/CAA4eK1JhLatVcQ2OvwA_3s0ih6Hx9+kZbq107cXVsSWWukH7vA@mail.gmail.com

Author: Amit Kapila

src/backend/storage/file/sharedfileset.c

@@ -266,12 +266,16 @@ SharedFileSetOnDetach(dsm_segment *segment, Datum datum)
 static void
 SharedFileSetDeleteOnProcExit(int status, Datum arg)
 {
-	ListCell   *l;
-
-	/* Loop over all the pending shared fileset entry */
-	foreach(l, filesetlist)
+	/*
+	 * Remove all the pending shared fileset entries. We don't use foreach() here
+	 * because SharedFileSetDeleteAll will remove the current element in
+	 * filesetlist. Though we have used foreach_delete_current() to remove the
+	 * element from filesetlist it could only fix up the state of one of the
+	 * loops, see SharedFileSetUnregister.
+	 */
+	while (list_length(filesetlist) > 0)
 	{
-		SharedFileSet *fileset = (SharedFileSet *) lfirst(l);
+		SharedFileSet *fileset = (SharedFileSet *) linitial(filesetlist);
 
 		SharedFileSetDeleteAll(fileset);
 	}
@@ -301,7 +305,7 @@ SharedFileSetUnregister(SharedFileSet *input_fileset)
 		/* Remove the entry from the list */
 		if (input_fileset == fileset)
 		{
-			filesetlist = list_delete_cell(filesetlist, l);
+			filesetlist = foreach_delete_current(filesetlist, l);
 			return;
 		}
 	}