ttlser ListRanker fix infinite loop case for cyclical lists

Any cyclical linked list would go infinite while traversing rdf:rest,
this resolves the issue and provides a simplified and clear test case.

ttlser/test/evil.ttl

@@ -1,35 +1,18 @@
-@prefix TEMP: <http://uri.interlex.org/temp/uris/> .
 @prefix ilxtr: <http://uri.interlex.org/tgbugs/uris/readable/> .
-@prefix owl: <http://www.w3.org/2002/07/owl#> .
 @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
-@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
 
-TEMP:271 a owl:Class ;
-    rdfs:label "electrical stimulation technique" ;
-    owl:equivalentClass [ a owl:Class ;
-            owl:intersectionOf [ rdf:first ilxtr:technique ;
-                    rdf:rest [ rdf:first [ a owl:Restriction ;
-                                    owl:onProperty ilxtr:hasPrimaryAspect ;
-                                    owl:someValuesFrom <http://uri.interlex.org/tgbugs/uris/readable/aspect/electrical> ] ;
-                            rdf:rest _:sg_0_4,
-                                _:sg_0_6 ] ] ] .
+ilxtr:b0 rdf:first ilxtr:x ;
+    rdf:rest
+    ilxtr:b1 .
 
-_:sg_0_5 a owl:Restriction ;
-    owl:onProperty ilxtr:hasProbe,
-        ilxtr:hasSomething ;
-    owl:someValuesFrom TEMP:272,
-        ilxtr:electricalField .
+ilxtr:b1 rdf:first ilxtr:y ;
+    rdf:rest
+    ilxtr:b2 .
 
-_:sg_0_7 a owl:Restriction ;
-    owl:onProperty ilxtr:hasProbe,
-        ilxtr:hasSomething ;
-    owl:someValuesFrom TEMP:272,
-        ilxtr:electricalField .
+ilxtr:b2 rdf:first ilxtr:z ;
+    rdf:rest
+    ilxtr:b3 .
 
-_:sg_0_4 rdf:first _:sg_0_5 ;
-    rdf:rest _:sg_0_6,
-        () .
-
-_:sg_0_6 rdf:first _:sg_0_7 ;
-    rdf:rest _:sg_0_4,
-        () .
+ilxtr:b3 rdf:first ilxtr:e ;
+    rdf:rest
+    ilxtr:b1 .

ttlser/ttlser/serializers.py

@@ -102,10 +102,25 @@ def __init__(self, node, serializer):
         self.vals = []
         self.nodes = []  # list helper nodes
         l = self.node
-        while l:
+        previous = set()
+        #count = 0
+        #enough = 99999
+        while l:  # infinite loop posibility
             item = self.serializer.store.value(l, RDF.first)
             self.add(item, l)
+            # if there is a self referential node anywhere in a list
+            # beyond the first element this goes infinite
+            previous.add(l)
             l = self.serializer.store.value(l, RDF.rest)
+            if l in previous:  # cyclical reference case
+                break
+            # XXX beware lists where there are multiple rdf:rest
+            # values for a node, I don't think they will break
+            # serialization, but they may produce unexpected ranking
+            #count += 1
+            #if count > enough:
+                #print(self.serializer.store)
+                #breakpoint()
         self.vis_vals = [v for v in self.vals if not isinstance(v, BNode)]
         self.bvals = [v for v in self.vals if isinstance(v, BNode)]