Strange bug of notation3 serialization that occur with “probability”

[blackwint3r]

Version:
rdflib 6.1.1
python 3.8.10

I have a test.py:

import rdflib
data1='''
@prefix : <http://www.example.com/dev#>.
:a :b :c.
'''
data2='''
@prefix : <http://www.example.com/dev#>.
{:a :b :c}=>{:d :e :f}.
'''
g = rdflib.Graph()
g.parse(data=data1, format = 'n3')
g.parse(data=data2, format = 'n3')
r = g.serialize(format='n3')
print(r)

The output of this test.py has 2 possibilities.
The first one (which is correct):

@prefix : <http://www.example.com/dev#> .

:a :b :c .

{
    :a :b :c .

} => {
        :d :e :f .

    } .

The second one (missing the triple parsed at the first time):

@prefix : <http://www.example.com/dev#> .

{
    :a :b :c .

} => {
        :d :e :f .

    } .

This strange bug appears in a completely random way. In order to count the "probability" of its occurrence, I wrote a loop like this：

num = 10000
error =0
for i in range(num):
    g = rdflib.Graph()
    g.parse(data=data1, format = 'n3')
    g.parse(data=data2, format = 'n3')
    r = g.serialize(format='n3')
    #print(g.serialize(format='n3'))
    index = (r.find(":a :b :c"))
    #print(index)
    if index > 43:  #second case occur
        error +=1
print("error =" ,error)

Each execution gives a different result, like error= 8533, 5693, 437... As the num increase, the error rate do not converge to a specific value. Can't find any patterns.

[aucampia]

Seems to be the same issue as #1807

[aucampia]

This seems to happen every time the first element in subjects_list here is the quoted graph:

rdflib/rdflib/plugins/serializers/turtle.py

Line 239 in e9f5c22

subjects_list = self.orderSubjects()

I'm guessing this may be because because the serializer thinks it already serialized the triple as it has been serialized inside the quoted graph. Checking further.

[aucampia]

Indeed it is, debugging inside master...aucampia:iwana-20220419T2343-n3_serialize_quoted_graph

20220419T234518 iwana@iwana-pc00.coop.no:~/sw/d/github.com/iafork/rdflib.cleanish
$ .venv/bin/python3 -m pytest  'test/test_issues/test_issue1701.py::test_issue1701_a' -rA --log-cli-level DEBUG
============================================================================ test session starts ============================================================================
platform linux -- Python 3.7.13, pytest-7.1.1, pluggy-1.0.0
rootdir: /home/iwana/sw/d/github.com/iafork/rdflib.cleanish, configfile: pyproject.toml
plugins: subtests-0.7.0, md-report-0.2.0, cov-3.0.0
collected 1 item                                                                                                                                                            

test/test_issues/test_issue1701.py::test_issue1701_a 
------------------------------------------------------------------------------- live log call -------------------------------------------------------------------------------
2022-04-19T23:45:23 DEBUG    root         turtle.py:228:serialize entry ...
2022-04-19T23:45:23 DEBUG    root         turtle.py:242:serialize subjects_list = [<Graph identifier=_:Formula3 (<class 'rdflib.graph.QuotedGraph'>)>, rdflib.term.URIRef('http://example.com/a'), rdflib.term.URIRef('http://example.com/d')]
2022-04-19T23:45:23 DEBUG    root         turtle.py:249:serialize subject (isDone=False) = {this rdflib.identifier _:_:Formula3;rdflib:storage [a rdflib:Store;rdfs:label 'Memory']}
2022-04-19T23:45:23 DEBUG    root         turtle.py:228:serialize entry ...
2022-04-19T23:45:23 DEBUG    root         turtle.py:242:serialize subjects_list = [rdflib.term.URIRef('http://example.com/a')]
2022-04-19T23:45:23 DEBUG    root         turtle.py:249:serialize subject (isDone=False) = http://example.com/a
2022-04-19T23:45:23 DEBUG    root         turtle.py:228:serialize entry ...
2022-04-19T23:45:23 DEBUG    root         turtle.py:242:serialize subjects_list = [rdflib.term.URIRef('http://example.com/d')]
2022-04-19T23:45:23 DEBUG    root         turtle.py:249:serialize subject (isDone=False) = http://example.com/d
2022-04-19T23:45:23 DEBUG    root         turtle.py:249:serialize subject (isDone=True) = http://example.com/a
2022-04-19T23:45:23 DEBUG    root         turtle.py:249:serialize subject (isDone=True) = http://example.com/d
2022-04-19T23:45:23 DEBUG    root         test_issue1701.py:42:test_issue1701_a data_s = @prefix : <http://example.com/> .

{
    :a :b :c .

} => {
        :d :e :f .

    } .


XFAIL ()                                                                                                                                                              [100%]

========================================================================== short test summary info ==========================================================================
XFAIL test/test_issues/test_issue1701.py::test_issue1701_a
  
https://github.com/RDFLib/rdflib/issues/1701

============================================================================ 1 xfailed in 0.11s =============================================================================

[aucampia]

Still digging around here, but I suspect this is wrong:

rdflib/rdflib/plugins/serializers/n3.py

Lines 66 to 73 in e9f5c22

	def preprocessTriple(self, triple):
	super(N3Serializer, self).preprocessTriple(triple)
	if isinstance(triple[0], Graph):
	for t in triple[0]:
	self.preprocessTriple(t)
	if isinstance(triple[2], Graph):
	for t in triple[2]:
	self.preprocessTriple(t)

I don't think this should be processing the triples in quoted graphs.

[aucampia]

This seems to do the trick, will look further tomorrow:

diff --git a/rdflib/plugins/serializers/n3.py b/rdflib/plugins/serializers/n3.py
index f82a08a2..1e91d8f3 100644
--- a/rdflib/plugins/serializers/n3.py
+++ b/rdflib/plugins/serializers/n3.py
@@ -25,13 +25,14 @@ class N3Serializer(TurtleSerializer):
 
     def subjectDone(self, subject):
         super(N3Serializer, self).subjectDone(subject)
-        if self.parent:
-            self.parent.subjectDone(subject)
+        # if self.parent:
+        #     self.parent.subjectDone(subject)
 
     def isDone(self, subject):
-        return super(N3Serializer, self).isDone(subject) and (
-            not self.parent or self.parent.isDone(subject)
-        )
+        return super(N3Serializer, self).isDone(subject)
+        # return super(N3Serializer, self).isDone(subject) and (
+        #     not self.parent or self.parent.isDone(subject)
+        # )
 
     def startDocument(self):
         super(N3Serializer, self).startDocument()
@@ -65,12 +66,12 @@ class N3Serializer(TurtleSerializer):
 
     def preprocessTriple(self, triple):
         super(N3Serializer, self).preprocessTriple(triple)
-        if isinstance(triple[0], Graph):
-            for t in triple[0]:
-                self.preprocessTriple(t)
-        if isinstance(triple[2], Graph):
-            for t in triple[2]:
-                self.preprocessTriple(t)
+        # if isinstance(triple[0], Graph):
+        #     for t in triple[0]:
+        #         self.preprocessTriple(t)
+        # if isinstance(triple[2], Graph):
+        #     for t in triple[2]:
+        #         self.preprocessTriple(t)
 
     def getQName(self, uri, gen_prefix=True):
         qname = None