Fixed #15849 -- Made IfChanged node thread safe.

Previously, the ifchanged node stored state on `self._last_seen`,
thereby giving undesired results when the node is reused by another
thread at the same time (e.g. globally caching a Template object).

Thanks to akaihola for the report and Diederik van der Boor and
Bas Peschier for the patch.

django/template/defaulttags.py

@@ -215,32 +215,44 @@ class IfChangedNode(Node):
 
     def __init__(self, nodelist_true, nodelist_false, *varlist):
         self.nodelist_true, self.nodelist_false = nodelist_true, nodelist_false
-        self._last_seen = None
         self._varlist = varlist
-        self._id = str(id(self))
 
     def render(self, context):
-        if 'forloop' in context and self._id not in context['forloop']:
-            self._last_seen = None
-            context['forloop'][self._id] = 1
+        # Init state storage
+        state_frame = self._get_context_stack_frame(context)
+        if self not in state_frame:
+            state_frame[self] = None
+
         try:
             if self._varlist:
                 # Consider multiple parameters.  This automatically behaves
                 # like an OR evaluation of the multiple variables.
                 compare_to = [var.resolve(context, True) for var in self._varlist]
             else:
+                # The "{% ifchanged %}" syntax (without any variables) compares the rendered output.
                 compare_to = self.nodelist_true.render(context)
         except VariableDoesNotExist:
             compare_to = None
 
-        if compare_to != self._last_seen:
-            self._last_seen = compare_to
-            content = self.nodelist_true.render(context)
-            return content
+        if compare_to != state_frame[self]:
+            state_frame[self] = compare_to
+            return self.nodelist_true.render(context)
         elif self.nodelist_false:
             return self.nodelist_false.render(context)
         return ''
 
+    def _get_context_stack_frame(self, context):
+        # The Context object behaves like a stack where each template tag can create a new scope.
+        # Find the place where to store the state to detect changes.
+        if 'forloop' in context:
+            # Ifchanged is bound to the local for loop.
+            # When there is a loop-in-loop, the state is bound to the inner loop,
+            # so it resets when the outer loop continues.
+            return context['forloop']
+        else:
+            # Using ifchanged outside loops. Effectively this is a no-op because the state is associated with 'self'.
+            return context.render_context
+
 class IfEqualNode(Node):
     child_nodelists = ('nodelist_true', 'nodelist_false')
 

tests/regressiontests/templates/tests.py

@@ -420,6 +420,27 @@ class Templates(TestCase):
         except TemplateSyntaxError as e:
             self.assertEqual(e.args[0], "Invalid block tag: 'endblock', expected 'elif', 'else' or 'endif'")
 
+    def test_ifchanged_concurrency(self):
+        # Tests for #15849
+        template = Template('[0{% for x in foo %},{% with var=get_value %}{% ifchanged %}{{ var }}{% endifchanged %}{% endwith %}{% endfor %}]')
+
+        # Using generator to mimic concurrency.
+        # The generator is not passed to the 'for' loop, because it does a list(values)
+        # instead, call gen.next() in the template to control the generator.
+        def gen():
+            yield 1
+            yield 2
+            # Simulate that another thread is now rendering.
+            # When the IfChangeNode stores state at 'self' it stays at '3' and skip the last yielded value below.
+            iter2 = iter([1, 2, 3])
+            output2 = template.render(Context({'foo': range(3), 'get_value': lambda: next(iter2)}))
+            self.assertEqual(output2, '[0,1,2,3]', 'Expected [0,1,2,3] in second parallel template, got {0}'.format(output2))
+            yield 3
+
+        gen1 = gen()
+        output1 = template.render(Context({'foo': range(3), 'get_value': lambda: next(gen1)}))
+        self.assertEqual(output1, '[0,1,2,3]', 'Expected [0,1,2,3] in first template, got {0}'.format(output1))
+
     def test_templates(self):
         template_tests = self.get_template_tests()
         filter_tests = filters.get_filter_tests()