Fix CombineGlobally with GlobalWindows (apache#26922)

Adds a better error message when this is used in an unsupported way as well as more testing.

sdks/python/apache_beam/transforms/combiners_test.py

@@ -20,6 +20,7 @@
 
 import itertools
 import random
+import time
 import unittest
 
 import hamcrest as hc
@@ -44,6 +45,7 @@
 from apache_beam.transforms.core import Map
 from apache_beam.transforms.display import DisplayData
 from apache_beam.transforms.display_test import DisplayDataItemMatcher
+from apache_beam.transforms.periodicsequence import PeriodicImpulse
 from apache_beam.transforms.ptransform import PTransform
 from apache_beam.transforms.trigger import AfterAll
 from apache_beam.transforms.trigger import AfterCount
@@ -977,5 +979,47 @@ def test_combiner_latest(self):
           label='assert per window')
 
 
+class CombineGloballyTest(unittest.TestCase):
+  def test_combine_globally_for_unbounded_source_with_default(self):
+    # this error is logged since the below combination is ill-defined.
+    with self.assertLogs() as captured_logs:
+      with TestPipeline() as p:
+        _ = (
+            p
+            | PeriodicImpulse(
+                start_timestamp=time.time(),
+                stop_timestamp=time.time() + 4,
+                fire_interval=1,
+                apply_windowing=False,
+            )
+            | beam.Map(lambda x: ('c', 1))
+            | beam.WindowInto(
+                window.GlobalWindows(),
+                trigger=trigger.Repeatedly(trigger.AfterCount(2)),
+                accumulation_mode=trigger.AccumulationMode.DISCARDING,
+            )
+            | beam.combiners.Count.Globally())
+    self.assertIn('unbounded collections', '\n'.join(captured_logs.output))
+
+  def test_combine_globally_for_unbounded_source_without_defaults(self):
+    # this is the supported case
+    with TestPipeline() as p:
+      _ = (
+          p
+          | PeriodicImpulse(
+              start_timestamp=time.time(),
+              stop_timestamp=time.time() + 4,
+              fire_interval=1,
+              apply_windowing=False,
+          )
+          | beam.Map(lambda x: 1)
+          | beam.WindowInto(
+              window.GlobalWindows(),
+              trigger=trigger.Repeatedly(trigger.AfterCount(2)),
+              accumulation_mode=trigger.AccumulationMode.DISCARDING,
+          )
+          | beam.CombineGlobally(sum).without_defaults())
+
+
 if __name__ == '__main__':
   unittest.main()

sdks/python/apache_beam/transforms/core.py

@@ -2665,6 +2665,15 @@ def add_input_types(transform):
             "or CombineGlobally().as_singleton_view() to get the default "
             "output of the CombineFn if the input PCollection is empty.")
 
+      # log the error for this ill-defined streaming case now
+      if not pcoll.is_bounded and not pcoll.windowing.is_default():
+        _LOGGER.error(
+            "When combining elements in unbounded collections with "
+            "the non-default windowing strategy, you must explicitly "
+            "specify how to define the combined result of an empty window. "
+            "Please use CombineGlobally().without_defaults() to output "
+            "an empty PCollection if the input PCollection is empty.")
+
       def typed(transform):
         # TODO(robertwb): We should infer this.
         if combined.element_type:
@@ -2676,6 +2685,11 @@ def typed(transform):
 
       def inject_default(_, combined):
         if combined:
+          if len(combined) > 1:
+            _LOGGER.error(
+                "Multiple combined values unexpectedly provided"
+                " for a global combine: %s",
+                combined)
           assert len(combined) == 1
           return combined[0]
         else: