indexing lister: Avoid generating empty or duplicate ranges when partitioning

swh/lister/core/indexing_lister.py

@@ -139,18 +139,22 @@ class IndexingLister(ListerBase):
 
         partition_width = (max_index - min_index) / n_partitions
 
-        partitions = [
-            [
-                format_bound(min_index + i * partition_width),
-                format_bound(min_index + (i+1) * partition_width),
-            ] for i in range(n_partitions)
+        # Generate n_partitions + 1 bounds for n_partitions partitons
+        bounds = [
+            format_bound(min_index + i * partition_width)
+            for i in range(n_partitions + 1)
         ]
 
-        # Remove bounds for lowest and highest partition
-        partitions[0][0] = None
-        partitions[-1][1] = None
+        # Trim duplicate bounds
+        bounds.append(None)
+        bounds = [cur
+                  for cur, next in zip(bounds[:-1], bounds[1:])
+                  if cur != next]
 
-        return [tuple(partition) for partition in partitions]
+        # Remove bounds for lowest and highest partition
+        bounds[0] = bounds[-1] = None
+
+        return list(zip(bounds[:-1], bounds[1:]))
 
     def db_first_index(self):
         """Look in the db for the smallest indexable value

swh/lister/core/tests/test_indexing_lister.py

@@ -63,6 +63,19 @@ def test_db_partition_indices_zero_first():
     assert partitions[-1] == (9000, None)
 
 
+def test_db_partition_indices_small_index_range():
+    m = MockedIndexingListerDbPartitionIndices(
+        num_entries=5000,
+        first_index=0,
+        last_index=5,
+    )
+    assert m
+
+    partitions = m.db_partition_indices(100)
+
+    assert partitions == [(None, 1), (1, 2), (2, 3), (3, 4), (4, None)]
+
+
 def test_db_partition_indices_date_indices():
     # 24 hour delta
     first = datetime.datetime.fromisoformat('2019-11-01T00:00:00+00:00')