fix: avoid recursion limit when concatenating EmptyArrays

src/awkward/contents/emptyarray.py

@@ -274,6 +274,7 @@ def _mergeable_next(self, other: Content, mergebool: bool) -> bool:
         return True
 
     def _mergemany(self, others: Sequence[Content]) -> Content:
+        others = [other for other in others if not other.is_unknown]
         if len(others) == 0:
             return self
         elif len(others) == 1:

tests/test_3664_concatenate_emptyarays.py

@@ -0,0 +1,74 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+from __future__ import annotations
+
+import sys
+
+import numpy as np
+
+import awkward as ak
+
+
+def test_all_empty():
+    result = ak.concatenate([ak.Array([None]) for _ in range(5000)], axis=0)
+    expected = ak.Array([None] * 5000)
+    assert ak.array_equal(result, expected)
+
+    result = ak.concatenate([ak.Array([{"x": None}]) for _ in range(5000)], axis=0)
+    expected = ak.Array([{"x": None}] * 5000)
+    assert ak.array_equal(result, expected)
+
+    result = ak.concatenate(
+        [ak.Array([{"x": i, "y": None}]) for i in range(5000)], axis=0
+    )
+    expected = ak.Array([{"x": i, "y": None} for i in range(5000)])
+    assert ak.array_equal(result, expected)
+
+
+def test_empty_and_nonempty():
+    N = sys.getrecursionlimit()
+    rng = np.random.default_rng(42)
+    choices1 = np.concatenate(
+        (np.array([True, True]), rng.choice([True, False], size=N))
+    )
+    choices2 = np.concatenate(
+        (np.array([False, False]), rng.choice([True, False], size=N))
+    )
+    choices3 = np.concatenate(
+        (np.array([True, False]), rng.choice([True, False], size=N))
+    )
+    choices4 = np.concatenate(
+        (np.array([False, True]), rng.choice([True, False], size=N))
+    )
+    all_choices = [choices1, choices2, choices3, choices4]
+
+    for choices in all_choices:
+        rows = []
+        for i in range(N):
+            if choices[i]:
+                rows.append(None)
+            else:
+                rows.append(i)
+        result = ak.concatenate([ak.Array([row]) for row in rows], axis=0)
+        expected = ak.Array(rows)
+        assert ak.array_equal(result, expected)
+
+        rows = []
+        for i in range(N):
+            if choices[i]:
+                rows.append({"x": None})
+            else:
+                rows.append({"x": i})
+        result = ak.concatenate([ak.Array([row]) for row in rows], axis=0)
+        expected = ak.Array(rows)
+        assert ak.array_equal(result, expected)
+
+        rows = []
+        for i in range(N):
+            if choices[i]:
+                rows.append({"x": i, "y": None})
+            else:
+                rows.append({"x": i, "y": i})
+        result = ak.concatenate([ak.Array([row]) for row in rows], axis=0)
+        expected = ak.Array(rows)
+        assert ak.array_equal(result, expected)