add hashtable stress tests

Using NSIndex (repo index) HashIndex, but they all are very similar.

src/borg/testsuite/hashindex_stress.py

@@ -0,0 +1,32 @@
+import os
+import random
+
+import pytest
+
+from ..hashindex import NSIndex
+
+
+@pytest.mark.skipif("BORG_TESTS_SLOW" not in os.environ, reason="slow tests not enabled, use BORG_TESTS_SLOW=1")
+def test_hashindex_stress():
+    """checks if the hashtable behaves as expected"""
+    ENTRIES = 10000
+    LOOPS = 100
+    idx = NSIndex()
+    kv = {}
+    for i in range(LOOPS):
+        # put some entries
+        for j in range(ENTRIES):
+            k = random.randbytes(32)
+            v = random.randint(0, NSIndex.MAX_VALUE - 1)
+            idx[k] = (v, v, v)
+            kv[k] = v
+        # check and delete a random amount of entries
+        delete_keys = random.sample(list(kv), k=random.randint(0, len(kv)))
+        for k in delete_keys:
+            v = kv.pop(k)
+            assert idx.pop(k) == (v, v, v)
+        # check if remaining entries are as expected
+        for k, v in kv.items():
+            assert idx[k] == (v, v, v)
+        # check entry count
+        assert len(kv) == len(idx)