fix: guard against IndexError in decompress_list on empty string

metaflow/util.py

@@ -383,6 +383,8 @@ def compress_list(lst, separator=",", rangedelim=":", zlibmarker="!", zlibmin=50
 
 
 def decompress_list(lststr, separator=",", rangedelim=":", zlibmarker="!"):
+    if not lststr:
+        return []
     # Three input modes:
     if lststr[0] == zlibmarker:
         # 3. zlib-compressed, base64-encoded

test/unit/test_util_compress.py

@@ -0,0 +1,81 @@
+"""
+Unit tests for compress_list / decompress_list in metaflow/util.py
+
+Bug: decompress_list("") raised IndexError on lststr[0] because there
+was no guard for an empty input string.  compress_list([]) legitimately
+produces "" (joining an empty list), so the round-trip
+    decompress_list(compress_list([]))
+crashed instead of returning [].
+
+Fix: add `if not lststr: return []` before the index access.
+"""
+
+import sys
+from unittest.mock import MagicMock
+
+# fcntl is Linux/macOS-only.  Stub it out so the test runs on Windows too.
+if "fcntl" not in sys.modules:
+    sys.modules["fcntl"] = MagicMock()
+if "metaflow.sidecar.sidecar_subprocess" not in sys.modules:
+    sys.modules["metaflow.sidecar.sidecar_subprocess"] = MagicMock()
+
+from metaflow.util import compress_list, decompress_list  # noqa: E402
+
+# ---------------------------------------------------------------------------
+# Core regression tests
+# ---------------------------------------------------------------------------
+
+
+def test_compress_empty_list_returns_empty_string():
+    """compress_list([]) must produce '' (join of zero elements)."""
+    assert compress_list([]) == ""
+
+
+def test_decompress_empty_string_returns_empty_list():
+    """
+    decompress_list("") must return [] without raising.
+
+    This is the direct crash case: before the fix, lststr[0] on an
+    empty string raised IndexError: string index out of range.
+    Fails without the fix, passes with it.
+    """
+    result = decompress_list("")
+    assert result == []
+
+
+def test_roundtrip_empty_list():
+    """
+    decompress_list(compress_list([])) must return [].
+
+    This is the end-to-end round-trip that exposed the bug: the output
+    of compress_list is fed straight into decompress_list, so an empty
+    list must survive the round-trip unharmed.
+    Fails without the fix, passes with it.
+    """
+    assert decompress_list(compress_list([])) == []
+
+
+# ---------------------------------------------------------------------------
+# Regression guard: existing non-empty round-trips must still work
+# ---------------------------------------------------------------------------
+
+
+def test_roundtrip_single_item():
+    """A single-element list survives the round-trip."""
+    items = ["abc"]
+    assert decompress_list(compress_list(items)) == items
+
+
+def test_roundtrip_multiple_items_with_common_prefix():
+    """
+    Lists with a long common prefix use the prefix-encoding path;
+    verify the round-trip is correct after the guard was added.
+    """
+    items = ["MyFlow/1234/start/1", "MyFlow/1234/start/2", "MyFlow/1234/start/3"]
+    assert decompress_list(compress_list(items)) == items
+
+
+def test_roundtrip_items_without_common_prefix():
+    """Lists with no common prefix use the plain comma-separated path."""
+    items = ["alpha", "beta", "gamma"]
+    assert decompress_list(compress_list(items)) == items