swaglog: delete oldest (not newest) logs on rotation after restart

common/swaglog.py

@@ -45,7 +45,8 @@ def get_existing_logfiles(self):
       fp = os.path.join(base_dir, fn)
       if fp.startswith(self.base_filename) and os.path.isfile(fp):
         log_files.append(fp)
-    return sorted(log_files)
+    # newest first, matching _open()'s insert(0, ...) so doRollover()'s pop() deletes the oldest
+    return sorted(log_files, reverse=True)
 
   def shouldRollover(self, record):
     size_exceeded = self.max_bytes > 0 and self.stream.tell() >= self.max_bytes

common/tests/test_swaglog.py

@@ -0,0 +1,37 @@
+from openpilot.common.swaglog import SwaglogRotatingFileHandler
+
+
+def seed(base: str, indexes) -> None:
+  for i in indexes:
+    with open(f"{base}.{i:010}", "w") as f:
+      f.write("x")
+
+
+def existing(tmp_path) -> list[str]:
+  return sorted(f.name for f in tmp_path.iterdir())
+
+
+class TestSwaglogRotation:
+  def test_rollover_deletes_oldest_first(self, tmp_path):
+    base = str(tmp_path / "swaglog")
+    seed(base, range(8))  # swaglog.0000000000 .. swaglog.0000000007, exceeds backup_count
+
+    # restart over pre-existing files: __init__ rolls over, opening .8 and pruning to backup_count
+    handler = SwaglogRotatingFileHandler(base, backup_count=4)
+    try:
+      assert existing(tmp_path) == [f"swaglog.{i:010}" for i in (5, 6, 7, 8)]
+
+      # subsequent rollovers keep deleting oldest-first
+      handler.doRollover()
+      assert existing(tmp_path) == [f"swaglog.{i:010}" for i in (6, 7, 8, 9)]
+      handler.doRollover()
+      assert existing(tmp_path) == [f"swaglog.{i:010}" for i in (7, 8, 9, 10)]
+    finally:
+      handler.close()
+
+    # a second restart over the survivors keeps the same invariant
+    handler = SwaglogRotatingFileHandler(base, backup_count=4)
+    try:
+      assert existing(tmp_path) == [f"swaglog.{i:010}" for i in (8, 9, 10, 11)]
+    finally:
+      handler.close()