standardise logs (#1442)

## Motivation

Standardises exo.log and event_log

## Changes

- exo.log and exo.log.zst are now in a exo_log directory.
- event_log is now timestamped and not numbered. The timestamps can be
sorted as they are YYYY-MM-DD-HH-MM

## Test Plan

### Manual Testing
Nothing crashes.

src/exo/master/event_log.py

@@ -2,6 +2,7 @@ import contextlib
 import json
 from collections import OrderedDict
 from collections.abc import Iterator
+from datetime import datetime, timezone
 from io import BufferedRandom, BufferedReader
 from pathlib import Path
 
@@ -159,35 +160,26 @@ class DiskEventLog:
         elif self._active_path.exists():
             self._active_path.unlink()
 
-    @staticmethod
-    def _archive_path(directory: Path, n: int) -> Path:
-        return directory / f"events.{n}.bin.zst"
-
     @staticmethod
     def _rotate(source: Path, directory: Path) -> None:
-        """Compress source into a numbered archive, shifting older archives.
+        """Compress source into a timestamped archive.
 
-        Keeps at most ``_MAX_ARCHIVES`` compressed copies.  The most recent
-        archive is always ``events.1.bin.zst``; older ones are shifted up
-        (2, 3, …) and the oldest beyond the limit is deleted.
+        Keeps at most ``_MAX_ARCHIVES`` compressed copies.  Oldest beyond
+        the limit are deleted.
         """
         try:
-            # Shift existing archives
-            oldest = DiskEventLog._archive_path(directory, _MAX_ARCHIVES)
-            with contextlib.suppress(FileNotFoundError):
-                oldest.unlink()
-            for i in range(_MAX_ARCHIVES - 1, 0, -1):
-                current = DiskEventLog._archive_path(directory, i)
-                if current.exists():
-                    current.rename(DiskEventLog._archive_path(directory, i + 1))
-
-            # Compress source into slot 1
-            dest = DiskEventLog._archive_path(directory, 1)
+            stamp = datetime.now(timezone.utc).strftime("%Y-%m-%d_%H-%M-%S_%f")
+            dest = directory / f"events.{stamp}.bin.zst"
             compressor = zstandard.ZstdCompressor()
             with open(source, "rb") as f_in, open(dest, "wb") as f_out:
                 compressor.copy_stream(f_in, f_out)
             source.unlink()
             logger.info(f"Rotated event log: {source} -> {dest}")
+
+            # Prune oldest archives beyond the limit
+            archives = sorted(directory.glob("events.*.bin.zst"))
+            for old in archives[:-_MAX_ARCHIVES]:
+                old.unlink()
         except Exception as e:
             logger.opt(exception=e).warning(f"Failed to rotate event log {source}")
             # Clean up the source even if compression fails

src/exo/master/tests/test_event_log.py

@@ -66,26 +66,31 @@ def test_empty_log(log_dir: Path):
     log.close()
 
 
+def _archives(log_dir: Path) -> list[Path]:
+    return sorted(log_dir.glob("events.*.bin.zst"))
+
+
 def test_rotation_on_close(log_dir: Path):
     log = DiskEventLog(log_dir)
     log.append(TestEvent())
     log.close()
 
     active = log_dir / "events.bin"
-    archive = log_dir / "events.1.bin.zst"
     assert not active.exists()
-    assert archive.exists()
-    assert archive.stat().st_size > 0
+
+    archives = _archives(log_dir)
+    assert len(archives) == 1
+    assert archives[0].stat().st_size > 0
 
 
 def test_rotation_on_construction_with_stale_file(log_dir: Path):
     log_dir.mkdir(parents=True, exist_ok=True)
-    active = log_dir / "events.bin"
-    active.write_bytes(b"stale data")
+    (log_dir / "events.bin").write_bytes(b"stale data")
 
     log = DiskEventLog(log_dir)
-    archive = log_dir / "events.1.bin.zst"
-    assert archive.exists()
+    archives = _archives(log_dir)
+    assert len(archives) == 1
+    assert archives[0].exists()
     assert len(log) == 0
 
     log.close()
@@ -97,19 +102,19 @@ def test_empty_log_no_archive(log_dir: Path):
     log.close()
 
     active = log_dir / "events.bin"
-    archive = log_dir / "events.1.bin.zst"
+
     assert not active.exists()
-    assert not archive.exists()
+    assert _archives(log_dir) == []
 
 
 def test_close_is_idempotent(log_dir: Path):
     log = DiskEventLog(log_dir)
     log.append(TestEvent())
     log.close()
+    archive = _archives(log_dir)
     log.close()  # should not raise
 
-    archive = log_dir / "events.1.bin.zst"
-    assert archive.exists()
+    assert _archives(log_dir) == archive
 
 
 def test_successive_sessions(log_dir: Path):
@@ -118,7 +123,7 @@ def test_successive_sessions(log_dir: Path):
     log1.append(TestEvent())
     log1.close()
 
-    assert (log_dir / "events.1.bin.zst").exists()
+    first_archive = _archives(log_dir)[-1]
 
     log2 = DiskEventLog(log_dir)
     log2.append(TestEvent())
@@ -126,18 +131,25 @@ def test_successive_sessions(log_dir: Path):
     log2.close()
 
     # Session 1 archive shifted to slot 2, session 2 in slot 1
-    assert (log_dir / "events.1.bin.zst").exists()
-    assert (log_dir / "events.2.bin.zst").exists()
+    second_archive = _archives(log_dir)[-1]
+    should_be_first_archive = _archives(log_dir)[-2]
+
+    assert first_archive.exists()
+    assert second_archive.exists()
+    assert first_archive != second_archive
+    assert should_be_first_archive == first_archive
 
 
 def test_rotation_keeps_at_most_5_archives(log_dir: Path):
     """After 7 sessions, only the 5 most recent archives should remain."""
+    all_archives: list[Path] = []
     for _ in range(7):
         log = DiskEventLog(log_dir)
         log.append(TestEvent())
         log.close()
+        all_archives.append(_archives(log_dir)[-1])
 
-    for i in range(1, 6):
-        assert (log_dir / f"events.{i}.bin.zst").exists()
-    assert not (log_dir / "events.6.bin.zst").exists()
-    assert not (log_dir / "events.7.bin.zst").exists()
+    for old in all_archives[:2]:
+        assert not old.exists()
+    for recent in all_archives[2:]:
+        assert recent.exists()

src/exo/shared/constants.py

@@ -43,7 +43,8 @@ DASHBOARD_DIR = (
 )
 
 # Log files (data/logs or cache)
-EXO_LOG = EXO_CACHE_HOME / "exo.log"
+EXO_LOG_DIR = EXO_CACHE_HOME / "exo_log"
+EXO_LOG = EXO_LOG_DIR / "exo.log"
 EXO_TEST_LOG = EXO_CACHE_HOME / "exo_test.log"
 
 # Identity (config)