Stop keeping old copies of eventbus files (#941)

We've never restored from these "backups", and as long as we ensure that
the current file points to a fully written file, it's fine to only keep
at most 2 files around: the actual current file and a temporary "new"
file.

To ensure that we're not pointing at a half-written file, we continue
using the age-old pattern for atomic file updates: write to another file
and swap a symlink once the write is complete

tests/eventbus_test.py

@@ -100,7 +100,10 @@ def test_sync_save_log_time(self, time):
         self.eventbus.sync_save_log("test")
         new_link = os.readlink(self.eventbus.log_current)
         assert_equal(new_link, os.path.join(self.log_dir.name, "2.pickle"))
-        assert os.path.exists(current_link)
+        # we clean up the previous link so as not to have a million pickles
+        # on disk
+        assert not os.path.exists(current_link)
+        # so at this point, we should only have the new link
         assert os.path.exists(new_link)
 
     @mock.patch("tron.eventbus.time", autospec=True)

tron/eventbus.py

@@ -5,6 +5,7 @@
 import time
 from collections import defaultdict
 from collections import deque
+from typing import Optional
 
 from twisted.internet import reactor
 
@@ -144,9 +145,18 @@ def sync_load_log(self):
         duration = time.time() - started
         log.info(f"log read from disk, took {duration:.4}s")
 
-    def sync_save_log(self, reason):
+    def sync_save_log(self, reason: str) -> bool:
         started = time.time()
         new_file = os.path.join(self.log_dir, f"{int(started)}.pickle")
+        previous_file: Optional[str] = os.path.realpath(os.path.join(self.log_dir, "current"))
+        # if we're starting  a fresh Tron server, there won't be a current symlink
+        # and the above line will give us the path to what will eventually be the current
+        # symlink...which is undesirable since we clean up whatever this points to :p
+        # we can tell if this is happening since this previous_file variable should
+        # always point to a file that ends in .pickle under normal operation
+        if previous_file and previous_file.endswith("current"):
+            previous_file = None
+
         try:
             with open(new_file, "xb") as f:
                 pickle.dump(self.event_log, f)
@@ -165,7 +175,16 @@ def sync_save_log(self, reason):
         except FileNotFoundError:
             pass
         os.symlink(new_file, tmplink)
-        os.replace(tmplink, self.log_current)
+        os.replace(src=tmplink, dst=self.log_current)
+        # once we get here, `self.log_current` is now pointing to `new_file`
+        # so we can safely delete the previous `self.log_current` target without
+        # fear of losing data
+        if previous_file:
+            try:
+                os.remove(previous_file)
+            except Exception:
+                # this shouldn't happen - but we also shouldn't crash if the impossible happens
+                log.exception(f"unable to delete {previous_file} - continuing anyway.")
 
         duration = time.time() - started
         log.info(f"log dumped to disk because {reason}, took {duration:.4}s")