sync: Keep a moving average of last fetch times

Try to more accurately estimate which projects take the longest to
sync by keeping an exponentially weighted moving average (a=0.5) of
fetch times, rather than just recording the last observation. This
should discount individual outliers (e.g. an unusually large project
update) and hopefully allow truly slow repos to bubble to the top.

Change-Id: I72b2508cb1266e8a19cf15b616d8a7fc08098cb3

subcmds/sync.py

@@ -510,7 +510,6 @@ uncommitted changes are present' % project.relpath
         to_fetch.append(rp)
       to_fetch.extend(all_projects)
       to_fetch.sort(key=self._fetch_times.Get, reverse=True)
-      self._fetch_times.Clear()
 
       self._Fetch(to_fetch, opt)
       _PostRepoFetch(rp, opt.no_repo_verify)
@@ -613,19 +612,24 @@ warning: Cannot automatically authenticate repo."""
   return True
 
 class _FetchTimes(object):
+  _ALPHA = 0.5
+
   def __init__(self, manifest):
     self._path = os.path.join(manifest.repodir, '.repopickle_fetchtimes')
     self._times = None
-
-  def Clear(self):
-    self._times = {}
+    self._seen = set()
 
   def Get(self, project):
     self._Load()
     return self._times.get(project.name, _ONE_DAY_S)
 
   def Set(self, project, t):
-    self._times[project.name] = t
+    self._Load()
+    name = project.name
+    old = self._times.get(name, t)
+    self._seen.add(name)
+    a = self._ALPHA
+    self._times[name] = (a*t) + ((1-a) * old)
 
   def _Load(self):
     if self._times is None:
@@ -650,6 +654,14 @@ class _FetchTimes(object):
   def Save(self):
     if self._times is None:
       return
+
+    to_delete = []
+    for name in self._times:
+      if name not in self._seen:
+        to_delete.append(name)
+    for name in to_delete:
+      del self._times[name]
+
     try:
       f = open(self._path, 'wb')
       try: