--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/repocache.py	Wed Oct 31 22:43:08 2018 +0900
@@ -0,0 +1,131 @@
+# repocache.py - in-memory repository cache for long-running services
+#
+# Copyright 2018 Yuya Nishihara <yuya@tcha.org>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import collections
+import gc
+import threading
+
+from . import (
+    error,
+    hg,
+    obsolete,
+    scmutil,
+    util,
+)
+
+class repoloader(object):
+    """Load repositories in background thread
+
+    This is designed for a forking server. A cached repo cannot be obtained
+    until the server fork()s a worker and the loader thread stops.
+    """
+
+    def __init__(self, ui, maxlen):
+        self._ui = ui.copy()
+        self._cache = util.lrucachedict(max=maxlen)
+        # use deque and Event instead of Queue since deque can discard
+        # old items to keep at most maxlen items.
+        self._inqueue = collections.deque(maxlen=maxlen)
+        self._accepting = False
+        self._newentry = threading.Event()
+        self._thread = None
+
+    def start(self):
+        assert not self._thread
+        if self._inqueue.maxlen == 0:
+            # no need to spawn loader thread as the cache is disabled
+            return
+        self._accepting = True
+        self._thread = threading.Thread(target=self._mainloop)
+        self._thread.start()
+
+    def stop(self):
+        if not self._thread:
+            return
+        self._accepting = False
+        self._newentry.set()
+        self._thread.join()
+        self._thread = None
+        self._cache.clear()
+        self._inqueue.clear()
+
+    def load(self, path):
+        """Request to load the specified repository in background"""
+        self._inqueue.append(path)
+        self._newentry.set()
+
+    def get(self, path):
+        """Return a cached repo if available
+
+        This function must be called after fork(), where the loader thread
+        is stopped. Otherwise, the returned repo might be updated by the
+        loader thread.
+        """
+        if self._thread and self._thread.is_alive():
+            raise error.ProgrammingError(b'cannot obtain cached repo while '
+                                         b'loader is active')
+        return self._cache.peek(path, None)
+
+    def _mainloop(self):
+        while self._accepting:
+            # Avoid heavy GC after fork(), which would cancel the benefit of
+            # COW. We assume that GIL is acquired while GC is underway in the
+            # loader thread. If that isn't true, we might have to move
+            # gc.collect() to the main thread so that fork() would never stop
+            # the thread where GC is in progress.
+            gc.collect()
+
+            self._newentry.wait()
+            while self._accepting:
+                self._newentry.clear()
+                try:
+                    path = self._inqueue.popleft()
+                except IndexError:
+                    break
+                scmutil.callcatch(self._ui, lambda: self._load(path))
+
+    def _load(self, path):
+        start = util.timer()
+        # TODO: repo should be recreated if storage configuration changed
+        try:
+            # pop before loading so inconsistent state wouldn't be exposed
+            repo = self._cache.pop(path)
+        except KeyError:
+            repo = hg.repository(self._ui, path).unfiltered()
+        _warmupcache(repo)
+        repo.ui.log(b'repocache', b'loaded repo into cache: %s (in %.3fs)\n',
+                    path, util.timer() - start)
+        self._cache.insert(path, repo)
+
+# TODO: think about proper API of preloading cache
+def _warmupcache(repo):
+    repo.invalidateall()
+    repo.changelog
+    repo.obsstore._all
+    repo.obsstore.successors
+    repo.obsstore.predecessors
+    repo.obsstore.children
+    for name in obsolete.cachefuncs:
+        obsolete.getrevs(repo, name)
+    repo._phasecache.loadphaserevs(repo)
+
+# TODO: think about proper API of attaching preloaded attributes
+def copycache(srcrepo, destrepo):
+    """Copy cached attributes from srcrepo to destrepo"""
+    destfilecache = destrepo._filecache
+    srcfilecache = srcrepo._filecache
+    if 'changelog' in srcfilecache:
+        destfilecache['changelog'] = ce = srcfilecache['changelog']
+        ce.obj.opener = ce.obj._realopener = destrepo.svfs
+    if 'obsstore' in srcfilecache:
+        destfilecache['obsstore'] = ce = srcfilecache['obsstore']
+        ce.obj.svfs = destrepo.svfs
+    if '_phasecache' in srcfilecache:
+        destfilecache['_phasecache'] = ce = srcfilecache['_phasecache']
+        ce.obj.opener = destrepo.svfs