hgweb: use separate repo instances per thread

Before this change, multiple threads/requests could share a
localrepository instance. This meant that all of localrepository needed
to be thread safe. Many bugs have been reported telling us that
localrepository isn't actually thread safe.

While making localrepository thread safe is a noble cause, it is a lot
of work. And there is little gain from doing so. Due to Python's GIL,
only 1 thread may be processing Python code at a time. The benefits
to multi-threaded servers are marginal.

Thread safety would be a lot of work for little gain. So, we're not
going to even attempt it.

This patch establishes a pool of repos in hgweb. When a request arrives,
we obtain the most recently used repository from the pool or create a
new one if none is available. When the request has finished, we put that
repo back in the pool.

We start with a pool size of 1. For servers using a single thread, the
pool will only ever be of size 1. For multi-threaded servers, the pool
size will grow to the max number of simultaneous requests the server
processes.

No logic for pruning the pool has been implemented. We assume server
operators either limit the number of threads to something they can
handle or restart the Mercurial process after a certain amount of
requests or time has passed.

mercurial/hgweb/hgweb_mod.py

@@ -6,6 +6,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+import contextlib
 import os
 from mercurial import ui, hg, hook, error, encoding, templater, util, repoview
 from mercurial.templatefilters import websub
@@ -208,7 +209,8 b' class hgweb(object):'
         # break some wsgi implementation.
         r.ui.setconfig('progress', 'disable', 'true', 'hgweb')
         r.baseui.setconfig('progress', 'disable', 'true', 'hgweb')
-        self._repo = hg.cachedlocalrepo(self._webifyrepo(r))
+        self._repos = [hg.cachedlocalrepo(self._webifyrepo(r))]
+        self._lastrepo = self._repos[0]
         hook.redirect(True)
         self.reponame = name
 
@@ -217,13 +219,34 b' class hgweb(object):'
         self.websubtable = webutil.getwebsubs(repo)
         return repo
 
-    def _getrepo(self):
-        r, created = self._repo.fetch()
-        if created:
-            r = self._webifyrepo(r)
+    @contextlib.contextmanager
+    def _obtainrepo(self):
+        """Obtain a repo unique to the caller.
+
+        Internally we maintain a stack of cachedlocalrepo instances
+        to be handed out. If one is available, we pop it and return it,
+        ensuring it is up to date in the process. If one is not available,
+        we clone the most recently used repo instance and return it.
 
-        self.mtime = self._repo.mtime
-        return r
+        It is currently possible for the stack to grow without bounds
+        if the server allows infinite threads. However, servers should
+        have a thread limit, thus establishing our limit.
+        """
+        if self._repos:
+            cached = self._repos.pop()
+            r, created = cached.fetch()
+            if created:
+                r = self._webifyrepo(r)
+        else:
+            cached = self._lastrepo.copy()
+            r, created = cached.fetch()
+
+        self._lastrepo = cached
+        self.mtime = cached.mtime
+        try:
+            yield r
+        finally:
+            self._repos.append(cached)
 
     def run(self):
         """Start a server from CGI environment.
@@ -251,7 +274,10 b' class hgweb(object):'
         This is typically only called by Mercurial. External consumers
         should be using instances of this class as the WSGI application.
         """
-        repo = self._getrepo()
+        with self._obtainrepo() as repo:
+            return self._runwsgi(req, repo)
+
+    def _runwsgi(self, req, repo):
         rctx = requestcontext(self, repo)
 
         # This state is global across all threads.

tests/test-hgweb-non-interactive.t

@@ -64,7 +64,8 b' by the WSGI standard and strictly implem'
   > print '---- OS.ENVIRON wsgi variables'
   > print sorted([x for x in os.environ if x.startswith('wsgi')])
   > print '---- request.ENVIRON wsgi variables'
-  > print sorted([x for x in i._getrepo().ui.environ if x.startswith('wsgi')])
+  > with i._obtainrepo() as repo:
+  >     print sorted([x for x in repo.ui.environ if x.startswith('wsgi')])
   > EOF
   $ python request.py
   ---- STATUS