--- a/changes/hgpoller.py
+++ b/changes/hgpoller.py
@@ -228,16 +228,17 @@ class BaseHgPoller(BasePoller):
             hgURL = hgURL[:-1]
         fragments = [hgURL, branch]
         if tree is not None:
             fragments.append(tree)
         self.baseURL = "/".join(fragments)
         self.pushlogUrlOverride = pushlogUrlOverride
         self.tipsOnly = tipsOnly
         self.lastChangeset = None
+        self.lastPushID = None
         self.startLoad = 0
         self.loadTime = None
         self.repo_branch = repo_branch
         self.maxChanges = maxChanges
         # With mergePushChanges=True we get one buildbot change per push to hg.
         # The files from all changes in the push will be accumulated in the buildbot change
         # and the comments of tipmost change of the push will be used
         self.mergePushChanges = mergePushChanges
@@ -286,20 +287,47 @@ class BaseHgPoller(BasePoller):
 
     def processData(self, query):
         push_data = parse_pushlog_json(query)
 
         # The payload tells us the most recent push ID. If it is the empty
         # string, the pushlog is empty and there is no data to consume.
         if not push_data['lastpushid']:
             self.emptyRepo = True
+            self.lastPushID = None
             if self.verbose:
                 log.msg('%s is empty' % self.baseURL)
             return
 
+        # If nothing has changed and we're fully caught up, the remote
+        # lastpushid will be the same as self.lastPushID.
+        #
+        # If the remote lastpushid is less than a previously observed value,
+        # this could mean one of the following:
+        #
+        #    a) Data from the pushlog was removed (perhaps the repo was
+        #       stripped)
+        #    b) The repo/pushlog was reset.
+        #
+        # These scenarios should be rare. In both of them, our assumption
+        # about the behavior of the pushlog always being monotonically
+        # increasing have been invalidated. So we reset state and start
+        # again.
+        #
+        # It's worth noting that a reset repo's pushlog could have *more*
+        # entries than the former repo. In this case, this code will fail
+        # to detect a reset repo from the pushlog alone.
+        if self.lastPushID and push_data['lastpushid'] < self.lastPushID:
+            self.emptyRepo = False
+            self.lastPushID = None
+            self.lastChangeset = None
+            log.msg('%s appears to have been reset; clearing state' %
+                    self.baseURL)
+            return
+
         # We want to add at most self.maxChanges changes per push. If
         # mergePushChanges is True, then we'll get up to maxChanges pushes,
         # each with up to maxChanges changes.
         # Go through the list of pushes backwards, since we want to keep the
         # latest ones and possibly discard earlier ones.
         change_list = []
         too_many = False
         for push in reversed(push_data['pushes']):
@@ -416,16 +444,17 @@ class BaseHgPoller(BasePoller):
                 self.parent.addChange(c)
 
         # The repository isn't empty any more!
         self.emptyRepo = False
         # Use the last change found by the poller, regardless of if it's on our
         # branch or not. This is so we don't have to constantly ignore it in
         # future polls.
         self.lastChangeset = push_data['pushes'][-1]['changesets'][-1]['node']
+        self.lastPushID = push_data['pushes'][-1]['pushid']
         if self.verbose:
             log.msg("last changeset %s on %s" %
                     (self.lastChangeset, self.baseURL))
 
     def changeHook(self, change):
         pass
 
 

--- a/test/test_hgpoller.py
+++ b/test/test_hgpoller.py
@@ -337,16 +337,57 @@ class EmptyLastPushID(PollingTest):
         {
             "lastpushid": "",
             "pushes": {}
         }
         """)
         self.assertTrue(poller.emptyRepo, 'repo marked as empty')
 
 
+class PushlogReset(PollingTest):
+    def testDecreasingLastPushID(self):
+        poller = self.doTest(data=validPushlog)
+        self.assertEqual(poller.lastPushID, 15227)
+
+        poller.processData("""
+        {
+            "lastpushid": 15225,
+            "pushes": {
+                "15200": {
+                    "changesets": [
+                        {
+                            "author": "Jim Chen <jchen@mozilla.com>",
+                            "branch": "GECKO20b5pre_20100820_RELBRANCH",
+                            "desc": "Bug 588456 - Properly commit Android IME composition on blur; r=mwu a=blocking-fennec",
+                            "files": [
+                                "embedding/android/GeckoInputConnection.java",
+                                "embedding/android/GeckoSurfaceView.java",
+                                "widget/src/android/nsWindow.cpp",
+                                "widget/src/android/nsWindow.h"
+                            ],
+                            "node": "4c23e51a484f077ea27af3ea4a4ee13da5aeb5e6",
+                            "parents": [
+                                "935c15d506516a2269cee35a1a80748aaec1ae08"
+                            ],
+                            "tags": []
+                        }
+                    ],
+                    "date": 1282358416,
+                    "user": "dougt@mozilla.com"
+                }
+            }
+        }
+        """)
+        self.assertIsNone(poller.lastPushID, 'last push ID should be None')
+        self.assertIsNone(poller.lastChangeset, 'last changeset should be None')
+        self.assertEqual(poller._make_url(),
+                         'http://localhost/whatever/json-pushes?version=2&full=1',
+                         'pushlog URL should start from the end')
+
+
 class RepoBranchHandling(PollingTest):
     def testNoRepoBranch(self):
         self.doTest(repo_branch=None)
 
         self.assertEquals(len(self.changes), 2)
 
     def testDefaultRepoBranch(self):
         self.doTest(repo_branch='default')