consider page completed after 3 failures

https://github.com/internetarchive/brozzler/pull/183#issuecomment-560562807

"We've had a number of cases where a page kept failing for one reason or
another, and it's bad. We can end up with tons of duplicate captures,
the crawl is not able to make progress, and the overall performance of
the cluster is impacted in cases like yours, where a browser is sitting
there doing nothing for five minutes."

tests/test_units.py

@@ -34,6 +34,7 @@ import socket
 import time
 import sys
 import threading
+from unittest import mock
 
 logging.basicConfig(
         stream=sys.stderr, level=logging.INFO, format=(
@@ -439,3 +440,55 @@ def test_seed_redirect():
         {'ssurt': 'com,foo,//http:/',},
         {'ssurt': 'com,bar,//https:/a/b/c',}]}
 
+def test_limit_failures():
+    page = mock.Mock()
+    page.failed_attempts = None
+    page.brozzle_count = 0
+
+    site = mock.Mock()
+    site.status = 'ACTIVE'
+    site.active_brozzling_time = 0
+    site.starts_and_stops = [{'start':datetime.datetime.utcnow()}]
+
+    rr = mock.Mock()
+    rr.servers = [mock.Mock()]
+    rethink_query = mock.Mock(run=mock.Mock(return_value=[]))
+    rr.db_list = mock.Mock(return_value=rethink_query)
+    rr.table_list = mock.Mock(return_value=rethink_query)
+    rr.table = mock.Mock(
+            return_value=mock.Mock(
+                between=mock.Mock(
+                    return_value=mock.Mock(
+                        limit=mock.Mock(
+                            return_value=rethink_query)))))
+    assert rr.table().between().limit().run() == []
+    frontier = brozzler.RethinkDbFrontier(rr)
+    frontier.enforce_time_limit = mock.Mock()
+    frontier.honor_stop_request = mock.Mock()
+    frontier.claim_page = mock.Mock(return_value=page)
+    frontier._maybe_finish_job = mock.Mock()
+
+    browser = mock.Mock()
+
+    worker = brozzler.BrozzlerWorker(frontier)
+    worker.brozzle_page = mock.Mock(side_effect=Exception)
+
+    assert page.failed_attempts is None
+    assert page.brozzle_count == 0
+    assert site.status == 'ACTIVE'
+
+    worker.brozzle_site(browser, site)
+    assert page.failed_attempts == 1
+    assert page.brozzle_count == 0
+    assert site.status == 'ACTIVE'
+
+    worker.brozzle_site(browser, site)
+    assert page.failed_attempts == 2
+    assert page.brozzle_count == 0
+    assert site.status == 'ACTIVE'
+
+    worker.brozzle_site(browser, site)
+    assert page.failed_attempts == 3
+    assert page.brozzle_count == 1
+    assert site.status == 'FINISHED'
+