Add option for using a shared r/o results.cache

This is needed by the CI train, where we

  (1) don't want to cache intermediate results for PPA runs, as they might
      "accidentally" pass in between and fail again for the final silo,

  (2) want to seed britney with the Ubuntu results.cache, to detect regressions
      relative to Ubuntu.

Introduce ADT_SHARED_RESULTS_CACHE option which can point to a path to
results.cache. This will then not be updated by britney.

autopkgtest.py

@@ -76,7 +76,10 @@ class AutoPackageTest(object):
         #   This is also used for tracking the latest seen time stamp for
         #   requesting only newer results.
         self.test_results = {}
-        self.results_cache_file = os.path.join(self.test_state_dir, 'results.cache')
+        if self.britney.options.adt_shared_results_cache:
+            self.results_cache_file = self.britney.options.adt_shared_results_cache
+        else:
+            self.results_cache_file = os.path.join(self.test_state_dir, 'results.cache')
 
         self.swift_container = 'autopkgtest-' + self.series
         if self.britney.options.adt_ppas:
@@ -528,11 +531,12 @@ class AutoPackageTest(object):
                 for (testsrc, _) in self.tests_for_source(src, ver, arch):
                     self.pkg_test_request(testsrc, arch, src + '/' + ver)
 
-        # update the results on-disk cache
-        self.log_verbose('Updating results cache')
-        with open(self.results_cache_file + '.new', 'w') as f:
-            json.dump(self.test_results, f, indent=2)
-        os.rename(self.results_cache_file + '.new', self.results_cache_file)
+        # update the results on-disk cache, unless we are using a r/o shared one
+        if not self.britney.options.adt_shared_results_cache:
+            self.log_verbose('Updating results cache')
+            with open(self.results_cache_file + '.new', 'w') as f:
+                json.dump(self.test_results, f, indent=2)
+            os.rename(self.results_cache_file + '.new', self.results_cache_file)
 
         # update the pending tests on-disk cache
         self.log_verbose('Updated pending requested tests in %s' % self.pending_tests_file)

britney.conf

@@ -72,3 +72,6 @@ ADT_SWIFT_URL     = https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e
 # space separate list of PPAs to add for test requests and for polling results;
 # the *last* one determines the swift container name
 ADT_PPAS          =
+# set this to the path of a (r/o) results.cache for running many parallel
+# britney instances for PPAs without updating the cache
+ADT_SHARED_RESULTS_CACHE =

britney_nobreakall.conf

@@ -69,3 +69,9 @@ ADT_ARCHES        = amd64 i386 armhf ppc64el
 ADT_AMQP          = amqp://test_request:password@162.213.33.228
 # Swift base URL with the results (must be publicly readable and browsable)
 ADT_SWIFT_URL     = https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac
+# space separate list of PPAs to add for test requests and for polling results;
+# the *last* one determines the swift container name
+ADT_PPAS          =
+# set this to the path of a (r/o) results.cache for running many parallel
+# britney instances for PPAs without updating the cache
+ADT_SHARED_RESULTS_CACHE =

tests/test_autopkgtest.py

@@ -1663,6 +1663,51 @@ class T(TestBase):
 
         self.assertFalse(os.path.exists(os.path.join(self.data.path, 'output', 'series', 'output.txt')))
 
+    def test_shared_results_cache(self):
+        '''Run with shared r/o results.cache'''
+
+        # first run to create results.cache
+        self.swift.set_results({'autopkgtest-series': {
+            'series/i386/l/lightgreen/20150101_100000@': (0, 'lightgreen 2', tr('lightgreen/2')),
+            'series/amd64/l/lightgreen/20150101_100000@': (0, 'lightgreen 2', tr('lightgreen/2')),
+        }})
+
+        self.do_test(
+            [('lightgreen', {'Version': '2', 'Depends': 'libc6'}, 'autopkgtest')],
+            {'lightgreen': (True, {'lightgreen 2': {'i386': 'PASS', 'amd64': 'PASS'}})},
+            )
+
+        # move and remember original contents
+        local_path = os.path.join(self.data.path, 'data/series-proposed/autopkgtest/results.cache')
+        shared_path = os.path.join(self.data.path, 'shared_results.cache')
+        os.rename(local_path, shared_path)
+        with open(shared_path) as f:
+            orig_contents = f.read()
+
+        # enable shared cache
+        for line in fileinput.input(self.britney_conf, inplace=True):
+            if 'ADT_SHARED_RESULTS_CACHE' in line:
+                print('ADT_SHARED_RESULTS_CACHE = %s' % shared_path)
+            else:
+                sys.stdout.write(line)
+
+        # second run, should now not update cache
+        self.swift.set_results({'autopkgtest-series': {
+            'series/i386/l/lightgreen/20150101_100100@': (0, 'lightgreen 3', tr('lightgreen/3')),
+            'series/amd64/l/lightgreen/20150101_100100@': (0, 'lightgreen 3', tr('lightgreen/3')),
+        }})
+
+        self.data.remove_all(True)
+        self.do_test(
+            [('lightgreen', {'Version': '3', 'Depends': 'libc6'}, 'autopkgtest')],
+            {'lightgreen': (True, {'lightgreen 3': {'i386': 'PASS', 'amd64': 'PASS'}})},
+            )
+
+        # leaves results.cache untouched
+        self.assertFalse(os.path.exists(local_path))
+        with open(shared_path) as f:
+            self.assertEqual(orig_contents, f.read())
+
 
 if __name__ == '__main__':
     unittest.main()