Fix check for whether /tracker/data/json needs to be recomputed

In commit 49e287d2574ed385e31ce3d36cb3cb19f4c7785f "tracker_service:
send a Last-Modified header for /tracker/data/json" I made the
json_timestamp update conditional on whether the data had changed.  That
meant that after a few minutes, we would consider the data to be always
stale, and recompute it every single time, as long as it didn't change.

To fix this, use separate timestamps for "when did we check last"
(json_timestamp) and "when did it change last" (json_last_modified).

1 files changed, 4 insertions, 2 deletions

diff --git a/bin/tracker_service.py b/bin/tracker_service.py
index 6dda2d48d0..38e01022ae 100755
--- a/bin/tracker_service.py
+++ b/bin/tracker_service.py
@@ -137,6 +137,7 @@ class TrackerService(webservice_base_class):
         self.db = security_db.DB(db_name)
         self.json_data = None # the JSON dump itself
         self.json_timestamp = None # timestamp of JSON generation
+        self.json_last_modified = None
         self.register('', self.page_home)
         self.register('*', self.page_object)
         self.register('redirect/*', self.page_redirect)
@@ -1452,14 +1453,15 @@ Debian bug number.'''),
         # store the JSON dump in memory, and update the generation
         # timestamp before returning
         new_data = json.dumps(data, separators=(',', ':'))
+        self.json_timestamp = time.time()
         if new_data != self.json_data:
             self.json_data = new_data
-            self.json_timestamp = time.time()
+            self.json_last_modified = self.json_timestamp
         return self.json_data
 
     def page_json(self, path, params, url):
         result = BinaryResult(self._get_json(),'application/json')
-        result.headers['Last-Modified'] = email.utils.formatdate(self.json_timestamp, usegmt=True)
+        result.headers['Last-Modified'] = email.utils.formatdate(self.json_last_modified, usegmt=True)
         return result
 
     def page_debsecan(self, path, params, url):