Implement leap-second caching. Not tested as root yet.

1 files changed, 51 insertions, 3 deletions

diff --git a/leapsecond.py b/leapsecond.py
index 565ad5bb..3148687e 100755
--- a/leapsecond.py
+++ b/leapsecond.py
@@ -4,7 +4,7 @@
 # GPS time, which changes occasionally due to variations in the Earth's
 # rotation.
 #
-import urllib, re, random
+import os, urllib, re, random, time, calendar, stat
 
 __locations = [
     (
@@ -23,6 +23,11 @@ __locations = [
     ),
 ]
 
+# File contasining cached offset data.
+# Two fields: the offset, and the start of the current six-month span
+# between times it might change, in seconds since Unix epoch GMT.
+__cachepath = "/var/run/leapsecond"
+
 def retrieve():
     "Retrieve current leap-second from Web sources."
     random.shuffle(__locations)	# To spread the load
@@ -37,7 +42,50 @@ def retrieve():
         except:
             pass
     else:
-        raise ValueError
+        return None
+
+def last_insertion_time():
+    "Give last potential insertion time for a leap second."
+    # We need the Unix times for midnights Jan 1 and Jul 1 this year.
+    when = time.gmtime()
+    when.tm_mday = 1
+    when.tm_hour = when.tm_min = when.tm_sec = 0
+    when.tm_mon = 1; jan = int(calendar.timegm(when))
+    when.tm_mon = 7; jul = int(calendar.timegm(when))
+    # We have the UTC times of the potential insertion points this year.
+    now = time()
+    if now > jul:
+        return jul
+    else:
+        return jan
+
+def get():
+    "Fetch GPS offset, from local cache file if possible."
+    stale = False
+    last_insertion = last_insertion_time()
+    if not os.path.exists(__cachepath):
+        stale = True
+    else:
+        cfp = open(__cachepath)
+        txt = cfp.read()
+        cfp.close()
+    (offset, valid_from) = map(int, txt.split())
+    if valid_from < last_insertion:
+        stale = True
+    # We now know whether the cached data is stale
+    if not stale:
+        return offset
+    else:
+        current_offset = retrieve()
+        # Try to cache this for later
+        if current_offset != None:
+            try:
+                cfp = open(__cachepath, "w")
+                cfp.write("%d %d\n" % (offset, last_insertion))
+                cfp.close()
+            except (IOError, OSError):
+                pass
+        return current_offset
 
 if __name__ == '__main__':
-    print "GPS offset is: %d" % retrieve()
+    print retrieve()