Lock the cache file

ianardap.py

@@ -12,6 +12,7 @@ import datetime
 import json
 import os
 import sys
+import fcntl
 
 IANABASE = "https://data.iana.org/rdap/dns.json"
 CACHE = os.environ["HOME"] + "/.ianardapcache.json" 
@@ -22,7 +23,9 @@ class IanaRDAPDatabase():
     def __init__(self, maxage=MAXAGE, cachefile=CACHE):
         """ Retrieves the IANA databse, if not already cached. maxage is in hours. """
         cache_valid = False
-        # TODO we should lock it
+        self.cachefile = cachefile
+        self.lockname = self.cachefile + ".lock"
+        self.lock()
         if os.path.exists(cachefile) and \
            datetime.datetime.fromtimestamp(os.path.getmtime(cachefile)) >= \
            (datetime.datetime.utcnow() - datetime.timedelta(hours = maxage)):
@@ -30,7 +33,9 @@ class IanaRDAPDatabase():
             content = cache.read()
             cache.close()
             cache_valid = True
+            self.unlock()
         else:
+            self.unlock()
             response = requests.get(IANABASE)
             if response.status_code != 200:
                 raise Exception("Invalid HTTPS return code when trying to get %s: %s" % (IANABASE, response.status_code))
@@ -45,11 +50,20 @@ class IanaRDAPDatabase():
                 for server in service[1]:
                     self.services[tld] = server
         if not cache_valid:
-            # TODO we should lock it
+            self.lock()
             cache = open(cachefile, "wb")
             cache.write(content)
             cache.close()
+            self.unlock()
         
+    def lock(self):
+        self.lockhandle = open(self.lockname, 'w')
+        fcntl.lockf(self.lockhandle, fcntl.LOCK_EX)
+
+    def unlock(self):
+        fcntl.lockf(self.lockhandle, fcntl.LOCK_UN)
+        self.lockhandle.close()
+
     def find(self, domain):
         """ Get the RDAP server for a given domain name. None if there is none."""
         labels = domain.split(".")