add a fairly basic file locking mechanism with src/LockFile.py

closes #96

1 files changed, 40 insertions, 0 deletions

diff --git a/src/LockFile.py b/src/LockFile.py
new file mode 100644
index 00000000..9f53b8d5
--- /dev/null
+++ b/src/LockFile.py
@@ -0,0 +1,40 @@
+import datetime, os
+from src import PollHook, utils
+
+EXPIRATION = 60 # 1 minute
+
+class LockFile(PollHook.PollHook):
+    def __init__(self, database_location: str):
+        self._database_location = database_location
+        self._lock_location = "%s.lock" % database_location
+        self._next_lock = None
+
+    def available(self):
+        now = utils.datetime_utcnow()
+        if os.path.exists(self._lock_location):
+            with open(self._lock_location, "r") as lock_file:
+                timestamp_str = lock_file.read().strip().split(" ", 1)[0]
+
+            timestamp = utils.iso8601_parse(timestamp_str)
+
+            if (now-timestamp).total_seconds() < EXPIRATION:
+                return False
+
+        return True
+
+    def lock(self):
+        with open(self._lock_location, "w") as lock_file:
+            last_lock = utils.datetime_utcnow()
+            lock_file.write("%s" % utils.iso8601_format(last_lock))
+            self._next_lock = last_lock+datetime.timedelta(
+                seconds=EXPIRATION/2)
+
+    def next(self):
+        return max(0, (self._next_lock-utils.datetime_utcnow()).total_seconds())
+    def call(self):
+        if self.next() == 0:
+            self.lock()
+
+    def unlock(self):
+        if os.path.isfile(self._lock_location):
+            os.remove(self._lock_location)