backend.py: Introduce protocol for alternative locking schemes.

It sounds fancier than it is.  There's now a method for FlatFileBackend
subclasses to override if they want to apply different locking
semantics.

diff --git a/backend.py b/backend.py
index 576486c..fda32e0 100644 (file)
--- a/backend.py
+++ b/backend.py
@@ -25,6 +25,8 @@
 
 from __future__ import with_statement
 
 
 from __future__ import with_statement
 

+from auto import HOME
+import errno as E

 import itertools as I
 import os as OS; ENV = OS.environ
 
 import itertools as I
 import os as OS; ENV = OS.environ
 


@@ -206,10 +208,13 @@ class FlatFileBackend (object):
   specified by the DELIM constructor argument.
 
   The file is updated by writing a new version alongside, as `FILE.new', and
   specified by the DELIM constructor argument.
 
   The file is updated by writing a new version alongside, as `FILE.new', and

-  renaming it over the old version.  If a LOCK file is named then an
-  exclusive fcntl(2)-style lock is taken out on `LOCKDIR/LOCK' (creating the
-  file if necessary) during the update operation.  Use of a lockfile is
-  strongly recommended.
+  renaming it over the old version.  If a LOCK is provided then this is done
+  while holding a lock.  By default, an exclusive fcntl(2)-style lock is
+  taken out on `LOCKDIR/LOCK' (creating the file if necessary) during the
+  update operation, but subclasses can override the `dolocked' method to
+  provide alternative locking behaviour; the LOCK parameter is not
+  interpreted by any other methods.  Use of a lockfile is strongly
+  recommended.

 
   The DELIM constructor argument specifies the delimiter character used when
   splitting lines into fields.  The USER and PASSWD arguments give the field
 
   The DELIM constructor argument specifies the delimiter character used when
   splitting lines into fields.  The USER and PASSWD arguments give the field


@@ -329,11 +334,21 @@ class FlatFileBackend (object):
 
     ## If there's a lockfile, then acquire it around the meat of this
     ## function; otherwise just do the job.
 
     ## If there's a lockfile, then acquire it around the meat of this
     ## function; otherwise just do the job.

-    if me._lock is None:
-      doit()
-    else:
-      with U.lockfile(OS.path.join(CFG.LOCKDIR, me._lock), 5):
-        doit()
+    if me._lock is None: doit()
+    else: me.dolocked(me._lock, doit)
+
+  def dolocked(me, lock, func):
+    """
+    Call FUNC with the LOCK held.
+
+    Subclasses can override this method in order to provide alternative
+    locking functionality.
+    """
+    try: OS.mkdir(CFG.LOCKDIR)
+    except OSError, e:
+      if e.errno != E.EEXIST: raise
+    with U.lockfile(OS.path.join(CFG.LOCKDIR, lock), 5):
+      func()

 
   def _parse(me, line):
     """Convenience function for constructing a record."""
 
   def _parse(me, line):
     """Convenience function for constructing a record."""