util: add base class for transactional context managers

We have at least three types with a close() and a release() method
where the close() method is supposed to be called on success and the
release() method is supposed to be called last, whether successful or
not. Two of them (transaction and dirstateguard) already have
identical implementations of __enter__ and __exit__. Let's extract a
base class for this, so we reuse the code and so the third type
(transactionmanager) can also be used as a context manager.

Differential Revision: https://phab.mercurial-scm.org/D392

mercurial/dirstateguard.py

@@ -11,9 +11,10 b' from .i18n import _'
 
 from . import (
     error,
+    util,
 )
 
-class dirstateguard(object):
+class dirstateguard(util.transactional):
     '''Restore dirstate at unexpected failure.
 
     At the construction, this class does:
@@ -43,16 +44,6 b' class dirstateguard(object):'
             # ``release(tr, ....)``.
             self._abort()
 
-    def __enter__(self):
-        return self
-
-    def __exit__(self, exc_type, exc_val, exc_tb):
-        try:
-            if exc_type is None:
-                self.close()
-        finally:
-            self.release()
-
     def close(self):
         if not self._active: # already inactivated
             msg = (_("can't close already inactivated backup: %s")

mercurial/exchange.py

@@ -1168,7 +1168,7 b' class pulloperation(object):'
         # deprecated; talk to trmanager directly
         return self.trmanager.transaction()
 
-class transactionmanager(object):
+class transactionmanager(util.transactional):
     """An object to manage the life cycle of a transaction
 
     It creates the transaction on demand and calls the appropriate hooks when

mercurial/transaction.py

@@ -101,7 +101,7 b' def _playback(journal, report, opener, v'
         # only pure backup file remains, it is sage to ignore any error
         pass
 
-class transaction(object):
+class transaction(util.transactional):
     def __init__(self, report, opener, vfsmap, journalname, undoname=None,
                  after=None, createmode=None, validator=None, releasefn=None,
                  checkambigfiles=None):
@@ -376,16 +376,6 b' class transaction(object):'
         if self.count > 0 and self.usages == 0:
             self._abort()
 
-    def __enter__(self):
-        return self
-
-    def __exit__(self, exc_type, exc_val, exc_tb):
-        try:
-            if exc_type is None:
-                self.close()
-        finally:
-            self.release()
-
     def running(self):
         return self.count > 0
 

mercurial/util.py

@@ -15,6 +15,7 b' hide platform-specific details from the '
 
 from __future__ import absolute_import
 
+import abc
 import bz2
 import calendar
 import codecs
@@ -592,6 +593,31 b' class sortdict(collections.OrderedDict):'
             for k, v in src:
                 self[k] = v
 
+class transactional(object):
+    """Base class for making a transactional type into a context manager."""
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def close(self):
+        """Successfully closes the transaction."""
+
+    @abc.abstractmethod
+    def release(self):
+        """Marks the end of the transaction.
+
+        If the transaction has not been closed, it will be aborted.
+        """
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        try:
+            if exc_type is None:
+                self.close()
+        finally:
+            self.release()
+
 @contextlib.contextmanager
 def acceptintervention(tr=None):
     """A context manager that closes the transaction on InterventionRequired