[PATCH 1 of 5] transaction: introduce scope

[Jun Wu]

# HG changeset patch
# User Jun Wu <quark at fb.com>
# Date 1488185788 28800
#      Mon Feb 27 00:56:28 2017 -0800
# Node ID 5dac612ec9a87553fcd329100846bcb01bae9d80
# Parent  b4cb86ab4c719eb615a4308eafd8b1386a511eeb
# Available At https://bitbucket.org/quark-zju/hg-draft
#              hg pull https://bitbucket.org/quark-zju/hg-draft -r 5dac612ec9a8
transaction: introduce scope

Previously, transaction tries to cover everything: bookmarks, dirstate,
phaseroots, and branch. It backs them up unconditionally.

That could be inefficient if we still back up the dirstate in a giant repo,
where the transaction is only intended to change bookmarks.

This patch introduces "scope" to transactions, which is a set of things that
the transaction should cover. For bookmark-only update, it could avoid
specifying the "dirstate" scope to be more efficient.

diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -102,5 +102,6 @@ def _playback(journal, report, opener, v
 class transaction(object):
     def __init__(self, report, opener, vfsmap, journalname, undoname=None,
-                 after=None, createmode=None, validator=None, releasefn=None):
+                 after=None, createmode=None, validator=None, releasefn=None,
+                 scope=None):
         """Begin a new transaction
 
@@ -111,4 +112,5 @@ class transaction(object):
         * `createmode`: the mode of the journal file that will be created
         * `releasefn`: called after releasing (with transaction and result)
+        * `scope`: a set-like, nested transaction's scope must be a subset
         """
         self.count = 1
@@ -171,4 +173,7 @@ class transaction(object):
         self._abortcallback = {}
 
+        # a set indicating what's covered
+        self._scope = scope or frozenset()
+
     def __del__(self):
         if self.journal:
@@ -342,5 +347,10 @@ class transaction(object):
 
     @active
-    def nest(self):
+    def nest(self, scope=None):
+        scope = scope or frozenset()
+        if not scope.issubset(self._scope):
+            raise error.ProgrammingError(
+                'nested transaction has a superset scope (%s > %s)'
+                % (scope, self._scope))
         self.count += 1
         self.usages += 1
@@ -555,4 +565,7 @@ class transaction(object):
             self.releasefn(self, False) # notify failure of transaction
 
+    def scope(self):
+        return self._scope
+
 def rollback(opener, vfsmap, file, report):
     """Rolls back the transaction contained in the given file