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localrepo: iteratively derive local repository type...
localrepo: iteratively derive local repository type This commit implements the dynamic local repository type derivation that was explained in the recent commit bfeab472e3c0 "localrepo: create new function for instantiating a local repo object." Instead of a static localrepository class/type which must be customized after construction, we now dynamically construct a type by building up base classes/types to represent specific repository interfaces. Conceptually, the end state is similar to what was happening when various extensions would monkeypatch the __class__ of newly-constructed repo instances. However, the approach is inverted. Instead of making the instance then customizing it, we do the customization up front by influencing the behavior of the type then we instantiate that custom type. This approach gives us much more flexibility. For example, we can use completely separate classes for implementing different aspects of the repository. For example, we could have one class representing revlog-based file storage and another representing non-revlog based file storage. When then choose which implementation to use based on the presence of repo requirements. A concern with this approach is that it creates a lot more types and complexity and that complexity adds overhead. Yes, it is true that this approach will result in more types being created. Yes, this is more complicated than traditional "instantiate a static type." However, I believe the alternatives to supporting alternate storage backends are just as complicated. (Before I arrived at this solution, I had patches storing factory functions on local repo instances for e.g. constructing a file storage instance. We ended up having a handful of these. And this was logically identical to assigning custom methods. Since we were logically changing the type of the instance, I figured it would be better to just use specialized types instead of introducing levels of abstraction at run-time.) On the performance front, I don't believe that having N base classes has any significant performance overhead compared to just a single base class. Intuition says that Python will need to iterate the base classes to find an attribute. However, CPython caches method lookups: as long as the __class__ or MRO isn't changing, method attribute lookup should be constant time after first access. And non-method attributes are stored in __dict__, of which there is only 1 per object, so the number of base classes for __dict__ is irrelevant. Anyway, this commit splits up the monolithic completelocalrepository interface into sub-interfaces: 1 for file storage and 1 representing everything else. We've taught ``makelocalrepository()`` to call a series of factory functions which will produce types implementing specific interfaces. It then calls type() to create a new type from the built-up list of base types. This commit should be considered a start and not the end state. I suspect we'll hit a number of problems as we start to implement alternate storage backends: * Passing custom arguments to __init__ and setting custom attributes on __dict__. * Customizing the set of interfaces that are needed. e.g. the "readonly" intent could translate to not requesting an interface providing methods related to writing. * More ergonomic way for extensions to insert themselves so their callbacks aren't unconditionally called. * Wanting to modify vfs instances, other arguments passed to __init__. That being said, this code is usable in its current state and I'm convinced future commits will demonstrate the value in this approach. Differential Revision: https://phab.mercurial-scm.org/D4642

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narrowrevlog.py
80 lines | 3.0 KiB | text/x-python | PythonLexer
# narrowrevlog.py - revlog storing irrelevant nodes as "ellipsis" nodes
#
# Copyright 2017 Google, Inc.
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
from __future__ import absolute_import
from mercurial import (
revlog,
util,
)
def readtransform(self, text):
return text, False
def writetransform(self, text):
return text, False
def rawtransform(self, text):
return False
revlog.addflagprocessor(revlog.REVIDX_ELLIPSIS,
(readtransform, writetransform, rawtransform))
def setup():
# We just wanted to add the flag processor, which is done at module
# load time.
pass
def makenarrowfilelog(fl, narrowmatch):
class narrowfilelog(fl.__class__):
def renamed(self, node):
# Renames that come from outside the narrowspec are
# problematic at least for git-diffs, because we lack the
# base text for the rename. This logic was introduced in
# 3cd72b1 of narrowhg (authored by martinvonz, reviewed by
# adgar), but that revision doesn't have any additional
# commentary on what problems we can encounter.
m = super(narrowfilelog, self).renamed(node)
if m and not narrowmatch(m[0]):
return None
return m
def size(self, rev):
# We take advantage of the fact that remotefilelog
# lacks a node() method to just skip the
# rename-checking logic when on remotefilelog. This
# might be incorrect on other non-revlog-based storage
# engines, but for now this seems to be fine.
#
# TODO: when remotefilelog is in core, improve this to
# explicitly look for remotefilelog instead of cheating
# with a hasattr check.
if util.safehasattr(self, 'node'):
node = self.node(rev)
# Because renamed() is overridden above to
# sometimes return None even if there is metadata
# in the revlog, size can be incorrect for
# copies/renames, so we need to make sure we call
# the super class's implementation of renamed()
# for the purpose of size calculation.
if super(narrowfilelog, self).renamed(node):
return len(self.read(node))
return super(narrowfilelog, self).size(rev)
def cmp(self, node, text):
different = super(narrowfilelog, self).cmp(node, text)
if different:
# Similar to size() above, if the file was copied from
# a file outside the narrowspec, the super class's
# would have returned True because we tricked it into
# thinking that the file was not renamed.
if super(narrowfilelog, self).renamed(node):
t2 = self.read(node)
return t2 != text
return different
fl.__class__ = narrowfilelog