revlog: change generaldelta delta parent heuristic
The old generaldelta heuristic was "if p1 (or p2) was closer than the last full text,
use it, otherwise use prev". This was problematic when a repo contained multiple
branches that were very different. If commits to branch A were pushed, and the
last full text was branch B, it would generate a fulltext. Then if branch B was
pushed, it would generate another fulltext. The problem is that the last
fulltext (and delta'ing against `prev` in general) has no correlation with the
contents of the incoming revision, and therefore will always have degenerate
cases.
According to the blame, that algorithm was chosen to minimize the chain length.
Since there is already code that protects against that (the delta-vs-fulltext
code), and since it has been improved since the original generaldelta algorithm
went in (2011), I believe the chain length criteria will still be preserved.
The new algorithm always diffs against p1 (or p2 if it's closer), unless the
resulting delta will fail the delta-vs-fulltext check, in which case we delta
against prev.
Some before and after stats on manifest.d size.
internal large repo
old heuristic - 2.0 GB
new heuristic - 1.2 GB
mozilla-central
old heuristic - 242 MB
new heuristic - 261 MB
The regression in mozilla central is due to the new heuristic choosing p2r as
the delta when it's closer to the tip. Switching the algorithm to always prefer
p1r brings the size back down (242 MB). This is result of the way in which
mozilla does merges and pushes, and the result could easily swing the other
direction in other repos (depending on if they merge X into Y or Y into X), but
will never be as degenerate as before.
I future patch will address the regression by introducing an optional, even more
aggressive delta heuristic which will knock the mozilla manifest size down
dramatically.
# error.py - Mercurial exceptions
#
# Copyright 2005-2008 Matt Mackall <mpm@selenic.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
"""Mercurial exceptions.
This allows us to catch exceptions at higher levels without forcing
imports.
"""
from __future__ import absolute_import
# Do not import anything here, please
class HintException(Exception):
def __init__(self, *args, **kw):
Exception.__init__(self, *args)
self.hint = kw.get('hint')
class RevlogError(HintException):
pass
class FilteredIndexError(IndexError):
pass
class LookupError(RevlogError, KeyError):
def __init__(self, name, index, message):
self.name = name
self.index = index
# this can't be called 'message' because at least some installs of
# Python 2.6+ complain about the 'message' property being deprecated
self.lookupmessage = message
if isinstance(name, str) and len(name) == 20:
from .node import short
name = short(name)
RevlogError.__init__(self, '%s@%s: %s' % (index, name, message))
def __str__(self):
return RevlogError.__str__(self)
class FilteredLookupError(LookupError):
pass
class ManifestLookupError(LookupError):
pass
class CommandError(Exception):
"""Exception raised on errors in parsing the command line."""
class InterventionRequired(Exception):
"""Exception raised when a command requires human intervention."""
class Abort(HintException):
"""Raised if a command needs to print an error and exit."""
pass
class HookAbort(Abort):
"""raised when a validation hook fails, aborting an operation
Exists to allow more specialized catching."""
pass
class ConfigError(Abort):
"""Exception raised when parsing config files"""
class OutOfBandError(Exception):
"""Exception raised when a remote repo reports failure"""
def __init__(self, *args, **kw):
Exception.__init__(self, *args)
self.hint = kw.get('hint')
class ParseError(Exception):
"""Raised when parsing config files and {rev,file}sets (msg[, pos])"""
class UnknownIdentifier(ParseError):
"""Exception raised when a {rev,file}set references an unknown identifier"""
def __init__(self, function, symbols):
from .i18n import _
ParseError.__init__(self, _("unknown identifier: %s") % function)
self.function = function
self.symbols = symbols
class RepoError(HintException):
pass
class RepoLookupError(RepoError):
pass
class FilteredRepoLookupError(RepoLookupError):
pass
class CapabilityError(RepoError):
pass
class RequirementError(RepoError):
"""Exception raised if .hg/requires has an unknown entry."""
pass
class LockError(IOError):
def __init__(self, errno, strerror, filename, desc):
IOError.__init__(self, errno, strerror, filename)
self.desc = desc
class LockHeld(LockError):
def __init__(self, errno, filename, desc, locker):
LockError.__init__(self, errno, 'Lock held', filename, desc)
self.locker = locker
class LockUnavailable(LockError):
pass
class ResponseError(Exception):
"""Raised to print an error with part of output and exit."""
class UnknownCommand(Exception):
"""Exception raised if command is not in the command table."""
class AmbiguousCommand(Exception):
"""Exception raised if command shortcut matches more than one command."""
# derived from KeyboardInterrupt to simplify some breakout code
class SignalInterrupt(KeyboardInterrupt):
"""Exception raised on SIGTERM and SIGHUP."""
class SignatureError(Exception):
pass
class PushRaced(RuntimeError):
"""An exception raised during unbundling that indicate a push race"""
# bundle2 related errors
class BundleValueError(ValueError):
"""error raised when bundle2 cannot be processed"""
class UnsupportedPartError(BundleValueError):
def __init__(self, parttype=None, params=()):
self.parttype = parttype
self.params = params
if self.parttype is None:
msg = 'Stream Parameter'
else:
msg = parttype
if self.params:
msg = '%s - %s' % (msg, ', '.join(self.params))
ValueError.__init__(self, msg)
class ReadOnlyPartError(RuntimeError):
"""error raised when code tries to alter a part being generated"""
pass
class PushkeyFailed(Abort):
"""error raised when a pushkey part failed to update a value"""
def __init__(self, partid, namespace=None, key=None, new=None, old=None,
ret=None):
self.partid = partid
self.namespace = namespace
self.key = key
self.new = new
self.old = old
self.ret = ret
# no i18n expected to be processed into a better message
Abort.__init__(self, 'failed to update value for "%s/%s"'
% (namespace, key))
class CensoredNodeError(RevlogError):
"""error raised when content verification fails on a censored node
Also contains the tombstone data substituted for the uncensored data.
"""
def __init__(self, filename, node, tombstone):
from .node import short
RevlogError.__init__(self, '%s:%s' % (filename, short(node)))
self.tombstone = tombstone
class CensoredBaseError(RevlogError):
"""error raised when a delta is rejected because its base is censored
A delta based on a censored revision must be formed as single patch
operation which replaces the entire base with new content. This ensures
the delta may be applied by clones which have not censored the base.
"""