view mercurial/filelog.py @ 29322:66dbdd3cc2b9 stable

bdiff: extend matches across popular lines

For very large diffs that have large numbers of identical lines (JSON
dumps) that also have large blocks of identical text, bdiff could become
confused about which block matches which because it can only match
very limited regions. The result is very large diffs for small sets of edits.

The earlier recursion rebalancing fix made this behavior more frequent because
it's now more prone to match block 1 to block 2. One frequent user of
large JSON files reported being unable to pass the resulting diffs
through their code review system.

Prior to this change, bdiff would calculate the length of a match at
(i, j) as 1 + length found at (i-1, j-1). With large number of popular
(ignored) lines, this often meant matches couldn't be extended
backwards at all and thus all matching regions were very small.
Disabling the popularity threshold is not an option because it brings
back quadratic behavior.

Instead, we extend a match backwards until we either found a previously
discovered match or we find a mismatching line. This thus successfully
bridges over any popular lines inside and before a matching region.
The larger regions then significant reduce the probability of confusion.

# filelog.py - file history class for mercurial
#
# Copyright 2005-2007 Matt Mackall <[e-mail]>
#
# 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

import re
import struct

from . import (
    error,
    mdiff,
    revlog,
)

_mdre = re.compile('\1\n')
def parsemeta(text):
    """return (metadatadict, keylist, metadatasize)"""
    # text can be buffer, so we can't use .startswith or .index
    if text[:2] != '\1\n':
        return None, None
    s = _mdre.search(text, 2).start()
    mtext = text[2:s]
    meta = {}
    for l in mtext.splitlines():
        k, v = l.split(": ", 1)
        meta[k] = v
    return meta, (s + 2)

def packmeta(meta, text):
    keys = sorted(meta.iterkeys())
    metatext = "".join("%s: %s\n" % (k, meta[k]) for k in keys)
    return "\1\n%s\1\n%s" % (metatext, text)

def _censoredtext(text):
    m, offs = parsemeta(text)
    return m and "censored" in m

class filelog(revlog.revlog):
    def __init__(self, opener, path):
        super(filelog, self).__init__(opener,
                        "/".join(("data", path + ".i")))

    def read(self, node):
        t = self.revision(node)
        if not t.startswith('\1\n'):
            return t
        s = t.index('\1\n', 2)
        return t[s + 2:]

    def add(self, text, meta, transaction, link, p1=None, p2=None):
        if meta or text.startswith('\1\n'):
            text = packmeta(meta, text)
        return self.addrevision(text, transaction, link, p1, p2)

    def renamed(self, node):
        if self.parents(node)[0] != revlog.nullid:
            return False
        t = self.revision(node)
        m = parsemeta(t)[0]
        if m and "copy" in m:
            return (m["copy"], revlog.bin(m["copyrev"]))
        return False

    def size(self, rev):
        """return the size of a given revision"""

        # for revisions with renames, we have to go the slow way
        node = self.node(rev)
        if self.renamed(node):
            return len(self.read(node))
        if self.iscensored(rev):
            return 0

        # XXX if self.read(node).startswith("\1\n"), this returns (size+4)
        return super(filelog, self).size(rev)

    def cmp(self, node, text):
        """compare text with a given file revision

        returns True if text is different than what is stored.
        """

        t = text
        if text.startswith('\1\n'):
            t = '\1\n\1\n' + text

        samehashes = not super(filelog, self).cmp(node, t)
        if samehashes:
            return False

        # censored files compare against the empty file
        if self.iscensored(self.rev(node)):
            return text != ''

        # renaming a file produces a different hash, even if the data
        # remains unchanged. Check if it's the case (slow):
        if self.renamed(node):
            t2 = self.read(node)
            return t2 != text

        return True

    def checkhash(self, text, p1, p2, node, rev=None):
        try:
            super(filelog, self).checkhash(text, p1, p2, node, rev=rev)
        except error.RevlogError:
            if _censoredtext(text):
                raise error.CensoredNodeError(self.indexfile, node, text)
            raise

    def iscensored(self, rev):
        """Check if a file revision is censored."""
        return self.flags(rev) & revlog.REVIDX_ISCENSORED

    def _peek_iscensored(self, baserev, delta, flush):
        """Quickly check if a delta produces a censored revision."""
        # Fragile heuristic: unless new file meta keys are added alphabetically
        # preceding "censored", all censored revisions are prefixed by
        # "\1\ncensored:". A delta producing such a censored revision must be a
        # full-replacement delta, so we inspect the first and only patch in the
        # delta for this prefix.
        hlen = struct.calcsize(">lll")
        if len(delta) <= hlen:
            return False

        oldlen = self.rawsize(baserev)
        newlen = len(delta) - hlen
        if delta[:hlen] != mdiff.replacediffheader(oldlen, newlen):
            return False

        add = "\1\ncensored:"
        addlen = len(add)
        return newlen >= addlen and delta[hlen:hlen + addlen] == add