tests/test-bdiff.py

cborutil: change buffering strategy

Profiling revealed that we were spending a lot of time on the
line that was concatenating the old buffer with the incoming data
when attempting to decode long byte strings, such as manifest
revisions.

Essentially, we were feeding N chunks of size len(X) << len(Y) into
decode() and continuously allocating a new, larger buffer to hold
the undecoded input. This created substantial memory churn and
slowed down execution.

Changing the code to aggregate pending chunks in a list until we
have enough data to fully decode the next atom makes things much
more efficient.

I don't have exact data, but I recall the old code spending >1s
on manifest fulltexts from the mozilla-unified repo. The new code
doesn't significantly appear in profile output.

Differential Revision: https://phab.mercurial-scm.org/D4854

from __future__ import absolute_import, print_function
import collections
import struct
import unittest

from mercurial import (
    mdiff,
)

class diffreplace(
    collections.namedtuple('diffreplace', 'start end from_ to')):
    def __repr__(self):
        return 'diffreplace(%r, %r, %r, %r)' % self

class BdiffTests(unittest.TestCase):

    def assert_bdiff_applies(self, a, b):
        d = mdiff.textdiff(a, b)
        c = a
        if d:
            c = mdiff.patches(a, [d])
        self.assertEqual(
            c, b, ("bad diff+patch result from\n  %r to\n  "
                   "%r: \nbdiff: %r\npatched: %r" % (a, b, d, c[:200])))

    def assert_bdiff(self, a, b):
        self.assert_bdiff_applies(a, b)
        self.assert_bdiff_applies(b, a)

    def test_bdiff_basic(self):
        cases = [
            (b"a\nc\n\n\n\n", b"a\nb\n\n\n"),
            (b"a\nb\nc\n", b"a\nc\n"),
            (b"", b""),
            (b"a\nb\nc", b"a\nb\nc"),
            (b"a\nb\nc\nd\n", b"a\nd\n"),
            (b"a\nb\nc\nd\n", b"a\nc\ne\n"),
            (b"a\nb\nc\n", b"a\nc\n"),
            (b"a\n", b"c\na\nb\n"),
            (b"a\n", b""),
            (b"a\n", b"b\nc\n"),
            (b"a\n", b"c\na\n"),
            (b"", b"adjfkjdjksdhfksj"),
            (b"", b"ab"),
            (b"", b"abc"),
            (b"a", b"a"),
            (b"ab", b"ab"),
            (b"abc", b"abc"),
            (b"a\n", b"a\n"),
            (b"a\nb", b"a\nb"),
        ]
        for a, b in cases:
            self.assert_bdiff(a, b)

    def showdiff(self, a, b):
        bin = mdiff.textdiff(a, b)
        pos = 0
        q = 0
        actions = []
        while pos < len(bin):
            p1, p2, l = struct.unpack(">lll", bin[pos:pos + 12])
            pos += 12
            if p1:
                actions.append(a[q:p1])
            actions.append(diffreplace(p1, p2, a[p1:p2], bin[pos:pos + l]))
            pos += l
            q = p2
        if q < len(a):
            actions.append(a[q:])
        return actions

    def test_issue1295(self):
        cases = [
            (b"x\n\nx\n\nx\n\nx\n\nz\n", b"x\n\nx\n\ny\n\nx\n\nx\n\nz\n",
             [b'x\n\nx\n\n',
              diffreplace(6, 6, b'', b'y\n\n'),
              b'x\n\nx\n\nz\n']),
            (b"x\n\nx\n\nx\n\nx\n\nz\n", b"x\n\nx\n\ny\n\nx\n\ny\n\nx\n\nz\n",
             [b'x\n\nx\n\n',
              diffreplace(6, 6, b'', b'y\n\n'),
              b'x\n\n',
              diffreplace(9, 9, b'', b'y\n\n'),
              b'x\n\nz\n']),
        ]
        for old, new, want in cases:
            self.assertEqual(self.showdiff(old, new), want)

    def test_issue1295_varies_on_pure(self):
            # we should pick up abbbc. rather than bc.de as the longest match
        got = self.showdiff(b"a\nb\nb\nb\nc\n.\nd\ne\n.\nf\n",
                            b"a\nb\nb\na\nb\nb\nb\nc\n.\nb\nc\n.\nd\ne\nf\n")
        want_c = [b'a\nb\nb\n',
                  diffreplace(6, 6, b'', b'a\nb\nb\nb\nc\n.\n'),
                  b'b\nc\n.\nd\ne\n',
                  diffreplace(16, 18, b'.\n', b''),
                  b'f\n']
        want_pure = [diffreplace(0, 0, b'', b'a\nb\nb\n'),
                     b'a\nb\nb\nb\nc\n.\n',
                     diffreplace(12, 12, b'', b'b\nc\n.\n'),
                     b'd\ne\n',
                     diffreplace(16, 18, b'.\n', b''), b'f\n']
        self.assertTrue(got in (want_c, want_pure),
                        'got: %r, wanted either %r or %r' % (
                            got, want_c, want_pure))

    def test_fixws(self):
        cases = [
            (b" \ta\r b\t\n", b"ab\n", 1),
            (b" \ta\r b\t\n", b" a b\n", 0),
            (b"", b"", 1),
            (b"", b"", 0),
        ]
        for a, b, allws in cases:
            c = mdiff.fixws(a, allws)
            self.assertEqual(
                c, b, 'fixws(%r) want %r got %r (allws=%r)' % (a, b, c, allws))

    def test_nice_diff_for_trivial_change(self):
        self.assertEqual(self.showdiff(
            b''.join(b'<%d\n-\n' % i for i in range(5)),
            b''.join(b'>%d\n-\n' % i for i in range(5))),
                         [diffreplace(0, 3, b'<0\n', b'>0\n'),
                          b'-\n',
                          diffreplace(5, 8, b'<1\n', b'>1\n'),
                          b'-\n',
                          diffreplace(10, 13, b'<2\n', b'>2\n'),
                          b'-\n',
                          diffreplace(15, 18, b'<3\n', b'>3\n'),
                          b'-\n',
                          diffreplace(20, 23, b'<4\n', b'>4\n'),
                          b'-\n'])

    def test_prefer_appending(self):
        # 1 line to 3 lines
        self.assertEqual(self.showdiff(b'a\n', b'a\n' * 3),
                         [b'a\n', diffreplace(2, 2, b'', b'a\na\n')])
        # 1 line to 5 lines
        self.assertEqual(self.showdiff(b'a\n', b'a\n' * 5),
                         [b'a\n', diffreplace(2, 2, b'', b'a\na\na\na\n')])

    def test_prefer_removing_trailing(self):
        # 3 lines to 1 line
        self.assertEqual(self.showdiff(b'a\n' * 3, b'a\n'),
                         [b'a\n', diffreplace(2, 6, b'a\na\n', b'')])
        # 5 lines to 1 line
        self.assertEqual(self.showdiff(b'a\n' * 5, b'a\n'),
                         [b'a\n', diffreplace(2, 10, b'a\na\na\na\n', b'')])

if __name__ == '__main__':
    import silenttestrunner
    silenttestrunner.main(__name__)