contrib/byteify-strings.py

rhg: Don’t compare ambiguous files one byte at a time

Even though the use of `BufReader` reduces the number of syscalls to read
the file from disk, `.bytes()` yields a separate `Result` for every byte.
Creating those results and dispatching on them is most likely costly.

Instead, this commit opts for simplicity by reading the entire file into memory
and comparing a single pair of byte strings. Note that memory already needs to
contain the entire previous contents of the file, as read from the filelog.
So with an extremely large file this doubles memory use but does not make it
grow by orders of magnitude.

At first I wrote code that still avoids reading the entire file into memory
and compares one buffer at a time with `BufReader`. Find this code below for
posterity. However its correctness is subtle. I ended up preferring the
simplicity of the obviously-correct single comparison.

```rust
let mut reader = BufReader::new(fobj);
let mut expected = &contents_in_p1[..];
loop {
    let buf = reader.fill_buf().when_reading_file(&fs_path)?;
    if buf.is_empty() {
        // Found EOF
        return Ok(expected.is_empty());
    } else if let Some(rest) = expected.drop_prefix(buf) {
        // What we read so far matches the expected content, continue reading
        let buf_len = buf.len();
        reader.consume(buf_len);
        expected = rest
    } else {
        // Found different content
        return Ok(false);
    }
}
```

Differential Revision: https://phab.mercurial-scm.org/D11412

#!/usr/bin/env python3
#
# byteify-strings.py - transform string literals to be Python 3 safe
#
# Copyright 2015 Gregory Szorc <[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, print_function

import argparse
import contextlib
import errno
import os
import sys
import tempfile
import token
import tokenize


def adjusttokenpos(t, ofs):
    """Adjust start/end column of the given token"""
    return t._replace(
        start=(t.start[0], t.start[1] + ofs), end=(t.end[0], t.end[1] + ofs)
    )


def replacetokens(tokens, opts):
    """Transform a stream of tokens from raw to Python 3.

    Returns a generator of possibly rewritten tokens.

    The input token list may be mutated as part of processing. However,
    its changes do not necessarily match the output token stream.
    """
    sysstrtokens = set()

    # The following utility functions access the tokens list and i index of
    # the for i, t enumerate(tokens) loop below
    def _isop(j, *o):
        """Assert that tokens[j] is an OP with one of the given values"""
        try:
            return tokens[j].type == token.OP and tokens[j].string in o
        except IndexError:
            return False

    def _findargnofcall(n):
        """Find arg n of a call expression (start at 0)

        Returns index of the first token of that argument, or None if
        there is not that many arguments.

        Assumes that token[i + 1] is '('.

        """
        nested = 0
        for j in range(i + 2, len(tokens)):
            if _isop(j, ')', ']', '}'):
                # end of call, tuple, subscription or dict / set
                nested -= 1
                if nested < 0:
                    return None
            elif n == 0:
                # this is the starting position of arg
                return j
            elif _isop(j, '(', '[', '{'):
                nested += 1
            elif _isop(j, ',') and nested == 0:
                n -= 1

        return None

    def _ensuresysstr(j):
        """Make sure the token at j is a system string

        Remember the given token so the string transformer won't add
        the byte prefix.

        Ignores tokens that are not strings. Assumes bounds checking has
        already been done.

        """
        k = j
        currtoken = tokens[k]
        while currtoken.type in (token.STRING, token.NEWLINE, tokenize.NL):
            k += 1
            if currtoken.type == token.STRING and currtoken.string.startswith(
                ("'", '"')
            ):
                sysstrtokens.add(currtoken)
            try:
                currtoken = tokens[k]
            except IndexError:
                break

    def _isitemaccess(j):
        """Assert the next tokens form an item access on `tokens[j]` and that
        `tokens[j]` is a name.
        """
        try:
            return (
                tokens[j].type == token.NAME
                and _isop(j + 1, '[')
                and tokens[j + 2].type == token.STRING
                and _isop(j + 3, ']')
            )
        except IndexError:
            return False

    def _ismethodcall(j, *methodnames):
        """Assert the next tokens form a call to `methodname` with a string
        as first argument on `tokens[j]` and that `tokens[j]` is a name.
        """
        try:
            return (
                tokens[j].type == token.NAME
                and _isop(j + 1, '.')
                and tokens[j + 2].type == token.NAME
                and tokens[j + 2].string in methodnames
                and _isop(j + 3, '(')
                and tokens[j + 4].type == token.STRING
            )
        except IndexError:
            return False

    coldelta = 0  # column increment for new opening parens
    coloffset = -1  # column offset for the current line (-1: TBD)
    parens = [(0, 0, 0, -1)]  # stack of (line, end-column, column-offset, type)
    ignorenextline = False  # don't transform the next line
    insideignoreblock = False  # don't transform until turned off
    for i, t in enumerate(tokens):
        # Compute the column offset for the current line, such that
        # the current line will be aligned to the last opening paren
        # as before.
        if coloffset < 0:
            lastparen = parens[-1]
            if t.start[1] == lastparen[1]:
                coloffset = lastparen[2]
            elif t.start[1] + 1 == lastparen[1] and lastparen[3] not in (
                token.NEWLINE,
                tokenize.NL,
            ):
                # fix misaligned indent of s/util.Abort/error.Abort/
                coloffset = lastparen[2] + (lastparen[1] - t.start[1])
            else:
                coloffset = 0

        # Reset per-line attributes at EOL.
        if t.type in (token.NEWLINE, tokenize.NL):
            yield adjusttokenpos(t, coloffset)
            coldelta = 0
            coloffset = -1
            if not insideignoreblock:
                ignorenextline = (
                    tokens[i - 1].type == token.COMMENT
                    and tokens[i - 1].string == "# no-py3-transform"
                )
            continue

        if t.type == token.COMMENT:
            if t.string == "# py3-transform: off":
                insideignoreblock = True
            if t.string == "# py3-transform: on":
                insideignoreblock = False

        if ignorenextline or insideignoreblock:
            yield adjusttokenpos(t, coloffset)
            continue

        # Remember the last paren position.
        if _isop(i, '(', '[', '{'):
            parens.append(t.end + (coloffset + coldelta, tokens[i + 1].type))
        elif _isop(i, ')', ']', '}'):
            parens.pop()

        # Convert most string literals to byte literals. String literals
        # in Python 2 are bytes. String literals in Python 3 are unicode.
        # Most strings in Mercurial are bytes and unicode strings are rare.
        # Rather than rewrite all string literals to use ``b''`` to indicate
        # byte strings, we apply this token transformer to insert the ``b``
        # prefix nearly everywhere.
        if t.type == token.STRING and t not in sysstrtokens:
            s = t.string

            # Preserve docstrings as string literals. This is inconsistent
            # with regular unprefixed strings. However, the
            # "from __future__" parsing (which allows a module docstring to
            # exist before it) doesn't properly handle the docstring if it
            # is b''' prefixed, leading to a SyntaxError. We leave all
            # docstrings as unprefixed to avoid this. This means Mercurial
            # components touching docstrings need to handle unicode,
            # unfortunately.
            if s[0:3] in ("'''", '"""'):
                # If it's assigned to something, it's not a docstring
                if not _isop(i - 1, '='):
                    yield adjusttokenpos(t, coloffset)
                    continue

            # If the first character isn't a quote, it is likely a string
            # prefixing character (such as 'b', 'u', or 'r'. Ignore.
            if s[0] not in ("'", '"'):
                yield adjusttokenpos(t, coloffset)
                continue

            # String literal. Prefix to make a b'' string.
            yield adjusttokenpos(t._replace(string='b%s' % t.string), coloffset)
            coldelta += 1
            continue

        # This looks like a function call.
        if t.type == token.NAME and _isop(i + 1, '('):
            fn = t.string

            # *attr() builtins don't accept byte strings to 2nd argument.
            if (
                fn
                in (
                    'getattr',
                    'setattr',
                    'hasattr',
                    'safehasattr',
                    'wrapfunction',
                    'wrapclass',
                    'addattr',
                )
                and (opts['allow-attr-methods'] or not _isop(i - 1, '.'))
            ):
                arg1idx = _findargnofcall(1)
                if arg1idx is not None:
                    _ensuresysstr(arg1idx)

            # .encode() and .decode() on str/bytes/unicode don't accept
            # byte strings on Python 3.
            elif fn in ('encode', 'decode') and _isop(i - 1, '.'):
                for argn in range(2):
                    argidx = _findargnofcall(argn)
                    if argidx is not None:
                        _ensuresysstr(argidx)

            # It changes iteritems/values to items/values as they are not
            # present in Python 3 world.
            elif opts['dictiter'] and fn in ('iteritems', 'itervalues'):
                yield adjusttokenpos(t._replace(string=fn[4:]), coloffset)
                continue

        if t.type == token.NAME and t.string in opts['treat-as-kwargs']:
            if _isitemaccess(i):
                _ensuresysstr(i + 2)
            if _ismethodcall(i, 'get', 'pop', 'setdefault', 'popitem'):
                _ensuresysstr(i + 4)

        # Looks like "if __name__ == '__main__'".
        if (
            t.type == token.NAME
            and t.string == '__name__'
            and _isop(i + 1, '==')
        ):
            _ensuresysstr(i + 2)

        # Emit unmodified token.
        yield adjusttokenpos(t, coloffset)


def process(fin, fout, opts):
    tokens = tokenize.tokenize(fin.readline)
    tokens = replacetokens(list(tokens), opts)
    fout.write(tokenize.untokenize(tokens))


def tryunlink(fname):
    try:
        os.unlink(fname)
    except OSError as err:
        if err.errno != errno.ENOENT:
            raise


@contextlib.contextmanager
def editinplace(fname):
    n = os.path.basename(fname)
    d = os.path.dirname(fname)
    fp = tempfile.NamedTemporaryFile(
        prefix='.%s-' % n, suffix='~', dir=d, delete=False
    )
    try:
        yield fp
        fp.close()
        if os.name == 'nt':
            tryunlink(fname)
        os.rename(fp.name, fname)
    finally:
        fp.close()
        tryunlink(fp.name)


def main():
    ap = argparse.ArgumentParser()
    ap.add_argument(
        '--version', action='version', version='Byteify strings 1.0'
    )
    ap.add_argument(
        '-i',
        '--inplace',
        action='store_true',
        default=False,
        help='edit files in place',
    )
    ap.add_argument(
        '--dictiter',
        action='store_true',
        default=False,
        help='rewrite iteritems() and itervalues()',
    ),
    ap.add_argument(
        '--allow-attr-methods',
        action='store_true',
        default=False,
        help='also handle attr*() when they are methods',
    ),
    ap.add_argument(
        '--treat-as-kwargs',
        nargs="+",
        default=[],
        help="ignore kwargs-like objects",
    ),
    ap.add_argument('files', metavar='FILE', nargs='+', help='source file')
    args = ap.parse_args()
    opts = {
        'dictiter': args.dictiter,
        'treat-as-kwargs': set(args.treat_as_kwargs),
        'allow-attr-methods': args.allow_attr_methods,
    }
    for fname in args.files:
        fname = os.path.realpath(fname)
        if args.inplace:
            with editinplace(fname) as fout:
                with open(fname, 'rb') as fin:
                    process(fin, fout, opts)
        else:
            with open(fname, 'rb') as fin:
                fout = sys.stdout.buffer
                process(fin, fout, opts)


if __name__ == '__main__':
    if sys.version_info[0:2] < (3, 7):
        print('This script must be run under Python 3.7+')
        sys.exit(3)
    main()