Mercurial > hg
view hgdemandimport/__init__.py @ 47965:f9e6f2bb721d
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
author | Simon Sapin <simon.sapin@octobus.net> |
---|---|
date | Mon, 13 Sep 2021 18:48:48 +0200 |
parents | 07731064ac41 |
children | 6000f5b25c9b fda7ec505dc5 |
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# hgdemandimport - global demand-loading of modules for Mercurial # # Copyright 2017 Facebook Inc. # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. '''demandimport - automatic demand-loading of modules''' # This is in a separate package from mercurial because in Python 3, # demand loading is per-package. Keeping demandimport in the mercurial package # would disable demand loading for any modules in mercurial. from __future__ import absolute_import import os import sys if sys.version_info[0] >= 3: from . import demandimportpy3 as demandimport else: from . import demandimportpy2 as demandimport # Full module names which can't be lazy imported. # Extensions can add to this set. IGNORES = { '__future__', '_hashlib', # ImportError during pkg_resources/__init__.py:fixup_namespace_package '_imp', '_xmlplus', 'fcntl', 'nt', # pathlib2 tests the existence of built-in 'nt' module 'win32com.gen_py', 'win32com.shell', # 'appdirs' tries to import win32com.shell '_winreg', # 2.7 mimetypes needs immediate ImportError 'pythoncom', # imported by tarfile, not available under Windows 'pwd', 'grp', # imported by profile, itself imported by hotshot.stats, # not available under Windows 'resource', # this trips up many extension authors 'gtk', # setuptools' pkg_resources.py expects "from __main__ import x" to # raise ImportError if x not defined '__main__', '_ast', # https://bugs.python.org/issue41631 '_ssl', # conditional imports in the stdlib, issue1964 '_sre', # issue4920 'rfc822', 'mimetools', 'sqlalchemy.events', # has import-time side effects (issue5085) 'sqlalchemy.dialects', # similar problems as above # setuptools 8 expects this module to explode early when not on windows 'distutils.msvc9compiler', '__builtin__', 'builtins', 'urwid.command_map', # for pudb 'lzma', } _pypy = '__pypy__' in sys.builtin_module_names if _pypy: # _ctypes.pointer is shadowed by "from ... import pointer" (PyPy 5) IGNORES.add('_ctypes.pointer') demandimport.init(IGNORES) # Re-export. isenabled = demandimport.isenabled disable = demandimport.disable deactivated = demandimport.deactivated def enable(): # chg pre-imports modules so do not enable demandimport for it if ( 'CHGINTERNALMARK' not in os.environ and os.environ.get('HGDEMANDIMPORT') != 'disable' ): demandimport.enable()