hgdemandimport: apply lazy module loading to sys.meta_path finders
Python's `sys.meta_path` finders are the primary objects whose job it
is to find a module at import time. When `import` is called, Python
iterates objects in this list and calls `o.find_spec(...)` to find
a `ModuleSpec` (or None if the module couldn't be found by that
finder). If no meta path finder can find a module, import fails.
One of the default meta path finders is `PathFinder`. Its job is to
import modules from the filesystem and is probably the most important
importer. This finder looks at `sys.path` and `sys.path_hooks` to do
its job.
The `ModuleSpec` returned by `MetaPathImporter.find_spec()` has a
`loader` attribute, which defines the concrete module loader to use.
`sys.path_hooks` is a hook point for teaching `PathFinder` to
instantiate custom loader types.
Previously, we injected a custom `sys.path_hook` that told `PathFinder`
to wrap the default loaders with a loader that creates a module object
that is lazy.
This approach worked. But its main limitation was that it only applied
to the `PathFinder` meta path importer. There are other meta path
importers that are registered. And in the case of PyOxidizer loading
modules from memory, `PathFinder` doesn't come into play since
PyOxidizer's own meta path importer was handling all imports.
This commit changes our approach to lazy module loading by proxying
all meta path importers. Specifically, we overload the `find_spec()`
method to swap in a wrapped loader on the `ModuleSpec` before it
is returned. The end result of this is all meta path importers should
be lazy.
As much as I would have loved to utilize .__class__ manipulation to
achieve this, some meta path importers are implemented in C/Rust
in such a way that they cannot be monkeypatched. This is why we
use __getattribute__ to define a proxy.
Also, this change could theoretically open us up to regressions in
meta path importers whose loader is creating module objects which
can't be monkeypatched. But I'm not aware of any of these in the
wild. So I think we'll be safe.
According to hyperfine, this change yields a decent startup time win of
5-6ms:
```
Benchmark #1: ~/.pyenv/versions/3.6.10/bin/python ./hg version
Time (mean ± σ): 86.8 ms ± 0.5 ms [User: 78.0 ms, System: 8.7 ms]
Range (min … max): 86.0 ms … 89.1 ms 50 runs
Time (mean ± σ): 81.1 ms ± 2.7 ms [User: 74.5 ms, System: 6.5 ms]
Range (min … max): 77.8 ms … 90.5 ms 50 runs
Benchmark #2: ~/.pyenv/versions/3.7.6/bin/python ./hg version
Time (mean ± σ): 78.9 ms ± 0.6 ms [User: 70.2 ms, System: 8.7 ms]
Range (min … max): 78.1 ms … 81.2 ms 50 runs
Time (mean ± σ): 73.4 ms ± 0.6 ms [User: 65.3 ms, System: 8.0 ms]
Range (min … max): 72.4 ms … 75.7 ms 50 runs
Benchmark #3: ~/.pyenv/versions/3.8.1/bin/python ./hg version
Time (mean ± σ): 78.1 ms ± 0.6 ms [User: 70.2 ms, System: 7.9 ms]
Range (min … max): 77.4 ms … 80.9 ms 50 runs
Time (mean ± σ): 72.1 ms ± 0.4 ms [User: 64.4 ms, System: 7.6 ms]
Range (min … max): 71.4 ms … 74.1 ms 50 runs
```
Differential Revision: https://phab.mercurial-scm.org/D7954
# Copyright (C) 2015 - Mike Edgar <adgar@google.com>
#
# This extension enables removal of file content at a given revision,
# rewriting the data/metadata of successive revisions to preserve revision log
# integrity.
"""erase file content at a given revision
The censor command instructs Mercurial to erase all content of a file at a given
revision *without updating the changeset hash.* This allows existing history to
remain valid while preventing future clones/pulls from receiving the erased
data.
Typical uses for censor are due to security or legal requirements, including::
* Passwords, private keys, cryptographic material
* Licensed data/code/libraries for which the license has expired
* Personally Identifiable Information or other private data
Censored nodes can interrupt mercurial's typical operation whenever the excised
data needs to be materialized. Some commands, like ``hg cat``/``hg revert``,
simply fail when asked to produce censored data. Others, like ``hg verify`` and
``hg update``, must be capable of tolerating censored data to continue to
function in a meaningful way. Such commands only tolerate censored file
revisions if they are allowed by the "censor.policy=ignore" config option.
A few informative commands such as ``hg grep`` will unconditionally
ignore censored data and merely report that it was encountered.
"""
from __future__ import absolute_import
from mercurial.i18n import _
from mercurial.node import short
from mercurial import (
error,
registrar,
scmutil,
)
cmdtable = {}
command = registrar.command(cmdtable)
# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
# be specifying the version(s) of Mercurial they are tested with, or
# leave the attribute unspecified.
testedwith = b'ships-with-hg-core'
@command(
b'censor',
[
(
b'r',
b'rev',
b'',
_(b'censor file from specified revision'),
_(b'REV'),
),
(b't', b'tombstone', b'', _(b'replacement tombstone data'), _(b'TEXT')),
],
_(b'-r REV [-t TEXT] [FILE]'),
helpcategory=command.CATEGORY_MAINTENANCE,
)
def censor(ui, repo, path, rev=b'', tombstone=b'', **opts):
with repo.wlock(), repo.lock():
return _docensor(ui, repo, path, rev, tombstone, **opts)
def _docensor(ui, repo, path, rev=b'', tombstone=b'', **opts):
if not path:
raise error.Abort(_(b'must specify file path to censor'))
if not rev:
raise error.Abort(_(b'must specify revision to censor'))
wctx = repo[None]
m = scmutil.match(wctx, (path,))
if m.anypats() or len(m.files()) != 1:
raise error.Abort(_(b'can only specify an explicit filename'))
path = m.files()[0]
flog = repo.file(path)
if not len(flog):
raise error.Abort(_(b'cannot censor file with no history'))
rev = scmutil.revsingle(repo, rev, rev).rev()
try:
ctx = repo[rev]
except KeyError:
raise error.Abort(_(b'invalid revision identifier %s') % rev)
try:
fctx = ctx.filectx(path)
except error.LookupError:
raise error.Abort(_(b'file does not exist at revision %s') % rev)
fnode = fctx.filenode()
heads = []
for headnode in repo.heads():
hc = repo[headnode]
if path in hc and hc.filenode(path) == fnode:
heads.append(hc)
if heads:
headlist = b', '.join([short(c.node()) for c in heads])
raise error.Abort(
_(b'cannot censor file in heads (%s)') % headlist,
hint=_(b'clean/delete and commit first'),
)
wp = wctx.parents()
if ctx.node() in [p.node() for p in wp]:
raise error.Abort(
_(b'cannot censor working directory'),
hint=_(b'clean/delete/update first'),
)
with repo.transaction(b'censor') as tr:
flog.censorrevision(tr, fnode, tombstone=tombstone)