view tests/test-oldcgi.t @ 44118:f81c17ec303c

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

#require no-msys # MSYS will translate web paths as if they were file paths

This tests if CGI files from before d0db3462d568 still work.

  $ hg init test
  $ cat >hgweb.cgi <<HGWEB
  > #!"$PYTHON"
  > #
  > # An example CGI script to use hgweb, edit as necessary
  > 
  > import cgitb, os, sys
  > cgitb.enable()
  > 
  > # sys.path.insert(0, "/path/to/python/lib") # if not a system-wide install
  > from mercurial import hgweb
  > 
  > h = hgweb.hgweb(b"test", b"Empty test repository")
  > h.run()
  > HGWEB

  $ chmod 755 hgweb.cgi

  $ cat >hgweb.config <<HGWEBDIRCONF
  > [paths]
  > test = test
  > HGWEBDIRCONF

  $ cat >hgwebdir.cgi <<HGWEBDIR
  > #!"$PYTHON"
  > #
  > # An example CGI script to export multiple hgweb repos, edit as necessary
  > 
  > import cgitb, sys
  > cgitb.enable()
  > 
  > # sys.path.insert(0, "/path/to/python/lib") # if not a system-wide install
  > from mercurial import hgweb
  > 
  > # The config file looks like this.  You can have paths to individual
  > # repos, collections of repos in a directory tree, or both.
  > #
  > # [paths]
  > # virtual/path = /real/path
  > # virtual/path = /real/path
  > #
  > # [collections]
  > # /prefix/to/strip/off = /root/of/tree/full/of/repos
  > #
  > # collections example: say directory tree /foo contains repos /foo/bar,
  > # /foo/quux/baz.  Give this config section:
  > #   [collections]
  > #   /foo = /foo
  > # Then repos will list as bar and quux/baz.
  > 
  > # Alternatively you can pass a list of ('virtual/path', '/real/path') tuples
  > # or use a dictionary with entries like 'virtual/path': '/real/path'
  > 
  > h = hgweb.hgwebdir(b"hgweb.config")
  > h.run()
  > HGWEBDIR

  $ chmod 755 hgwebdir.cgi

  $ . "$TESTDIR/cgienv"
  $ "$PYTHON" hgweb.cgi > page1
  $ "$PYTHON" hgwebdir.cgi > page2

  $ PATH_INFO="/test/"
  $ PATH_TRANSLATED="/var/something/test.cgi"
  $ REQUEST_URI="/test/test/"
  $ SCRIPT_URI="http://hg.omnifarious.org/test/test/"
  $ SCRIPT_URL="/test/test/"
  $ "$PYTHON" hgwebdir.cgi > page3

  $ grep -i error page1 page2 page3
  [1]