Setting --shadow-file makes dmypy reload daemon(perf issue)

[dnaaun]

This is a bug report.

• Minimal repro

$ touch test.py
$ cp test.py /tmp/test.py
$ dmypy run test.py
Daemon started
Success: no issues found in 22 source files
$ dmypy run test.py
Success: no issues found in 1 source file
$ dmypy run -- --shadow-file test.py /tmp/test.py test.py 
Restarting: configuration changed
Daemon stopped
Daemon started
Success: no issues found in 22 source files

• What is the actual behavior/output?
  dmypy interprets setting --shadow-file as a "configuration change" and reloads the mypy daemon. This effectively counteracts the whole point of having a daemon since it takes a long time to reload the daemon.
• What is the behavior/output you expect?
  dmypy should not interpret --shadow-file changes as configuration changes. It should use the existing daemon instead of reloading.
• What are the versions of mypy and Python you are using?
  Python 3.8.2.
  mypy 0.790+dev.2e9afec83a5214f4a0eaf57a96034230b8460379
  The mypy install is from Github master as of thirty minutes ago.
• What are the mypy flags you are using? (For example --strict-optional)
  The repro above describes this. There are no mypy.ini (or friends) files in the directory.

[dnaaun]

An additional note: Running mypy with -v shows that setting --shadow-file doesn't make mypy "invalidate it's cache".

Might it therefore be enough to modify the lines below to ignore the "shadow-file" option? (or perhaps create a new method on Options to achieve the same thing? )

mypy/mypy/dmypy_server.py

Lines 307 to 308 in d0711bd

	if self.options_snapshot != options.snapshot():
	return {'restart': 'configuration changed'}

[gvanrossum]

That sounds reasonable. Maybe you can help by submitting a PR? I'd be happy to review it.

[dnaaun]

a PR?

Id' be happy to. Thanks for offering to review. Will ping you when I make the PR.

[gvanrossum]

Excellent!

[dnaaun]

Well, getting dmypy to use the already existing options.select_options_affecting_cache() was easy.

But now, dmypy just keeps whichever path it cached when the daemon was started(in other words, it actually uses the --shadow-file flag only from when the daemon started).

Fixing that seems to be a lot more involved than I originally thought, I don't think I'll be able to manage that in the near future.

[gvanrossum]

That's too bad, but I see it's more than you signed up for. :-) Thanks for the bug report anyway.