tests/test-imports-checker.t

interfaces: add the optional `bdiff.xdiffblocks()` method

PyCharm flagged where this was called on the protocol class in `mdiff.py` in the
previous commit, but pytype completely missed it.  PyCharm is correct here, but
I'm committing this separately to highlight this potential problem- some of the
implementations don't implement _all_ of the methods the others do, and there's
not a great way to indicate on a protocol class that a method or attribute is
optional- that's kinda the opposite of what static typing is about.

Making the method an `Optional[Callable]` attribute works here, and keeps both
PyCharm and pytype happy, and the generated `mdiff.pyi` and `modules.pyi` look
reasonable.  We might be getting a little lucky, because the method isn't
invoked directly- it is returned from another method that selects which block
function to use.  Except since it is declared on the protocol class, every
module needs this attribute (in theory, but in practice this doesn't seem to be
checked), so the check for it on the module has to change from `hasattr()` to
`getattr(..., None)`.  We defer defining the optional attrs to the type checking
phase as an extra precaution- that way it isn't an attr with a `None` value at
runtime if someone is still using `hasattr()`.

As to why pytype missed this, I have no clue.  The generated `mdiff.pyi` even
has the global variable typed as `bdiff: intmod.BDiff`, so uses of it really
should comply with what is on the class, protocol class or not.

#require test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"
  $ testrepohgenv
  $ import_checker="$TESTDIR"/../contrib/import-checker.py

Run the doctests from the import checker, and make sure
it's working correctly.
  $ TERM=dumb
  $ export TERM
  $ "$PYTHON" -m doctest $import_checker

Run additional tests for the import checker

  $ mkdir testpackage
  $ touch testpackage/__init__.py

  $ cat > testpackage/multiple.py << EOF
  > import os, sys
  > EOF

  $ cat > testpackage/unsorted.py << EOF
  > import sys
  > import os
  > EOF

  $ cat > testpackage/stdafterlocal.py << EOF
  > from . import unsorted
  > import os
  > EOF

  $ cat > testpackage/requirerelative.py << EOF
  > import testpackage.unsorted
  > EOF

  $ cat > testpackage/importalias.py << EOF
  > import ui
  > EOF

  $ cat > testpackage/relativestdlib.py << EOF
  > from .. import os
  > EOF

  $ cat > testpackage/stdlibfrom.py << EOF
  > from collections import abc
  > EOF

  $ cat > testpackage/symbolimport.py << EOF
  > from .unsorted import foo
  > EOF

  $ cat > testpackage/latesymbolimport.py << EOF
  > from . import unsorted
  > from mercurial.node import hex
  > EOF

  $ cat > testpackage/multiplegroups.py << EOF
  > from . import unsorted
  > from . import more
  > EOF

  $ mkdir testpackage/subpackage
  $ cat > testpackage/subpackage/levelpriority.py << EOF
  > from . import foo
  > from .. import parent
  > EOF

  $ touch testpackage/subpackage/foo.py
  $ cat > testpackage/subpackage/__init__.py << EOF
  > from . import levelpriority  # should not cause cycle
  > EOF

  $ cat > testpackage/subpackage/localimport.py << EOF
  > from . import foo
  > def bar():
  >     # should not cause "higher-level import should come first"
  >     from .. import unsorted
  >     # but other errors should be detected
  >     from .. import more
  >     import testpackage.subpackage.levelpriority
  > EOF

  $ cat > testpackage/importmodulefromsub.py << EOF
  > from .subpackage import foo  # not a "direct symbol import"
  > EOF

  $ cat > testpackage/importsymbolfromsub.py << EOF
  > from .subpackage import foo, nonmodule
  > EOF

  $ cat > testpackage/sortedentries.py << EOF
  > from . import (
  >     foo,
  >     bar,
  > )
  > EOF

  $ cat > testpackage/importfromalias.py << EOF
  > from . import ui
  > EOF

  $ cat > testpackage/importfromrelative.py << EOF
  > from testpackage.unsorted import foo
  > EOF

  $ mkdir testpackage2
  $ touch testpackage2/__init__.py

  $ cat > testpackage2/latesymbolimport.py << EOF
  > from testpackage import unsorted
  > from mercurial.node import hex
  > EOF

# Shadowing a stdlib module to test "relative import of stdlib module" is
# allowed if the module is also being checked

  $ mkdir email
  $ touch email/__init__.py
  $ touch email/errors.py
  $ cat > email/utils.py << EOF
  > from . import errors
  > EOF

  $ "$PYTHON" "$import_checker" testpackage*/*.py testpackage/subpackage/*.py \
  >   email/*.py
  testpackage/importalias.py:1: ui module must be "as" aliased to uimod
  testpackage/importfromalias.py:1: ui from testpackage must be "as" aliased to uimod
  testpackage/importfromrelative.py:1: import should be relative: testpackage.unsorted
  testpackage/importfromrelative.py:1: direct symbol import foo from testpackage.unsorted
  testpackage/importsymbolfromsub.py:1: direct symbol import nonmodule from testpackage.subpackage
  testpackage/latesymbolimport.py:2: symbol import follows non-symbol import: mercurial.node
  testpackage/multiple.py:1: multiple imported names: os, sys
  testpackage/multiplegroups.py:2: multiple "from . import" statements
  testpackage/relativestdlib.py:1: relative import of stdlib module
  testpackage/requirerelative.py:1: import should be relative: testpackage.unsorted
  testpackage/sortedentries.py:1: imports from testpackage not lexically sorted: bar < foo
  testpackage/stdafterlocal.py:2: stdlib import "os" follows local import: testpackage
  testpackage/stdlibfrom.py:1: direct symbol import abc from collections
  testpackage/subpackage/levelpriority.py:2: higher-level import should come first: testpackage
  testpackage/subpackage/localimport.py:6: multiple "from .. import" statements
  testpackage/subpackage/localimport.py:7: import should be relative: testpackage.subpackage.levelpriority
  testpackage/symbolimport.py:1: direct symbol import foo from testpackage.unsorted
  testpackage/unsorted.py:2: imports not lexically sorted: os < sys
  testpackage2/latesymbolimport.py:2: symbol import follows non-symbol import: mercurial.node
  [1]