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

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.

--- a/mercurial/cffi/bdiff.py	Sat Sep 28 19:12:18 2024 -0400
+++ b/mercurial/cffi/bdiff.py	Sun Sep 29 02:03:20 2024 -0400
@@ -8,13 +8,20 @@
 from __future__ import annotations
 
 import struct
+import typing
 
 from typing import (
     List,
+    Optional,
     Tuple,
 )
 
 from ..pure.bdiff import *
+
+from ..interfaces import (
+    modules as intmod,
+)
+
 from . import _bdiff  # pytype: disable=import-error
 
 ffi = _bdiff.ffi
@@ -87,3 +94,11 @@
         lib.free(b[0])
         lib.bdiff_freehunks(l.next)
     return b"".join(rl)
+
+
+# In order to adhere to the module protocol, these functions must be visible to
+# the type checker, though they aren't actually implemented by this
+# implementation of the module protocol.  Callers are responsible for
+# checking that the implementation is available before using them.
+if typing.TYPE_CHECKING:
+    xdiffblocks: Optional[intmod.BDiffBlocksFnc] = None

--- a/mercurial/interfaces/modules.py	Sat Sep 28 19:12:18 2024 -0400
+++ b/mercurial/interfaces/modules.py	Sun Sep 29 02:03:20 2024 -0400
@@ -8,14 +8,19 @@
 import typing
 
 from typing import (
+    Callable,
     List,
+    Optional,
     Protocol,
     Tuple,
 )
 
 if typing.TYPE_CHECKING:
     BDiffBlock = Tuple[int, int, int, int]
-    """An entry in the list returned by bdiff.blocks()."""
+    """An entry in the list returned by bdiff.{xdiff,}blocks()."""
+
+    BDiffBlocksFnc = Callable[[bytes, bytes], List[BDiffBlock]]
+    """The signature of `bdiff.blocks()` and `bdiff.xdiffblocks()`."""
 
 
 class BDiff(Protocol):
@@ -32,3 +37,6 @@
 
     def fixws(self, text: bytes, allws: bool) -> bytes:
         ...
+
+    xdiffblocks: Optional[BDiffBlocksFnc]
+    """This method is currently only available in the ``cext`` module."""

--- a/mercurial/mdiff.py	Sat Sep 28 19:12:18 2024 -0400
+++ b/mercurial/mdiff.py	Sun Sep 29 02:03:20 2024 -0400
@@ -212,7 +212,11 @@
 
 
 def chooseblocksfunc(opts=None):
-    if opts is None or not opts.xdiff or not hasattr(bdiff, 'xdiffblocks'):
+    if (
+        opts is None
+        or not opts.xdiff
+        or not getattr(bdiff, 'xdiffblocks', None)
+    ):
         return bdiff.blocks
     else:
         return bdiff.xdiffblocks

--- a/mercurial/pure/bdiff.py	Sat Sep 28 19:12:18 2024 -0400
+++ b/mercurial/pure/bdiff.py	Sun Sep 29 02:03:20 2024 -0400
@@ -10,12 +10,18 @@
 import difflib
 import re
 import struct
+import typing
 
 from typing import (
     List,
+    Optional,
     Tuple,
 )
 
+from ..interfaces import (
+    modules as intmod,
+)
+
 
 def splitnewlines(text: bytes) -> List[bytes]:
     '''like str.splitlines, but only split on newlines.'''
@@ -107,3 +113,11 @@
         text = re.sub(b'[ \t\r]+', b' ', text)
         text = text.replace(b' \n', b'\n')
     return text
+
+
+# In order to adhere to the module protocol, these functions must be visible to
+# the type checker, though they aren't actually implemented by this
+# implementation of the module protocol.  Callers are responsible for
+# checking that the implementation is available before using them.
+if typing.TYPE_CHECKING:
+    xdiffblocks: Optional[intmod.BDiffBlocksFnc] = None