interfaces: introduce and use a protocol class for the `bdiff` module

interfaces: introduce and use a protocol class for the `bdiff` module

This is allowed by PEP 544[1], and we basically follow the example there.  The
class here is copied from `mercurial.pure.bdiff`, and the implementation
removed.

There are several modules that have a few different implementations, and the
implementation chosen is controlled by `HGMODULEPOLICY`.  The module is loaded
via `mercurial/policy.py`, and has been inferred by pytype as `Any` up to this
point.  Therefore it and PyCharm were blind to all functions on the module, and
their signatures.  Also, having multiple instances of the same module allows
their signatures to get out of sync.

Introducing a protocol class allows the loaded module that is stored in a
variable to be given type info, which cascades through the various places it is
used.  This change alters 11 *.pyi files, for example.  In theory, this would
also allow us to ensure the various implementations of the same module are kept
in alignment- simply import the module in a test module, attempt to pass it to a
function that uses the corresponding protocol as an argument, and run pytype on
it.

In practice, this doesn't work (yet).  PyCharm (erroneously) flags imported
modules being passed where a protocol class is used[2].  Pytype has problems the
other way- it fails to detect when a module that doesn't adhere to the protocol
is passed to a protocol argument.  The good news is that mypy properly detects
this case.  The bad news is that mypy spews a bunch of other errors when
importing even simple modules, like the various `bdiff` modules.  Therefore I'm
punting on the tests for now because the type info around a loaded module in
PyCharm is a clear win by itself.

[1] https://peps.python.org/pep-0544/#modules-as-implementations-of-protocols
[2] https://youtrack.jetbrains.com/issue/PY-58679/Support-modules-implementing-protocols

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/interfaces/modules.py	Sat Sep 28 19:12:18 2024 -0400
@@ -0,0 +1,34 @@
+# modules.py - protocol classes for dynamically loaded modules
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import annotations
+
+import typing
+
+from typing import (
+    List,
+    Protocol,
+    Tuple,
+)
+
+if typing.TYPE_CHECKING:
+    BDiffBlock = Tuple[int, int, int, int]
+    """An entry in the list returned by bdiff.blocks()."""
+
+
+class BDiff(Protocol):
+    """A Protocol class for the various bdiff module implementations."""
+
+    def splitnewlines(self, text: bytes) -> List[bytes]:
+        """like str.splitlines, but only split on newlines."""
+
+    def bdiff(self, a: bytes, b: bytes) -> bytes:
+        ...
+
+    def blocks(self, a: bytes, b: bytes) -> List[BDiffBlock]:
+        ...
+
+    def fixws(self, text: bytes, allws: bool) -> bytes:
+        ...

--- a/mercurial/mdiff.py	Sat Sep 28 19:11:39 2024 -0400
+++ b/mercurial/mdiff.py	Sat Sep 28 19:12:18 2024 -0400
@@ -20,9 +20,13 @@
     pycompat,
     util,
 )
+from .interfaces import (
+    modules as intmod,
+)
+
 from .utils import dateutil
 
-bdiff = policy.importmod('bdiff')
+bdiff: intmod.BDiff = policy.importmod('bdiff')
 mpatch = policy.importmod('mpatch')
 
 blocks = bdiff.blocks