fscache: handle trailing slash in exists_case prefix

mypy/fscache.py

@@ -230,7 +230,8 @@ def exists_case(self, path: str, prefix: str) -> bool:
         if path in self.exists_case_cache:
             return self.exists_case_cache[path]
         head, tail = os.path.split(path)
-        if not head.startswith(prefix) or not tail:
+        prefix = prefix.rstrip(os.sep)
+        if (prefix != "" and not (head.startswith(prefix + os.sep) or head == prefix)) or not tail:
             # Only perform the check for paths under prefix.
             self.exists_case_cache[path] = True
             return True

mypy/test/testfscache.py

@@ -88,6 +88,20 @@ def test_isfile_case_other_directory(self) -> None:
                 # this path is not under the prefix, case difference is fine.
                 assert self.isfile_case(os.path.join(other, "PKG/other_dir.py"))
 
+    def test_exists_case_1(self) -> None:
+        self.make_file(os.path.join("foo", "bar", "baz.py"))
+        # Run twice to test both cached and non-cached code paths.
+        for i in range(2):
+            assert self.exists_case(os.path.join("foo", "bar", "baz.py"), "foo")
+            assert self.exists_case(os.path.join("foo", "bar"), "foo")
+            assert self.exists_case(os.path.join("foo", "bar", "non_existent1.py"), "bar")
+            assert self.exists_case(os.path.join("foobar", "non_existent2.py"), "foo")
+            assert not self.exists_case(os.path.join("foo", "bar", "non_existent3.py"), "foo")
+            assert not self.exists_case(os.path.join("foo", "bar", "not_a_dir"), "foo")
+            assert not self.exists_case(
+                os.path.join("not_a_dir", "not_a_subdir"), "not_a_dir" + os.sep
+            )
+
     def make_file(self, path: str, base: str | None = None) -> None:
         if base is None:
             base = self.tempdir
@@ -99,3 +113,8 @@ def make_file(self, path: str, base: str | None = None) -> None:
 
     def isfile_case(self, path: str) -> bool:
         return self.fscache.isfile_case(os.path.join(self.tempdir, path), self.tempdir)
+
+    def exists_case(self, path: str, prefix: str) -> bool:
+        return self.fscache.exists_case(
+            os.path.join(self.tempdir, path), os.path.join(self.tempdir, prefix)
+        )