LSPFileBuffer: Ensure that file contents are never stale by monitoring file modification date

For every read access on .contents, check for external file modification
and re-read if necessary, invalidating the content hash as well.

Fixes #1013

test/solidlsp/test_ls_common.py

@@ -0,0 +1,44 @@
+import os
+import platform
+import time
+
+import pytest
+
+from solidlsp import SolidLanguageServer
+from solidlsp.ls_config import Language
+from test.conftest import is_ci
+
+
+class TestLanguageServerCommonFunctionality:
+    """Test common functionality of SolidLanguageServer base implementation (not language-specific behaviour)."""
+
+    @pytest.mark.parametrize("language_server", [Language.PYTHON], indirect=True)
+    def test_open_file_cache_invalidate(self, language_server: SolidLanguageServer) -> None:
+        """
+        Tests that the file buffer cache is invalidated when the file is changed on disk.
+        """
+        file_path = os.path.join(language_server.repository_root_path, "test_open_file.py")
+        test_string1 = "# foo"
+        test_string2 = "# bar"
+
+        with open(file_path, "w") as f:
+            f.write(test_string1)
+
+        try:
+            with language_server.open_file(file_path) as fb:
+                assert fb.contents == test_string1
+
+                # apply external change to file
+                with open(file_path, "w") as f:
+                    f.write(test_string2)
+
+                # ensure that mtime is updated for Windows CI (odd FS behaviour, updates lazily)
+                if is_ci and platform.system() == "Windows":
+                    now = time.time()
+                    os.utime(file_path, (now, now))
+
+                # check that the file buffer has been invalidated and reloaded
+                assert fb.contents == test_string2
+
+        finally:
+            os.remove(file_path)