fix(mcp_server): split get_or_create_collection on reopen (follow-up to #1262)

#1262 split `get_or_create_collection` into `get_collection` + fallback
`create_collection` inside `ChromaBackend.get_collection`, fixing the
chromadb 1.5.x Rust-binding SIGSEGV that fires when stored collection
metadata differs from the call-site's `_HNSW_BLOAT_GUARD` payload.

The MCP server's `_get_collection(create=True)` carries the same metadata
payload at `mcp_server.py:287` and routes through chromadb's Python
client directly, bypassing the backend layer. Both `tool_add_drawer`
and `tool_diary_write` reach this site on every invocation, and the
Stop hook fires `mempalace_diary_write` at session end — which was
exactly the crash path #1089 named.

Apply the same try/except split here so legacy palaces whose stored
metadata predates the bloat-guard expansion no longer crash on the
MCP-server reopen path. Regression test patches
`get_or_create_collection` at the chromadb client class level (not the
instance — chromadb's mtime-change detection rebuilds the client between
calls, so an instance-level spy doesn't survive) and asserts the second
`_get_collection(create=True)` call never reaches it.

mempalace/mcp_server.py

@@ -57,6 +57,8 @@ from .config import (  # noqa: E402
     sanitize_content,
 )
 from .version import __version__  # noqa: E402
+from chromadb.errors import NotFoundError as _ChromaNotFoundError  # noqa: E402
+
 from .backends.chroma import (  # noqa: E402
     ChromaBackend,
     ChromaCollection,
@@ -284,14 +286,22 @@ def _get_collection(create=False):
             # palaces whose collections were created before this fix (the
             # runtime config does not persist cross-process in chromadb 1.5.x,
             # so the retrofit runs every time _get_collection opens a cache).
-            raw = client.get_or_create_collection(
-                _config.collection_name,
-                metadata={
-                    "hnsw:space": "cosine",
-                    "hnsw:num_threads": 1,
-                    **_HNSW_BLOAT_GUARD,
-                },
-            )
+            #
+            # ChromaDB 1.5.x's Rust binding SIGSEGVs when get_or_create_collection
+            # is called with metadata that differs from what's stored. The split
+            # below skips the metadata-comparison codepath for existing
+            # collections, mirroring the backend-layer fix from #1262.
+            try:
+                raw = client.get_collection(_config.collection_name)
+            except _ChromaNotFoundError:
+                raw = client.create_collection(
+                    _config.collection_name,
+                    metadata={
+                        "hnsw:space": "cosine",
+                        "hnsw:num_threads": 1,
+                        **_HNSW_BLOAT_GUARD,
+                    },
+                )
             _pin_hnsw_threads(raw)
             _collection_cache = ChromaCollection(raw)
             _metadata_cache = None

tests/test_mcp_server.py

@@ -874,3 +874,48 @@ class TestCacheInvalidation:
         assert result["success"] is True
         assert "Reconnected" in result["message"]
         assert isinstance(result["drawers"], int)
+
+    def test_get_collection_create_true_avoids_get_or_create_on_reopen(
+        self, monkeypatch, config, palace_path, kg
+    ):
+        """Regression for the MCP-server half of #1262.
+
+        ChromaDB 1.5.x's Rust bindings SIGSEGV when
+        ``client.get_or_create_collection`` is called with metadata that
+        differs from the collection's stored metadata. The Stop hook
+        path (``tool_diary_write`` -> ``_get_collection(create=True)``)
+        was reaching that codepath on every session-end; #1262 fixed
+        the equivalent crash class in ``ChromaBackend`` but left this
+        site untouched. ``_get_collection(create=True)`` must call
+        ``client.get_collection`` first and only fall back to
+        ``client.create_collection`` when the collection does not yet
+        exist on disk.
+        """
+        _patch_mcp_server(monkeypatch, config, kg)
+        from mempalace import mcp_server
+
+        col1 = mcp_server._get_collection(create=True)
+        assert col1 is not None
+
+        client = mcp_server._client_cache
+        assert client is not None
+
+        # Patch at the class level — chromadb's mtime-change detection
+        # may rebuild the client between calls, so an instance-level
+        # spy would not survive.
+        client_cls = type(client)
+        calls: list[tuple] = []
+
+        def _spy(self, *args, **kwargs):
+            calls.append((args, kwargs))
+            raise AssertionError(
+                "get_or_create_collection must not be called on reopen "
+                "(SIGSEGV path on metadata mismatch)"
+            )
+
+        monkeypatch.setattr(client_cls, "get_or_create_collection", _spy)
+        mcp_server._collection_cache = None
+
+        col2 = mcp_server._get_collection(create=True)
+        assert col2 is not None
+        assert calls == [], f"get_or_create_collection was called: {calls}"