fix(memory): don't let an unreadable store get overwritten with an empty one (#5831)

* fix(memory): don't let an unreadable store get overwritten with an empty one

load_all() answered a failed read the same way it answered an empty store:
with []. Every mutation path is a read-modify-write (load the whole file,
change it, save it back), so a failed read became

    load_all() -> []  ->  [].append(new)  ->  save([new])

and save() is atomic, so the replacement stuck.

The case that actually destroys data is a store that is READABLE but not
parseable - a truncated file, or one holding {} instead of []. Nothing
obstructs the write, so adding a memory returns HTTP 200 and every memory
already stored is gone. Verified end-to-end against a running instance: on the
current code a truncated memory.json plus one add leaves the file holding only
the new entry. Truncation is reachable - core/database.py rewrites memory.json
during migration with a plain open(.., "w") + json.dump, which is not atomic.

A live exclusive lock is not the dangerous case: it blocks the read and the
os.replace alike, so the save fails too and the store survives. That path
currently 500s and loses nothing.

_read_entries() now returns [] only when the file genuinely does not exist and
raises MemoryStoreUnreadable for every other failure, including a store that
parses but is not a JSON array. load_all() keeps the old lenient behaviour so
display, search and context injection still degrade quietly instead of
breaking chat. The read-modify-write callers switch to load_all_for_update(),
which propagates the error: the memory routes turn it into a 503 and change
nothing, backup import refuses rather than saving only the incoming rows, and
auto-extraction and the audit merge skip the write. The audit merge mattered
most - it rebuilds the whole file from one owner's slice plus everyone else's
rows, so an empty read there dropped every other tenant's memories.

The corrupt-JSON path still gets its one shot at the legacy memory.txt
migration before raising, so that recovery is unchanged.

The two updated fakes gained load_all_for_update because the real class has it;
MagicMock would otherwise hand the import path a Mock instead of the seeded list.

Fixes #5673

* fix(memory): fail closed on the remaining read-modify-write add paths

The strict loader landed with the routes, the backup import and the extractor
converted, but three read-modify-write sinks still called load_all(), which
degrades an unreadable store to []. Two of them are the paths users actually
reach, so the data loss in #5673 stayed reproducible:

- src/ai_interaction.py do_manage_memory, action "add" — reached from ordinary
  chat via src/tool_execution.py:793 -> dispatch_ai_tool. "Remember that I
  prefer X" against an unreadable store wrote a one-entry file over it and
  reported success.
- mcp_servers/memory_server.py, action "add" — the same shape through
  _scope_entries(), registered as a built-in in src/builtin_mcp.py.
- src/memory_provider.py NativeMemoryProvider.remember and .delete — wired
  into app state in src/app_initializer.py but not consumed outside tests yet,
  converted here so the pattern is uniform before it goes live.

The MCP server takes _scope_entries(for_update=True) so list keeps the lenient
read. The edit and delete branches on both tool paths were already fail-closed
by accident — an empty view matches nothing and returns before the save — so
they are left alone.

The three new tests drive the real entry points rather than replaying the
shape, and use a truncated store, which is the case that reads back fine so
nothing stops the save. Each asserts memory.json is byte-identical afterwards;
all three fail on the previous commit with the store overwritten.

routes/memory/memory_routes.py

@@ -21,7 +21,7 @@ def _strip_list_prefix(text: str) -> str:
         return text
     return _LIST_PREFIX_RE.sub("", text, count=1).strip()
 
-from services.memory import MemoryManager
+from services.memory import MemoryManager, MemoryStoreUnreadable
 from core.session_manager import SessionManager
 from src.request_models import MemoryAddRequest
 from core.database import SessionLocal
@@ -35,6 +35,22 @@ from src.upload_limits import read_upload_limited, MEMORY_IMPORT_MAX_BYTES
 logger = logging.getLogger(__name__)
 
 
+def _load_for_update(memory_manager) -> List[Dict[str, Any]]:
+    """Load the whole store for a read-modify-write cycle.
+
+    A transient read failure must not look like an empty store: the caller
+    would append to ``[]`` and save that back, atomically destroying every
+    existing memory (issue #5673). Surface it as a 503 and change nothing.
+    """
+    try:
+        return memory_manager.load_all_for_update()
+    except MemoryStoreUnreadable as e:
+        logger.error("Refusing to rewrite the memory store: %s", e)
+        raise HTTPException(
+            503, "Memory store is temporarily unreadable — no changes were made."
+        )
+
+
 def setup_memory_routes(memory_manager: MemoryManager, session_manager: SessionManager, memory_vector=None):
     """Set up memory-related routes."""
     router = APIRouter(prefix="/api/memory", tags=["memory"])
@@ -116,7 +132,7 @@ def setup_memory_routes(memory_manager: MemoryManager, session_manager: SessionM
         new_entry = memory_manager.add_entry(text, memory_data.source, memory_data.category, owner=user)
         if memory_data.session_id:
             new_entry["session_id"] = memory_data.session_id
-        all_mem = memory_manager.load_all()
+        all_mem = _load_for_update(memory_manager)
         all_mem.append(new_entry)
         memory_manager.save(all_mem)
         # Sync vector index
@@ -487,7 +503,7 @@ def setup_memory_routes(memory_manager: MemoryManager, session_manager: SessionM
     def pin_memory(request: Request, memory_id: str, pinned: bool = Form(True)):
         """Pin or unpin a memory. Pinned memories are always included in context."""
         user = _owner(request)
-        all_mem = memory_manager.load_all()
+        all_mem = _load_for_update(memory_manager)
         for i, memory in enumerate(all_mem):
             if memory["id"] == memory_id:
                 _verify_memory_owner(memory, user)
@@ -512,7 +528,7 @@ def setup_memory_routes(memory_manager: MemoryManager, session_manager: SessionM
     def update_memory(request: Request, memory_id: str, text: str = Form(...), category: str = Form(None)):
         """Update an existing memory item with new text and optional category."""
         user = _owner(request)
-        all_mem = memory_manager.load_all()
+        all_mem = _load_for_update(memory_manager)
         for i, memory in enumerate(all_mem):
             if memory["id"] == memory_id:
                 _verify_memory_owner(memory, user)
@@ -534,7 +550,7 @@ def setup_memory_routes(memory_manager: MemoryManager, session_manager: SessionM
     def delete_memory(request: Request, memory_id: str):
         """Delete a memory item by its ID."""
         user = _owner(request)
-        all_mem = memory_manager.load_all()
+        all_mem = _load_for_update(memory_manager)
 
         # Find and verify ownership before deleting
         target = next((m for m in all_mem if m["id"] == memory_id), None)