fix(storage): address PR #798 round-3 review (SonarQube + pool sizing + RETURNING test)

Round-3 fixes. Two threads:
- 9 OPEN SonarQube issues caused the "E Security Rating on New Code"
  gate failure. The bot's diagnosis (sa.text(text_sql) → SQL injection)
  was a wrong guess; the actual SQ rules firing were different.
- Bot's substantive concerns: pool defaults too aggressive,
  delete_browser_session RETURNING path untested on Postgres,
  schema_version legacy table created on Postgres, stale module
  docstring.
- User's underlying question on the pool: "isn't 1 connection enough?"
  Right-sized to 2+5 and documented the concurrency model in ADR-026
  so the rationale is durable.

SonarQube quality-gate fixes (clears all 9 OPEN issues)
-------------------------------------------------------
- BLOCKER S6418: rename `SECRET` constant in test_storage_logging.py
  to `SENTINEL_PASSWORD_FRAGMENT` + NOSONAR with rationale.
- CRITICAL S3776: extract `_build_postgres_engine()` from
  `initialize()` (was complexity 26 > 15); incidentally creates a
  clean unit-test seam for engine args.
- CRITICAL S4423: `ssl.create_default_context(cafile=...)` is flagged
  as "weak protocol" — Python 3.10+ already negotiates the strongest
  available protocol. Explicitly pass `purpose=ssl.Purpose.SERVER_AUTH`
  and NOSONAR with the Python-version rationale.
- MAJOR S3358: split the TLS-mode nested ternary in the engine
  factory into a `_describe_ssl_arg()` helper.
- MAJOR S2068 ×3: bind test app-password literals to local vars and
  put `# NOSONAR S2068` on the same line as the literal (anchoring
  requirement) instead of on the closing paren.
- MINOR S7503 ×2: `# NOSONAR S7503` on `_Cursor.__aenter__/__aexit__`
  — they MUST be `async` per the context-manager protocol.

Pool sizing right-sized (answers "why so many connections?")
------------------------------------------------------------
- `DATABASE_POOL_SIZE` default 10 → **2**.
- `DATABASE_MAX_OVERFLOW` default 20 → **5**.
- Per-pod max drops from 30 to 7. With 3 replicas, total = 21
  connections (was 90) — well under managed-Postgres
  `max_connections=100`.
- New INFO log at startup: `Postgres engine ready: pool_size=N
  max_overflow=M (per-pod max K connections)`. Surfaces the active
  sizing without grepping config.
- New ADR-026 § "Concurrency model and pool sizing" explains
  asyncpg's single-flight connection semantics, the MCP workload
  shape (read-mostly point lookups), why-not-1 (multi-user
  serialization), and the tune-up/tune-down recipe.
- `docs/configuration.md` table updated with new defaults +
  homelab-vs-prod tuning guidance, linking the ADR.

RETURNING path covered on Postgres
----------------------------------
- New `test_browser_session_delete_returning` exercises the
  `DELETE … RETURNING user_id` path — the only RETURNING clause in
  the storage layer and the most dialect-sensitive SQL in this PR.
  Asserts both present-row (returns True, row gone) and absent-row
  (returns False) branches.

Schema portability polish
-------------------------
- `alembic 001`: gate `schema_version` table creation on
  `op.get_bind().dialect.name == "sqlite"`. The table exists purely
  to match the fingerprint of pre-Alembic SQLite databases; fresh
  Postgres installs no longer carry the dead legacy table.

Misc polish
-----------
- Module docstring: "SQLite-based" → "SQL-backed", with a sentence
  on the DATABASE_URL opt-in and an ADR-026 link.
- Comment on `_wrap_row` noting `row._mapping` is the documented
  RowMapping accessor in SQLAlchemy 2.x despite the underscore.

Skipped (rationale in PR reply)
-------------------------------
- `_qmark_to_named` SQL-comment handling: docstring already notes
  the limitation; no `?` in storage SQL comments today.
- Module-level `anyio.Lock()`: established precedent confirmed by
  the bot itself.
- `get_audit_logs` `SELECT *`: pre-existing pattern, out of scope.

Verification
------------
- `uv run pytest tests/unit/` — 1025 passed.
- `TEST_DATABASE_URL=… uv run pytest tests/integration/test_storage_postgres.py -m postgres` — 7 passed.
- `ruff check && ruff format --check && ty check` — clean.
- Confirmed `schema_version` absent on fresh Postgres, still present
  on fresh SQLite.

Tracked on Astrolabe Cloud POC board, card #99.

---

_This PR was generated with the help of AI, and reviewed by a Human_

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/alembic/versions/20251217_2200_001_initial_schema.py

@@ -111,14 +111,18 @@ def upgrade() -> None:
         ["mcp_authorization_code"],
     )
 
-    # Legacy schema-version table; superseded by alembic_version. Retained
-    # so pre-Alembic databases that get stamped into the migration chain
-    # still match the schema fingerprint they had on disk.
-    op.create_table(
-        "schema_version",
-        sa.Column("version", sa.Integer, primary_key=True, autoincrement=False),
-        sa.Column("applied_at", sa.Float, nullable=False),
-    )
+    # Legacy schema-version table; superseded by alembic_version. Only
+    # created on SQLite because it exists *purely* to match the
+    # fingerprint of pre-Alembic SQLite databases that get stamped into
+    # the migration chain (see ``RefreshTokenStorage.initialize()``).
+    # Fresh Postgres installs have no pre-Alembic history and don't
+    # need it. PR #798 round-3 review (#4).
+    if op.get_bind().dialect.name == "sqlite":
+        op.create_table(
+            "schema_version",
+            sa.Column("version", sa.Integer, primary_key=True, autoincrement=False),
+            sa.Column("applied_at", sa.Float, nullable=False),
+        )
 
     op.create_table(
         "registered_webhooks",
@@ -144,7 +148,9 @@ def downgrade() -> None:
     op.drop_index("idx_webhooks_created", table_name="registered_webhooks")
     op.drop_index("idx_webhooks_preset", table_name="registered_webhooks")
     op.drop_table("registered_webhooks")
-    op.drop_table("schema_version")
+    # ``schema_version`` is only created on SQLite (see ``upgrade()``).
+    if op.get_bind().dialect.name == "sqlite":
+        op.drop_table("schema_version")
     op.drop_index("idx_oauth_sessions_mcp_code", table_name="oauth_sessions")
     op.drop_table("oauth_sessions")
     op.drop_table("oauth_clients")

nextcloud_mcp_server/auth/storage.py

@@ -1,8 +1,14 @@
 """
 Persistent Storage for MCP Server State
 
-This module provides SQLite-based storage for multiple concerns across both
-BasicAuth and OAuth authentication modes:
+This module provides SQL-backed storage for multiple concerns across both
+BasicAuth and OAuth authentication modes. The default backend is SQLite
+(file-based or per-process tempfile); set ``DATABASE_URL`` to a
+``postgresql+asyncpg://...`` URL for HA k8s deployments where pods need
+to be stateless. See :doc:`ADR-026 </docs/ADR-026-pluggable-database-backend>`
+for the design.
+
+Concerns covered:
 
 1. **Refresh Tokens** (OAuth mode only, for background jobs)
    - Securely stores encrypted refresh tokens for offline access
@@ -145,9 +151,26 @@ class _Row:
 def _wrap_row(row) -> _Row | None:
     if row is None:
         return None
+    # ``row._mapping`` is the documented public RowMapping accessor in
+    # SQLAlchemy 2.x (the leading underscore is historical); it returns
+    # a column-name → value mapping that survives the row being
+    # tuple-iterated. See SQLAlchemy 2.x ``Row.mapping`` docs.
     return _Row(tuple(row), dict(row._mapping))
 
 
+def _describe_ssl_arg(ssl_arg: object) -> str:
+    """Render the ``ssl`` value for the startup log line.
+
+    Split out of the engine factory to avoid a nested-ternary
+    SonarQube finding (``S3358``) and to make the cases readable.
+    """
+    if ssl_arg is False:
+        return "disabled"
+    if isinstance(ssl_arg, bool):
+        return "verify-full (system CAs)"
+    return "custom CA bundle"
+
+
 def _wrap_rows(rows) -> list[_Row]:
     """Wrap a list of SQLAlchemy rows; iterator never yields ``None``."""
     return [_Row(tuple(r), dict(r._mapping)) for r in rows]
@@ -182,10 +205,14 @@ class _Cursor:
     async def fetchall(self) -> list[_Row]:
         return _wrap_rows(self._result.fetchall())
 
-    async def __aenter__(self) -> "_Cursor":
+    # NOSONAR S7503 on the next two methods — Python's async-context-manager
+    # protocol *requires* ``__aenter__`` / ``__aexit__`` to be coroutines
+    # even when the body has nothing to await; dropping ``async`` would
+    # break ``async with _Cursor(...)``.
+    async def __aenter__(self) -> "_Cursor":  # NOSONAR S7503
         return self
 
-    async def __aexit__(self, *exc: object) -> None:
+    async def __aexit__(self, *exc: object) -> None:  # NOSONAR S7503
         # SQLAlchemy Result closes when the connection closes; no-op here.
         return None
 
@@ -419,8 +446,8 @@ class RefreshTokenStorage:
 
         # Create the shared async engine for the chosen backend. SQLite uses
         # NullPool (per-call connections, matches the prior aiosqlite-direct
-        # behavior); Postgres uses the default pool with pre-ping so dropped
-        # connections from idle k8s networks are retried transparently.
+        # behavior); Postgres uses a small bounded pool — see
+        # ``_build_postgres_engine`` for sizing rationale.
         if is_sqlite:
             self.engine = create_async_engine(
                 self.database_url,
@@ -429,45 +456,7 @@ class RefreshTokenStorage:
                 future=True,
             )
         else:
-            # Postgres ships as an optional PyPI extra (`[postgres]`) so the
-            # default `pip install nextcloud-mcp-server` audience doesn't
-            # pull in asyncpg's C extension. The Docker image bundles it.
-            # Surface a clear actionable error when the driver is missing
-            # rather than the generic ModuleNotFoundError SQLAlchemy emits.
-            if "+asyncpg" in self.database_url.lower() and (
-                importlib.util.find_spec("asyncpg") is None
-            ):
-                raise RuntimeError(
-                    "DATABASE_URL points at Postgres via asyncpg but the "
-                    "'asyncpg' driver is not installed. Install with "
-                    "`pip install nextcloud-mcp-server[postgres]` or use "
-                    "the Docker image, which bundles it. See ADR-026."
-                )
-            # Conditionally pass TLS config through to asyncpg. When
-            # get_database_ssl() returns None we omit ``ssl`` entirely so
-            # asyncpg's default (``prefer``) applies — keeps cluster-local
-            # Postgres without TLS working out of the box. See ADR-026.
-            connect_args: dict[str, object] = {}
-            ssl_arg = get_database_ssl()
-            if ssl_arg is not None:
-                connect_args["ssl"] = ssl_arg
-                logger.info(
-                    "Postgres backend TLS: %s",
-                    "disabled"
-                    if ssl_arg is False
-                    else "custom CA bundle"
-                    if not isinstance(ssl_arg, bool)
-                    else "verify-full (system CAs)",
-                )
-            settings = get_settings()
-            self.engine = create_async_engine(
-                self.database_url,
-                pool_size=settings.database_pool_size,
-                max_overflow=settings.database_max_overflow,
-                pool_pre_ping=True,
-                connect_args=connect_args,
-                future=True,
-            )
+            self.engine = self._build_postgres_engine()
         self._dialect = self.engine.dialect.name
 
         # Check database state with the SQLAlchemy inspector so the legacy
@@ -511,6 +500,64 @@ class RefreshTokenStorage:
             mask_db_password(self.database_url),
         )
 
+    def _build_postgres_engine(self) -> AsyncEngine:
+        """Construct the AsyncEngine for a Postgres ``DATABASE_URL``.
+
+        Split out from :meth:`initialize` so cognitive complexity stays
+        under the SonarQube ``S3776`` threshold and so a future
+        engine-arg unit test has a single seam to mock.
+
+        Defaults to ``pool_size=2, max_overflow=5`` (max 7 connections
+        per pod) — see ADR-026 § "Concurrency model and pool sizing"
+        for the rationale. asyncpg connections are single-flight, so
+        the pool only needs to cover the typical multi-user MCP burst,
+        not every potential in-flight tool call.
+        """
+        # asyncpg ships as an optional PyPI extra (`[postgres]`) so the
+        # default `pip install nextcloud-mcp-server` audience doesn't
+        # pull in the C extension. The Docker image bundles it. Surface
+        # a clear actionable error when the driver is missing rather
+        # than the generic ``ModuleNotFoundError`` SQLAlchemy emits.
+        if "+asyncpg" in self.database_url.lower() and (
+            importlib.util.find_spec("asyncpg") is None
+        ):
+            raise RuntimeError(
+                "DATABASE_URL points at Postgres via asyncpg but the "
+                "'asyncpg' driver is not installed. Install with "
+                "`pip install nextcloud-mcp-server[postgres]` or use "
+                "the Docker image, which bundles it. See ADR-026."
+            )
+
+        # Conditionally pass TLS config through to asyncpg. When
+        # ``get_database_ssl()`` returns None we omit ``ssl`` entirely
+        # so asyncpg's default (``prefer``) applies — keeps
+        # cluster-local Postgres without TLS working out of the box.
+        connect_args: dict[str, object] = {}
+        ssl_arg = get_database_ssl()
+        if ssl_arg is not None:
+            connect_args["ssl"] = ssl_arg
+            logger.info("Postgres backend TLS: %s", _describe_ssl_arg(ssl_arg))
+
+        settings = get_settings()
+        engine = create_async_engine(
+            self.database_url,
+            pool_size=settings.database_pool_size,
+            max_overflow=settings.database_max_overflow,
+            pool_pre_ping=True,
+            connect_args=connect_args,
+            future=True,
+        )
+        # Log the configured sizing so operators can spot
+        # over-allocation at startup without grepping config.
+        logger.info(
+            "Postgres engine ready: pool_size=%d max_overflow=%d "
+            "(per-pod max %d connections)",
+            settings.database_pool_size,
+            settings.database_max_overflow,
+            settings.database_pool_size + settings.database_max_overflow,
+        )
+        return engine
+
     @asynccontextmanager
     async def _db(self):
         """Open a backend-agnostic connection.

nextcloud_mcp_server/config.py

@@ -67,12 +67,14 @@ _DEFAULTS: dict[str, Any] = {
     # homelab servers. DATABASE_CA_BUNDLE points at a private-CA PEM.
     "database_verify_ssl": None,
     "database_ca_bundle": None,
-    # Postgres connection pool sizing (ADR-026, reviewer feedback on #798).
-    # Per-pod pool defaults to 10 + 20 overflow = 30 max connections.
-    # With many replicas this can blow past managed-Postgres
-    # `max_connections=100`; tune down via env when needed.
-    "database_pool_size": 10,
-    "database_max_overflow": 20,
+    # Postgres connection pool sizing (ADR-026 → "Concurrency model and
+    # pool sizing"). Per-pod defaults to 2 + 5 overflow = 7 max
+    # connections. asyncpg connections are single-flight, so the pool
+    # only needs to cover typical multi-user MCP burst — not every
+    # potential in-flight tool call. Tune up with DATABASE_POOL_SIZE /
+    # DATABASE_MAX_OVERFLOW for high-traffic prod fleets.
+    "database_pool_size": 2,
+    "database_max_overflow": 5,
     # Webhook delivery authentication (ADR-010): when set, registrations
     # tell NC to add `Authorization: Bearer <secret>` to webhook deliveries
     # and the receiver rejects unauthenticated requests.
@@ -515,9 +517,12 @@ class Settings:
     database_ca_bundle: str | None = None
     # Postgres connection pool sizing (ADR-026). The asyncpg engine maps
     # these to its underlying QueuePool. Per-pod max = pool_size +
-    # max_overflow. Validate >= 1 in __post_init__.
-    database_pool_size: int = 10
-    database_max_overflow: int = 20
+    # max_overflow. Defaults are intentionally small (2 + 5 = 7) because
+    # asyncpg connections are single-flight and the typical MCP workload
+    # is light read-mostly point lookups. Validate >= 1 / >= 0 in
+    # __post_init__.
+    database_pool_size: int = 2
+    database_max_overflow: int = 5
 
     # ADR-005: Token Audience Validation (required for OAuth mode)
     nextcloud_mcp_server_url: str | None = None  # MCP server URL (used as audience)
@@ -1166,7 +1171,15 @@ def get_database_ssl() -> bool | ssl.SSLContext | None:
     if settings.database_verify_ssl is False:
         return False
     if settings.database_ca_bundle:
-        return ssl.create_default_context(cafile=settings.database_ca_bundle)
+        # ``ssl.create_default_context()`` on Python 3.10+ already negotiates
+        # the strongest available protocol (TLS 1.2+ with secure ciphers);
+        # we pin Python 3.11+ in pyproject.toml. ``purpose=SERVER_AUTH`` is
+        # the default but spelt out here so static analysers (SonarQube
+        # ``S4423``) can see it explicitly. NOSONAR S4423
+        return ssl.create_default_context(  # NOSONAR S4423
+            purpose=ssl.Purpose.SERVER_AUTH,
+            cafile=settings.database_ca_bundle,
+        )
     if settings.database_verify_ssl is True:
         return True
     return None