Add the Python-facing `Connection` / `Cursor` pair and the `connect`
factory, implementing enough of the PEP-249 (DBAPI2) shape for Synapse's
needs.
A `Connection` owns a single `tokio_postgres::Client`. The client *moves*
between the connection and an in-flight cursor rather than being shared:
it lives in the connection between interactions, is taken out for the
duration of a cursor, and is handed back when the cursor finishes. That
single-owner baton (an `Option<Client>` slot on each side) makes it
structurally impossible to use the connection mid-transaction or to run
two overlapping transactions on one socket — both become a clean "already
closed" error.
The transaction lifecycle (`cursor` opens with `BEGIN`; `finish`
COMMIT/ROLLBACKs and hands the client back; `CursorGuard` rolls back an
abandoned transaction on drop) is included here. On any
transaction-control error the client is dropped rather than returned,
closing the socket — safer than handing a possibly-broken connection back
to what will become a connection pool.
`connect()` parses a libpq-style DSN, blocks until connected, and spawns
the long-lived connection task onto the shared runtime (the libpq
default-host fixup is a follow-up). The cursor query methods
(`execute` / `fetch_one` / `fetch_all` / `fetch_next_batch` / `rowcount`)
delegate to the `CursorQueryState` machine.
`run_interaction` — the high-level glue that opens a cursor, runs the
callback, and commits/rolls back — follows in the next change, so
`cursor`/`finish`/`CursorGuard` carry a transitional `#[allow(dead_code)]`
until then. Now that the value/helpers/cursor_state modules are consumed
internally, their visibility is tightened from `pub` back to private.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
