Annotations for psycopg2.ConnectionInfo (#7834)

* Annotations for psycopg2.ConnectionInfo

These annotations come from the documentation here:

https://www.psycopg.org/docs/extensions.html#psycopg2.extensions.ConnectionInfo
If there was doubt, I referred to the libpq documentation cited by
psycopg2's docs.

I wasn't completely sure about `dsn_parameters`. Psycopg2's docs list it
as an `dict`, and the example suggests it's a `dict[str, str]` at that.
From psycopg2's source I found

    1d3a89a0bb/psycopg/conninfo_type.c (L183-L206)

which is implemented here:

    1d3a89a0bb/psycopg/utils.c (L251-L279)

I'm no expert in CPython's API, but this looks to me like it's building
a `dict[str, str]`.

Additionally, the libpq docs

https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQCONNINFO
https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQCONNDEFAULTS

show that the underlying data just consists of strings.

Additionally, I'm pretty sure from this chunk of source

    1d3a89a0bb/psycopg/conninfo_type.c (L581-L598)

That `ConnectionInfo.__init__` takes one positional-only argument, which
must be a `psycopg2.connection`. But I don't think users are intended to
be constructing this type, so I've not added that annotation.

* Annotate `connection.info` and related attributes

* Make ConnectionInfo attributes properties

According to 1d3a89a0bb/psycopg/conninfo_type.c (L534-L563)

* Mark connection attributes as readonly

according to 8ef195f2ff/psycopg/connection_type.c (L1244)

* Explain why some properties aren't `T | None`

stubs/psycopg2/psycopg2/_psycopg.pyi

@@ -157,27 +157,63 @@ class Column:
     def __setstate__(self, state): ...
 
 class ConnectionInfo:
-    backend_pid: Any
-    dbname: Any
-    dsn_parameters: Any
-    error_message: Any
-    host: Any
-    needs_password: Any
-    options: Any
-    password: Any
-    port: Any
-    protocol_version: Any
-    server_version: Any
-    socket: Any
-    ssl_attribute_names: Any
-    ssl_in_use: Any
-    status: Any
-    transaction_status: Any
-    used_password: Any
-    user: Any
+    # Note: the following properties can be None if their corresponding libpq function
+    # returns NULL. They're not annotated as such, because this is very unlikely in
+    # practice---the psycopg2 docs [1] don't even mention this as a possibility!
+    #
+    # - db_name
+    # - user
+    # - password
+    # - host
+    # - port
+    # - options
+    #
+    # (To prove this, one needs to inspect the psycopg2 source code [2], plus the
+    # documentation [3] and source code [4] of the corresponding libpq calls.)
+    #
+    # [1]: https://www.psycopg.org/docs/extensions.html#psycopg2.extensions.ConnectionInfo
+    # [2]: https://github.com/psycopg/psycopg2/blob/1d3a89a0bba621dc1cc9b32db6d241bd2da85ad1/psycopg/conninfo_type.c#L52 and below
+    # [3]: https://www.postgresql.org/docs/current/libpq-status.html
+    # [4]: https://github.com/postgres/postgres/blob/b39838889e76274b107935fa8e8951baf0e8b31b/src/interfaces/libpq/fe-connect.c#L6754 and below
+    @property
+    def backend_pid(self) -> int: ...
+    @property
+    def dbname(self) -> str: ...
+    @property
+    def dsn_parameters(self) -> dict[str, str]: ...
+    @property
+    def error_message(self) -> str | None: ...
+    @property
+    def host(self) -> str: ...
+    @property
+    def needs_password(self) -> bool: ...
+    @property
+    def options(self) -> str: ...
+    @property
+    def password(self) -> str: ...
+    @property
+    def port(self) -> int: ...
+    @property
+    def protocol_version(self) -> int: ...
+    @property
+    def server_version(self) -> int: ...
+    @property
+    def socket(self) -> int: ...
+    @property
+    def ssl_attribute_names(self) -> list[str]: ...
+    @property
+    def ssl_in_use(self) -> bool: ...
+    @property
+    def status(self) -> int: ...
+    @property
+    def transaction_status(self) -> int: ...
+    @property
+    def used_password(self) -> bool: ...
+    @property
+    def user(self) -> str: ...
     def __init__(self, *args, **kwargs) -> None: ...
-    def parameter_status(self, *args, **kwargs): ...
-    def ssl_attribute(self, *args, **kwargs): ...
+    def parameter_status(self, name: str) -> str | None: ...
+    def ssl_attribute(self, name: str) -> str | None: ...
 
 class DataError(psycopg2.DatabaseError): ...
 class DatabaseError(psycopg2.Error): ...
@@ -334,24 +370,35 @@ class connection:
     OperationalError: Any
     ProgrammingError: Any
     Warning: Any
-    async_: Any
+    @property
+    def async_(self) -> Any: ...
     autocommit: Any
-    binary_types: Any
-    closed: Any
+    @property
+    def binary_types(self) -> Any: ...
+    @property
+    def closed(self) -> Any: ...
     cursor_factory: Callable[..., _cursor]
     deferrable: Any
-    dsn: Any
-    encoding: Any
-    info: Any
+    @property
+    def dsn(self) -> Any: ...
+    @property
+    def encoding(self) -> Any: ...
+    @property
+    def info(self) -> ConnectionInfo: ...
     isolation_level: Any
     notices: Any
     notifies: Any
-    pgconn_ptr: Any
-    protocol_version: Any
+    @property
+    def pgconn_ptr(self) -> Any: ...
+    @property
+    def protocol_version(self) -> int: ...
     readonly: Any
-    server_version: Any
-    status: Any
-    string_types: Any
+    @property
+    def server_version(self) -> int: ...
+    @property
+    def status(self) -> Any: ...
+    @property
+    def string_types(self) -> Any: ...
     def __init__(self, *args, **kwargs) -> None: ...
     def cancel(self, *args, **kwargs): ...
     def close(self, *args, **kwargs): ...
@@ -361,11 +408,11 @@ class connection:
     @overload
     def cursor(self, name=..., cursor_factory: Callable[..., _T_cur] = ..., scrollable=..., withhold=...) -> _T_cur: ...
     def fileno(self, *args, **kwargs): ...
-    def get_backend_pid(self, *args, **kwargs): ...
-    def get_dsn_parameters(self, *args, **kwargs): ...
+    def get_backend_pid(self) -> int: ...
+    def get_dsn_parameters(self) -> dict[str, str]: ...
     def get_native_connection(self, *args, **kwargs): ...
-    def get_parameter_status(self, parameter): ...
-    def get_transaction_status(self): ...
+    def get_parameter_status(self, parameter: str) -> str | None: ...
+    def get_transaction_status(self) -> int: ...
     def isexecuting(self, *args, **kwargs): ...
     def lobject(self, oid=..., mode=..., new_oid=..., new_file=..., lobject_factory=...): ...
     def poll(self, *args, **kwargs): ...