pgsql: Avoid mixing custom and OpenSSL BIO functions

Avoid mixing custom and OpenSSL BIO functions

PostgreSQL has for a long time mixed two BIO implementations, which can
lead to subtle bugs and inconsistencies. This cleans up our BIO by just
just setting up the methods we need. This patch does not introduce any
functionality changes.

The following methods are no longer defined due to not being needed:

- gets: Not used by libssl
- puts: Not used by libssl
- create: Sets up state not used by libpq
- destroy: Not used since libpq use BIO_NOCLOSE, if it was used it close
the socket from underneath libpq
- callback_ctrl: Not implemented by sockets

The following methods are defined for our BIO:

- read: Used for reading arbitrary length data from the BIO. No change
in functionality from the previous implementation.
- write: Used for writing arbitrary length data to the BIO. No change
in functionality from the previous implementation.
- ctrl: Used for processing ctrl messages in the BIO (similar to ioctl).
The only ctrl message which matters is BIO_CTRL_FLUSH used for
writing out buffered data (or signal EOF and that no more data
will be written). BIO_CTRL_FLUSH is mandatory to implement and
is implemented as a no-op since there is no intermediate buffer
to flush.
BIO_CTRL_EOF is the out-of-band method for signalling EOF to
read_ex based BIO's. Our BIO is not read_ex based but someone
could accidentally call BIO_CTRL_EOF on us so implement mainly
for completeness sake.

As the implementation is no longer related to BIO_s_socket or calling
SSL_set_fd, methods have been renamed to reference the PGconn and Port
types instead.

This also reverts back to using BIO_set_data, with our fallback, as a small
optimization as BIO_set_app_data require the ex_data mechanism in OpenSSL.

Author: David Benjamin <davidben(at)google(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/CAF8qwaCZ97AZWXtg_y359SpOHe+HdJ+p0poLCpJYSUxL-8Eo8A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6f782a2a1738ab96ee948a4ab33ca3defd39327b

Modified Files
--------------
src/backend/libpq/be-secure-openssl.c | 107 ++++++++++++++++++-------------
src/include/libpq/libpq-be.h | 1 +
src/interfaces/libpq/fe-secure-openssl.c | 99 ++++++++++++++++------------
src/interfaces/libpq/libpq-int.h | 1 +
4 files changed, 122 insertions(+), 86 deletions(-)