pgsql: Ensure we discard unread/unsent data when abandoning a connectio

Ensure we discard unread/unsent data when abandoning a connection attempt.

There are assorted situations wherein PQconnectPoll() will abandon a
connection attempt and try again with different parameters (eg, SSL versus
not SSL). However, the code forgot to discard any pending data in libpq's
I/O buffers when doing this. In at least one case (server returns E
message during SSL negotiation), there is unread input data which bollixes
the next connection attempt. I have not checked to see whether this is
possible in the other cases where we close the socket and retry, but it
seems like a matter of good defensive programming to add explicit
buffer-flushing code to all of them.

This is one of several issues exposed by Daniel Farina's report of
misbehavior after a server-side fork failure.

This has been wrong since forever, so back-patch to all supported branches.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e5d2db5d22c98c9332919899d51f36d174b3990a

Modified Files
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src/interfaces/libpq/fe-connect.c | 15 +++++++++++++++
1 files changed, 15 insertions(+), 0 deletions(-)