pgsql: Fix unbounded recursive handling of SSL/GSS in ProcessStartupPac

Fix unbounded recursive handling of SSL/GSS in ProcessStartupPacket()

The handling of SSL and GSS negotiation messages in
ProcessStartupPacket() could cause a recursion of the backend,
ultimately crashing the server as the negotiation attempts were not
tracked across multiple calls processing startup packets.

A malicious client could therefore alternate rejected SSL and GSS
requests indefinitely, each adding a stack frame, until the backend
crashed with a stack overflow, taking down a server.

This commit addresses this issue by modifying ProcessStartupPacket() so
as processed negotiation attempts are tracked, preventing infinite
recursive attempts. A TAP test is added to check this problem, where
multiple SSL and GSS negotiated attempts are stacked.

Reported-by: Calif.io in collaboration with Claude and Anthropic
Research
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Security: CVE-2026-6479
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/32a4ce55ccabe4c1e9b1e45d4efc1e62e69fc754
Author: Michael Paquier <michael(at)paquier(dot)xyz>

Modified Files
--------------
src/backend/tcop/backend_startup.c | 23 +++++++++-
src/test/Makefile | 2 +-
src/test/meson.build | 1 +
src/test/postmaster/.gitignore | 2 +
src/test/postmaster/Makefile | 23 ++++++++++
src/test/postmaster/README | 27 +++++++++++
src/test/postmaster/meson.build | 12 +++++
src/test/postmaster/t/004_negotiate.pl | 82 ++++++++++++++++++++++++++++++++++
8 files changed, 169 insertions(+), 3 deletions(-)