pgsql: Avoid exposing WAL receiver raw conninfo during timeline jumps

Avoid exposing WAL receiver raw conninfo during timeline jumps

When reusing an existing WAL receiver after it has reached
WALRCV_WAITING for new instructions, RequestXLogStreaming() copied
PrimaryConnInfo into WalRcv->conninfo before switching the state to
WALRCV_RESTARTING. At that point ready_to_display could still be true,
so pg_stat_wal_receiver could expose the raw connection string,
including sensitive fields, but it should only show the user-displayable
version of the connection string.

WALRCV_RESTARTING does not establish a new connection. The waiting WAL
receiver reuses its existing connection and only needs a new startpoint
and timeline, so there is no need to copy the raw connection string into
shared memory again. Let's only copy conninfo when launching a new WAL
receiver after WALRCV_STOPPED, not while waiting for instructions.

This commit adds coverage for the case fixed by this commit to the
timeline-switch test by verifying that the WAL receiver conninfo remains
consistent across the jump.

Backpatch all the way down, as this issue is possible since
pg_stat_wal_receiver has been introduced.

Author: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/EF91FF76-1E2B-4F3B-9162-290B4DC517FF@gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e18b77153c740122a0eadde39ebfd5899156143e

Modified Files
--------------
src/backend/replication/walreceiverfuncs.c | 14 +++++++++-----
src/test/recovery/t/004_timeline_switch.pl | 17 ++++++++++++++---
2 files changed, 23 insertions(+), 8 deletions(-)