pgpool: Prevent pcp_node_info from hanging.

Prevent pcp_node_info from hanging.

The problem:
In inform_node_info() (src/pcp_con/pcp_worker.c), the PCP reply packet
reads bi->replication_state and bi->replication_sync_state directly
from shared memory twice: once via strlen() to compute the packet
length, and once via pcp_write() to write the payload.

The streaming-replication check worker rewrites those same
shared-memory strings without a lock (it clears them to "" then
repopulates them every check cycle and on state transitions,
src/streaming_replication/pool_worker_child.c). If the string's length
changes between the two reads, the declared wsize no longer matches
the bytes actually written, so the PCP byte stream desynchronises. The
client then blocks forever in pcp_read() waiting for bytes the server
never sends.

The fix:
Snapshot the two strings into local buffers once, right after bi =
pool_get_node_info(i), and use the locals for both the length and the
payload — so a single packet is always internally consistent. This
matches how every other field in the packet is already handled.

Author: Emond Papegaaij <emond(dot)papegaaij(at)gmail(dot)com>
Reviewed-by: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://www.postgresql.org/message-id/CAGXsc%2BZhGjwm%2BF42Xmt8Qn1qP_h7woipiV0WsY-e-P7W3ZG2OA%40mail.gmail.com
Backpatch-through: v4.3

Branch
------
master

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=7c918dc247613d16d590a9f30ecc747da6871796

Modified Files
--------------
src/pcp_con/pcp_worker.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)