b8bb8dbf20
SyncRepRequested() must check not only the value of the synchronous_replication GUC but also whether max_wal_senders > 0. Otherwise, we might end up waiting for sync rep even when there's no possibility of a standby ever managing to connect. There are some existing cross-checks to prevent this, but they're not quite sufficient: the user can start the server with max_wal_senders=0, synchronous_standby_names='', and synchronous_replication=off and then subsequent make synchronous_standby_names not empty using pg_ctl reload, and then SET synchronous_standby=on, leading to an indefinite hang. Along the way, rename the global variable for the synchronous_replication GUC to match the name of the GUC itself, for clarity. Report by Fujii Masao, though I didn't use his patch.
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