Re: Support for N synchronous standby servers - take 2

Hello,

Please find attached the WIP patch for the proposed feature. It is built based on the already discussed design.

Changes made:

- add new parameter "sync_file" to provide the location of the pg_syncinfo file. The default is 'ConfigDir/pg_syncinfo.conf', same as for pg_hba and pg_ident file.

- pg_syncinfo file will hold the sync rep information in the approved JSON format.

- synchronous_standby_names can be set to 'pg_syncinfo.conf'  to read the JSON value stored in the file.

- All the standbys mentioned in the s_s_names or the pg_syncinfo file currently get the priority as 1 and all others as  0 (async)

- Various functions in syncrep.c to read the json file and store the values in a struct to be used in checking the quorum status of syncrep standbys (SyncRepGetQuorumRecPtr function).

It does not support the current behavior for synchronous_standby_names = '*'. I am yet to thoroughly test the patch.