Fix no open transaction in clone --follow --snapshot

[ahmad-sanad]

Note: tests fail for me without #925

Issue

When calling snapshot then using the snapshot id with clone --follow --snapshot the code fails with BUG: call to pgsql_commit() without holding an open multi statement connection

This happens because:

1. clone_and_follow calls copydb_fetch_schema_and_prepare_specs which calls copydb_prepare_snapshot, since specs->sourceSnapshot.state is initially SNAPSHOT_STATE_UNKNOWN. copydb_fetch_schema_and_prepare_specs later calls copydb_close_snapshot which commits the transaction and sets the snapshot state to SNAPSHOT_STATE_CLOSED
2. clone_and_follow later calls start_clone_process which calls cloneDB which calls copydb_fetch_schema_and_prepare_specs again. This time specs->sourceSnapshot.state is SNAPSHOT_STATE_CLOSED so copydb_prepare_snapshot is not called. The code later tries to set the search path, which fails with a warning, then tries to commit the transaction, which fails with this error since no transaction was opened.

Fix:

copydb_prepare_snapshot checks if the source snapshot state is SNAPSHOT_STATE_UNKNOWN or SNAPSHOT_STATE_CLOSED in order to prepare the snapshshot and open a transaction.
copydb_fetch_schema_and_prepare_specs on the other hand, only calls copydb_prepare_snapshot if the snapshot state is SNAPSHOT_STATE_UNKNOWN.
This code changes copydb_fetch_schema_and_prepare_specs to match copydb_prepare_snapshot and call it in either state.

Full code invocation notes:

clone --snapshot --follow:

cli_clone_follow.cli_clone
  cli_clone_follow.clone_and_follow
    copydb_schema.copydb_fetch_schema_and_prepare_specs
      snapshot.copydb_prepare_snapshot (because specs->sourceSnapshot.state == SNAPSHOT_STATE_UNKNOWN)
        snapshot.copydb_set_snapshot
          pgsql.pgsql_init (setup db connection)
          pgsql.pgsql_begin (open transaction (BEGIN) and keep it open (PGSQL_CONNECTION_MULTI_STATEMENT))
          pgsql.pgsql_set_transaction (set transaction isolation level)
          pgsql.pgsql_set_snapshot (SET TRANSACTION SNAPSHOT)
      copydb_schema.copydb_fetch_source_schema
        pgsql.pgsql_prepend_search_path
          pgsql.pgsql_set_search_path
            successfully sets search path since the transaction is still open
      snapshot.copydb_close_snapshot (copySpecs->sourceSnapshot.state = SNAPSHOT_STATE_CLOSED)
        end transaction (COMMIT) and close DB connection and set the state to SNAPSHOT_STATE_CLOSED
        copySpecs->sourceSnapshot.state = SNAPSHOT_STATE_CLOSED;
    cli_clone_follow.start_clone_process
      cli_clone_follow.cloneDB
        copydb_schema.copydb_fetch_schema_and_prepare_specs (because specs->sourceSnapshot.state == SNAPSHOT_STATE_CLOSED so we do not call copydb_prepare_snapshot)
          copydb_schema.copydb_fetch_source_schema
            pgsql.pgsql_prepend_search_path
              pgsql.pgsql_set_search_path
                pgsql.pgsql_execute
                  pgsql.pgsql_execute_with_params
                    fails to sets search path since there is no open transaction
                    WARNING:  SET LOCAL can only be used in transaction blocks
          pgsql.pgsql_commit (specs->sourceSnapshot.state == SNAPSHOT_STATE_UNKNOWN is false since SNAPSHOT_STATE_CLOSED is set. preparedSnapshot is false)
            There is no open transaction since we closed it earlier and did not call copydb_prepare_snapshot again since the state was not SNAPSHOT_STATE_UNKNOWN
            BUG: call to pgsql_commit() without holding an open multi statement connection

clone --snapshot without follow, works without any issues because:

cli_clone_follow.cli_clone
  snapshot.copydb_should_export_snapshot (false since we have --snapshot so we do not call copydb_prepare_snapshot)
  cli_clone_follow.start_clone_process
    cli_clone_follow.cloneDB (same as above. We never ran copydb_prepare_snapshot. This means the state will be SNAPSHOT_STATE_UNKNOWN so we will run it later and properly have a transaction open)