Unique Contiguous Ranges

How to get unique, non-overlapping, contiguous ranges, usable by concurrent sessions?

DROP TABLE IF EXISTS single_row_table;

CREATE TABLE single_row_table ( id INTEGER NOT NULL );


-- This unique index enforces that only a single row can ever be in this table.
CREATE UNIQUE INDEX one_row_only_uidx ON single_row_table (( true ));

INSERT INTO single_row_table (id) VALUES (1);

-- Drop sequence if it already exists
DROP SEQUENCE IF EXISTS single_row_table_seq;

-- Create the sequence we will use to generate the ranges
CREATE SEQUENCE single_row_table_seq;


-- Let's assume we want 5 values at a time
-- Note: the "begin;commit;" is optional,
--  it is used to demonstrate multiple sessions waiting.
-- Alternatively, for testing, add "pg_sleep(10)" (no quotes)
--  to the SELECT below
begin;
SELECT string_agg(id::text,',' ORDER BY 1) as id_range
FROM (
    SELECT nextval('single_row_table_seq') AS id
    FROM generate_series(1, 5) n
    JOIN LATERAL (
        /* block access to the row in single_row_table */
        SELECT id
        FROM single_row_table
        FOR UPDATE) x ON true
    ) y;


-- session 1 immediately returns:
 id_range
-----------
 1,2,3,4,5

commit;

-- session 2 waits until session 1 commits,
-- then returns:
  id_range
------------
 6,7,8,9,10