BUG #17376: Adding unique column with a function() default results in "could not read block 0 in file" error

The following bug has been logged on the website:

Bug reference:      17376
Logged by:          akg
Email address:      adrien.gilmore+pg@gmail.com
PostgreSQL version: 13.5
Operating system:   Linux
Description:

Hello,

SQL demonstrating the issue on 13.5 is below. 

--
BEGIN;
CREATE TABLE t1 (id SERIAL PRIMARY KEY);
INSERT INTO t1 VALUES (default);

CREATE FUNCTION myfunc() RETURNS TEXT LANGUAGE plpgsql AS $$
  BEGIN
    SELECT r FROM t1;
    RETURN random()::text;
END $$;

ALTER TABLE t1 ADD COLUMN r TEXT NOT NULL UNIQUE DEFAULT myfunc();
--

Results in the error:
ERROR:  could not read block 0 in file "base/84505/84705": read only 0 of
8192 bytes

The error message content returned is what I suspect of being a bug, not so
much that this SQL didn't work.