sql: specific strings in table data cause job to never terminate

[knz]

Run the following SQL:

> CREATE TABLE t AS SELECT 'must be after GC replica threshold' AS x;
> ALTER TABLE t ADD COLUMN y INT AS (x::INT) STORED;

Observe: the job remains in status "running" forever

root@/defaultdb> show jobs;
-[ RECORD 1 ]
job_id             | 564125022800936961
job_type           | SCHEMA CHANGE
description        | ALTER TABLE defaultdb.public.t ADD COLUMN y INT8 AS (x::INT8) STORED
statement          |
user_name          | root
status             | running
running_status     | populating schema
created            | 2020-06-15 13:28:14.555411+00:00
started            | 2020-06-15 13:28:14.56666+00:00
finished           | NULL
modified           | 2020-06-15 13:30:13.527146+00:00
fraction_completed | 0
error              |
coordinator_id     | 1

This bug had been predicted all the way in this and that RFC sections.

The problem is this code in isPermanentSchemaChangeError():

  // Ignore error thrown because of a read at a very old timestamp.
  // The Backfill will grab a new timestamp to read at for the rest
  // of the backfill.
  if strings.Contains(⊙.Error(), "must be after replica GC threshold") {
    return false
  }

(With kudos to @ajwerner for helping me figuring out the exact example syntax)