BugReport: PostgreSQL 17.8. Processing UTF8 encoded strings

Greetings,

After updating PostgreSQL from version 17.7 to 17.8, we encountered a
problem when extracting a substring from a UTF8 encoded string:
*ERROR:  invalid byte sequence for encoding "UTF8": 0xe2*

*
*

_Server:_
Linux i-db-sandbox1.4vrs.com 6.1.0-43-cloud-amd64 #1 SMP PREEMPT_DYNAMIC
Debian 6.1.162-1 (2026-02-08) x86_64 GNU/Linux

$ cat /etc/apt/sources.list.d/pgdg.list
deb http://apt.postgresql.org/pub/repos/apt/ bookworm-pgdg main
deb-src http://apt.postgresql.org/pub/repos/apt/ bookworm-pgdg main
deb https://apt-archive.postgresql.org/pub/repos/apt
bookworm-pgdg-archive main

PostgreSQL 17.8 (Debian 17.8-1.pgdg12+1) on x86_64-pc-linux-gnu,
compiled by gcc (Debian 12.2.0-14+deb12u1) 12.2.0, 64-bit

_Steps to reproduce (psql):_

db_fev:vladimir(at)i-db-sandbox1 => create table test123(id integer, m text);
CREATE TABLE

db_fev:vladimir(at)i-db-sandbox1 => insert into test123 (id,m) values (1,
repeat('a', 1027)||E'\xe2\x80\x8d'||repeat('a', 1027));
INSERT 0 1

db_fev:vladimir(at)i-db-sandbox1 => select length(SUBSTRING(m from 1 for
256)) from test123;
*ERROR:  invalid byte sequence for encoding "UTF8": 0xe2*

db_fev:vladimir(at)i-db-sandbox1 => select length(SUBSTRING(substring(m
from 1 for length(m)) from 1 for 256)) from test123;
 length
--------
    256
(1 row)

_Database db_feb:_
    Name    | Encoding  | Locale Provider | LC_COLLATE | LC_CTYPE |
Locale | ICU Rules |
------------+-----------+-----------------+------------+----------+--------+-----------+
 db_feb     | UTF8      | libc            | C          | C   | [NULL] |
[NULL]    |

The problem does not appear on PostgreSQL 17.7. Also, the problem does
not occur if the string is fully loaded into memory:

db_feb:vladimir(at)i-db-sandbox1 =# select length(SUBSTRING(*substring(m
from 1 for length(m))* from 1 for 256)) from test123;
 length
--------
    256
(1 row)

The bugreport has also been sent to bugs(at)postgrespro(dot)ru

--
Best Regards,
Vladimir Valikaev
Streamline - Property Management Software