Failures in constraints regression test, "read only 0 of 8192 bytes"

These two animals seem to have got mixed up about about the size of
this relation in the same place:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=avocet&dt=2024-02-28%2017%3A34%3A30
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=trilobite&dt=2024-03-01%2006%3A47%3A53

+++ /home/buildfarm/trilobite/buildroot/HEAD/pgsql.build/src/test/regress/results/constraints.out
2024-03-01 08:22:11.624897033 +0100
@@ -573,42 +573,38 @@
  UNIQUE (i) DEFERRABLE INITIALLY DEFERRED;
 BEGIN;
 INSERT INTO unique_tbl VALUES (1, 'five');
+ERROR:  could not read blocks 0..0 in file "base/16384/21437": read
only 0 of 8192 bytes

That error message changed slightly in my smgrreadv() commit a couple
of months ago (it would have been "block 0" and now it's "blocks 0..0"
because now we can read more than one block at a time) but I don't
immediately see how anything at that low level could be responsible
for this.