buildfarm: could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes

We've seen several occurrences of $subject in the buildfarm in the past
month or so.  Scraping the logs, I find

 coypu    | 2018-06-14 21:17:49 | HEAD          | Check | 2018-06-14 23:31:44.505 CEST [5b22deb8.30e1:124] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 coypu    | 2018-06-14 21:17:49 | HEAD          | Check | 2018-06-14 23:31:44.653 CEST [5b22deb8.6d4d:132] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 gull     | 2018-06-21 04:02:11 | HEAD          | Check | 2018-06-20 21:27:06.843 PDT [31138:3] FATAL:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 mandrill | 2018-06-22 16:14:16 | HEAD          | Check | 2018-06-22 16:46:24.138 UTC [14353240:43] pg_regress/create_table_like ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 mandrill | 2018-06-22 16:14:16 | HEAD          | Check | 2018-06-22 16:46:24.137 UTC [20710034:1] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 lapwing  | 2018-07-05 07:20:02 | HEAD          | Check | 2018-07-05 07:21:45.585 UTC [24022:1] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 lapwing  | 2018-07-05 07:20:02 | HEAD          | Check | 2018-07-05 07:21:45.591 UTC [23941:39] pg_regress/roleattributes ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 mantid   | 2018-07-17 01:20:04 | REL_11_STABLE | Check | 2018-07-16 21:21:32.557 EDT [26072:1] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 mantid   | 2018-07-17 01:20:04 | REL_11_STABLE | Check | 2018-07-16 21:21:32.557 EDT [25990:82] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 coypu    | 2018-07-17 01:27:39 | HEAD          | Check | 2018-07-17 03:47:26.264 CEST [5b4d4aa4.3ac4:71] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes
 coypu    | 2018-07-20 11:18:02 | HEAD          | Check | 2018-07-20 13:32:14.104 CEST [5b51c833.4884:131] ERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes

While I didn't go back indefinitely far, there are no other occurrences of
"could not read block" failures in the last three months.  This suggests
strongly that we broke something in early June.  Don't know what though.
Ideas?

In case anyone's wondering, 2662 is pg_class_oid_index.

			regards, tom lane