Horton

Re: HOT chain validation in verify_heapam()

Aleksander Alekseev <aleksander@timescale.com>

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From: Aleksander Alekseev <aleksander@timescale.com>
To: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Cc: Himanshu Upadhyaya <upadhyaya.himanshu@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Peter Geoghegan <pg@bowt.ie>, Andrew Dunstan <andrew@dunslane.net>, Mark Dilger <mark.dilger@enterprisedb.com>
Date: 2022-09-06T12:29:22Z
Lists: pgsql-hackers 
Hi Himanshu,

Many thanks for working on this!

> Please find attached the patch with the above idea of HOT chain's validation

Please correct me if I'm wrong, but don't we have a race condition here:

```
+            if ((TransactionIdDidAbort(pred_xmin) ||
TransactionIdIsInProgress(pred_xmin))
+                && !TransactionIdEquals(pred_xmin, curr_xmin))
             {
```

The scenario that concerns me is the following:

1. TransactionIdDidAbort(pred_xmin) returns false
2. The transaction aborts
3. TransactionIdIsInProgress(pred_xmin) returns false
4. (false || false) gives us false. An error is reported, although
actually the condition should have been true.

-- 
Best regards,
Aleksander Alekseev

Commits

  1. amcheck: Generalize one of the recently-added update chain checks.

  2. amcheck: Tighten up validation of redirect line pointers.

  3. amcheck: Fix verify_heapam for tuples where xmin or xmax is 0.

  4. amcheck: Fix a few bugs in new update chain validation.

  5. Fix new test case to work on (some?) big-endian architectures.

  6. Don't test HEAP_XMAX_INVALID when freezing xmax.