[BUG] pg_basebackup produces wrong incremental files after relation truncation in segmented tables

Hello PostgreSQL developers,

I’ve encountered a bug in the incremental backup feature that prevents restoration of backups containing relations larger than 1 GB that were vacuum-truncated.



Problem Description

When taking incremental backups of relations that span multiple segments, if the relation is truncated during VACUUM (after the base backup but before the incremental one), pg_combinebackup fails with:

```

file "%s" has truncation block length %u in excess of segment size %u

```

pg_basebackup itself completes without errors, but the resulting incremental backup cannot be restored.



Root Cause

In segmented relations, a VACUUM that truncates blocks sets a limit_block in the WAL summary. The incremental restore logic miscalculates truncation_block_length when processing segment 0…N, because it compares the segment-local size with a relation-wide limit.

In src/backend/backup/basebackup_incremental.c:

```

*truncation_block_length = size / BLCKSZ;

if (BlockNumberIsValid(limit_block))

{

    unsigned relative_limit = limit_block - segno * RELSEG_SIZE;

    if (*truncation_block_length < relative_limit)   /* ← problematic */

        *truncation_block_length = relative_limit;

}

```

For example, if limit_block lies in segment 10, then relative_limit will be roughly 9 * RELSEG_SIZE while processing segment 0. This forces truncation_block_length far beyond the actual segment size, leading to a segment length larger than RELSEG_SIZE and eventually the restore error.



Reproduction Steps

Create a table larger than 1 GB (multiple segments).

Take a full base backup.

Delete rows that occupy the end of the relation.

Run VACUUM (VERBOSE, TRUNCATE) to ensure blocks are removed.

(optional) Confirm that the WAL summary includes a limit entry for the relation.

Take an incremental backup with pg_basebackup.

Attempt to restore using pg_combinebackup.

Observe the truncation block length error.



Patch

A patch correcting this logic is attached, and I’m happy to provide additional details or revisions if helpful.



Best regards,

Oleg Tkachenko