Fix for pageinspect bug in PG 17

[Tomas Vondra]

Hi,

Here's a fix for pageinspect bug in PG17, reported in [1]. The bug turns
out to be introduced by my commit

commit dae761a87edae444d11a411f711f1d679bed5941
Author: Tomas Vondra <tomas.von...@postgresql.org>
Date:   Fri Dec 8 17:07:30 2023 +0100

    Add empty BRIN ranges during CREATE INDEX

    ...

This adds an out argument to brin_page_items, but I failed to consider
the user may still run with an older version of the extension - either
after pg_upgrade (as in the report), or when the CREATE EXTENSION
command specifies VERSION.

The new "empty" field is added in the middle of the output tuple, which
shifts the values, causing segfault when accessing a text field at the
end of the array.

Of course, we add arguments to existing functions pretty often, and we
know how to do that in backwards-compatible way - pg_stat_statements is
a good example of how to do that nicely. Unfortunately, it's too late to
do that for brin_page_items :-( There may already be upgraded systems or
with installed pageinspect, etc.

The only fix I can think of is explicitly looking at TupleDesc->natts,
as in the attached fix.


regards

-- 
Tomas Vondra

diff --git a/contrib/pageinspect/brinfuncs.c b/contrib/pageinspect/brinfuncs.c
index 22621d584fa..6086b02445b 100644
--- a/contrib/pageinspect/brinfuncs.c
+++ b/contrib/pageinspect/brinfuncs.c
@@ -240,25 +240,29 @@ brin_page_items(PG_FUNCTION_ARGS)
 		else
 		{
 			int			att = attno - 1;
+			int			idx = 0;
 
-			values[0] = UInt16GetDatum(offset);
+			values[idx++] = UInt16GetDatum(offset);
 			switch (TupleDescAttr(rsinfo->setDesc, 1)->atttypid)
 			{
 				case INT8OID:
-					values[1] = Int64GetDatum((int64) dtup->bt_blkno);
+					values[idx++] = Int64GetDatum((int64) dtup->bt_blkno);
 					break;
 				case INT4OID:
 					/* support for old extension version */
-					values[1] = UInt32GetDatum(dtup->bt_blkno);
+					values[idx++] = UInt32GetDatum(dtup->bt_blkno);
 					break;
 				default:
 					elog(ERROR, "incorrect output types");
 			}
-			values[2] = UInt16GetDatum(attno);
-			values[3] = BoolGetDatum(dtup->bt_columns[att].bv_allnulls);
-			values[4] = BoolGetDatum(dtup->bt_columns[att].bv_hasnulls);
-			values[5] = BoolGetDatum(dtup->bt_placeholder);
-			values[6] = BoolGetDatum(dtup->bt_empty_range);
+			values[idx++] = UInt16GetDatum(attno);
+			values[idx++] = BoolGetDatum(dtup->bt_columns[att].bv_allnulls);
+			values[idx++] = BoolGetDatum(dtup->bt_columns[att].bv_hasnulls);
+			values[idx++] = BoolGetDatum(dtup->bt_placeholder);
+
+			if (rsinfo->setDesc->natts >= 8)
+				values[idx++] = BoolGetDatum(dtup->bt_empty_range);
+
 			if (!dtup->bt_columns[att].bv_allnulls)
 			{
 				BrinValues *bvalues = &dtup->bt_columns[att];
@@ -284,13 +288,15 @@ brin_page_items(PG_FUNCTION_ARGS)
 				}
 				appendStringInfoChar(&s, '}');
 
-				values[7] = CStringGetTextDatum(s.data);
+				values[idx++] = CStringGetTextDatum(s.data);
 				pfree(s.data);
 			}
 			else
 			{
-				nulls[7] = true;
+				nulls[idx++] = true;
 			}
+
+			Assert(idx == rsinfo->setDesc->natts);
 		}
 
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);