Fix possible crash with GENERATED ALWAYS columns

In some corner cases, this could also lead to corrupted values being
included in the tuple.

Users who are concerned that they are affected by this should first
upgrade and then perform a base backup of their database and restore onto
an off-line server. They should then query each table with generated
columns to ensure there are no rows where the generated expression does
not match a newly calculated version of the GENERATED ALWAYS expression.
If no crashes occur and no rows are returned then you're not affected.

Fixes bug #16369.

Reported-by: Cameron Ezell
Discussion: https://postgr.es/m/16369-5845a6f1bef59884@postgresql.org
Backpatch-through: 12 (where GENERATED ALWAYS columns were added.)

src/backend/executor/nodeModifyTable.c

@@ -311,6 +311,13 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 
 			val = ExecEvalExpr(resultRelInfo->ri_GeneratedExprs[i], econtext, &isnull);
 
+			/*
+			 * We must make a copy of val as we have no guarantees about where
+			 * memory for a pass-by-reference Datum is located.
+			 */
+			if (!isnull)
+				val = datumCopy(val, attr->attbyval, attr->attlen);
+
 			values[i] = val;
 			nulls[i] = isnull;
 		}

src/test/regress/expected/generated.out

@@ -320,6 +320,18 @@ SELECT * FROM gtest2;
  1 |  
 (1 row)
 
+-- simple column reference for varlena types
+CREATE TABLE gtest_varlena (a varchar, b varchar GENERATED ALWAYS AS (a) STORED);
+INSERT INTO gtest_varlena (a) VALUES('01234567890123456789');
+INSERT INTO gtest_varlena (a) VALUES(NULL);
+SELECT * FROM gtest_varlena ORDER BY a;
+          a           |          b           
+----------------------+----------------------
+ 01234567890123456789 | 01234567890123456789
+                      | 
+(2 rows)
+
+DROP TABLE gtest_varlena;
 -- composite types
 CREATE TYPE double_int as (a int, b int);
 CREATE TABLE gtest4 (

src/test/regress/sql/generated.sql

@@ -145,6 +145,13 @@ CREATE TABLE gtest2 (a int PRIMARY KEY, b int GENERATED ALWAYS AS (NULL) STORED)
 INSERT INTO gtest2 VALUES (1);
 SELECT * FROM gtest2;
 
+-- simple column reference for varlena types
+CREATE TABLE gtest_varlena (a varchar, b varchar GENERATED ALWAYS AS (a) STORED);
+INSERT INTO gtest_varlena (a) VALUES('01234567890123456789');
+INSERT INTO gtest_varlena (a) VALUES(NULL);
+SELECT * FROM gtest_varlena ORDER BY a;
+DROP TABLE gtest_varlena;
+
 -- composite types
 CREATE TYPE double_int as (a int, b int);
 CREATE TABLE gtest4 (