pg_upgrade: check for inconsistencies in not-null constraints w/inheritance

With tables defined like this,
  CREATE TABLE ip (id int PRIMARY KEY);
  CREATE TABLE ic (id int) INHERITS (ip);
  ALTER TABLE ic ALTER id DROP NOT NULL;

pg_upgrade fails during the schema restore phase due to this error:
  ERROR: column "id" in child table must be marked NOT NULL

This can only be fixed by marking the child column as NOT NULL before
the upgrade, which could take an arbitrary amount of time (because ic's
data must be scanned).  Have pg_upgrade's check mode warn if that
condition is found, so that users know what to adjust before running the
upgrade for real.

Author: Ali Akbar <the.apaan@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/CACQjQLoMsE+1pyLe98pi0KvPG2jQQ94LWJ+PTiLgVRK4B=i_jg@mail.gmail.com

src/bin/pg_upgrade/check.c

@@ -23,6 +23,7 @@ static void check_proper_datallowconn(ClusterInfo *cluster);
 static void check_for_prepared_transactions(ClusterInfo *cluster);
 static void check_for_isn_and_int8_passing_mismatch(ClusterInfo *cluster);
 static void check_for_tables_with_oids(ClusterInfo *cluster);
+static void check_for_not_null_inheritance(ClusterInfo *cluster);
 static void check_for_composite_data_type_usage(ClusterInfo *cluster);
 static void check_for_reg_data_type_usage(ClusterInfo *cluster);
 static void check_for_removed_data_type_usage(ClusterInfo *cluster,
@@ -123,6 +124,13 @@ check_and_dump_old_cluster(bool live_check)
 	if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1100)
 		check_for_tables_with_oids(&old_cluster);
 
+	/*
+	 * Pre-PG 18 allowed child tables to omit not-null constraints that their
+	 * parents columns have, but schema restore fails for them.  Verify there
+	 * are none.
+	 */
+	check_for_not_null_inheritance(&old_cluster);
+
 	/*
 	 * PG 12 changed the 'sql_identifier' type storage to be based on name,
 	 * not varchar, which breaks on-disk format for existing data. So we need
@@ -1021,6 +1029,83 @@ check_for_tables_with_oids(ClusterInfo *cluster)
 		check_ok();
 }
 
+/*
+ * check_for_not_null_inheritance()
+ *
+ * An attempt to create child tables lacking not-null constraints that are
+ * present in their parents errors out.  This can no longer occur since 18,
+ * but previously there were various ways for that to happen.  Check that
+ * the cluster to be upgraded doesn't have any of those problems.
+ */
+static void
+check_for_not_null_inheritance(ClusterInfo *cluster)
+{
+	FILE	   *script = NULL;
+	char		output_path[MAXPGPATH];
+	int			ntup;
+
+	prep_status("Checking for not-null constraint inconsistencies");
+
+	snprintf(output_path, sizeof(output_path),
+			 "not_null_inconsistent_columns.txt");
+	for (int dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
+	{
+		PGresult   *res;
+		bool		db_used = false;
+		int			i_nspname,
+					i_relname,
+					i_attname;
+		DbInfo	   *active_db = &old_cluster.dbarr.dbs[dbnum];
+		PGconn	   *conn = connectToServer(&old_cluster, active_db->db_name);
+
+		res = executeQueryOrDie(conn,
+								"SELECT cc.relnamespace::pg_catalog.regnamespace AS nspname, "
+								"       cc.relname, ac.attname "
+								"FROM pg_catalog.pg_inherits i, pg_catalog.pg_attribute ac, "
+								"     pg_catalog.pg_attribute ap, pg_catalog.pg_class cc "
+								"WHERE cc.oid = ac.attrelid AND i.inhrelid = ac.attrelid "
+								"      AND i.inhparent = ap.attrelid AND ac.attname = ap.attname "
+								"      AND ap.attnum > 0 and ap.attnotnull AND NOT ac.attnotnull");
+
+		ntup = PQntuples(res);
+		i_nspname = PQfnumber(res, "nspname");
+		i_relname = PQfnumber(res, "relname");
+		i_attname = PQfnumber(res, "attname");
+		for (int i = 0; i < ntup; i++)
+		{
+			if (script == NULL && (script = fopen_priv(output_path, "w")) == NULL)
+				pg_fatal("could not open file \"%s\": %m", output_path);
+			if (!db_used)
+			{
+				fprintf(script, "In database: %s\n", active_db->db_name);
+				db_used = true;
+			}
+
+			fprintf(script, "  %s.%s.%s\n",
+					PQgetvalue(res, i, i_nspname),
+					PQgetvalue(res, i, i_relname),
+					PQgetvalue(res, i, i_attname));
+		}
+
+		PQclear(res);
+		PQfinish(conn);
+	}
+
+	if (script)
+	{
+		fclose(script);
+		pg_log(PG_REPORT, "fatal");
+		pg_fatal("Your installation contains inconsistent NOT NULL constraints.\n"
+				 "If the parent column(s) are NOT NULL, then the child column must\n"
+				 "also be marked NOT NULL, or the upgrade will fail.\n"
+				 "You can fix this by running\n"
+				 "  ALTER TABLE tablename ALTER column SET NOT NULL;\n"
+				 "on each column listed in the file:\n"
+				 "    %s\n\n", output_path);
+	}
+	else
+		check_ok();
+}
 
 /*
  * check_for_composite_data_type_usage()