PG18: Add regression coverage for EXPLAIN (WAL) “WAL Buffers Full” (#8383)

PostgreSQL 18 extends `EXPLAIN (ANALYZE, WAL)` to report how many times
WAL buffers became full (`WAL Buffers Full`).
https://github.com/postgres/postgres/commit/320545bfc

This PR adds regression coverage to ensure Citus preserves the new PG18
field through the distributed EXPLAIN aggregation path:

* Creates a distributed table and forces per-task EXPLAIN output via
`citus.explain_all_tasks`.
* Captures `EXPLAIN (ANALYZE true, WAL true, FORMAT JSON)` into `jsonb`.
* Asserts the plan is distributed by checking for `"Task Count"`.
* Asserts the new PG18 `"WAL Buffers Full"` property exists, and
extracts its value.


#### Why we don’t assert `wal_buffers_full > 0`

Getting `wal_buffers_full > 0` reliably requires either shrinking
`wal_buffers` (postmaster restart) or generating enough WAL to exhaust
the buffer before a flush, which isn’t deterministic in this regression
harness across coordinators/workers. As-is we’re verifying Citus carries
the new field through EXPLAIN; pushing for non-zero would likely be
flaky.

src/test/regress/expected/pg18.out

@@ -2957,6 +2957,59 @@ SELECT result FROM run_command_on_all_nodes(
  DROP TABLESPACE
 (3 rows)
 
+-- PG18 Feature: EXPLAIN (WAL) reports WAL buffers becoming full.
+-- PG18 commit: https://github.com/postgres/postgres/commit/320545bfc
+SET search_path TO pg18_nn;
+SET citus.explain_all_tasks TO true;
+CREATE TABLE wal_explain_dist(id int, payload text);
+SELECT create_distributed_table('wal_explain_dist', 'id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+INSERT INTO wal_explain_dist VALUES
+  (1, 'one'), (2, 'two'), (3, 'three');
+-- Helper to capture distributed EXPLAIN ANALYZE (WAL) as JSON
+CREATE OR REPLACE FUNCTION explain_analyze_wal_json(query text)
+RETURNS jsonb
+AS $$
+DECLARE
+  result jsonb;
+BEGIN
+  EXECUTE format('EXPLAIN (ANALYZE true, WAL true, FORMAT JSON) %s', query)
+    INTO result;
+  RETURN result;
+END;
+$$ LANGUAGE plpgsql;
+CREATE TEMP TABLE wal_explain_plan(plan jsonb);
+INSERT INTO wal_explain_plan(plan)
+SELECT explain_analyze_wal_json($$SELECT count(*) FROM wal_explain_dist$$);
+-- Ensure Citus keeps distributed-task context in the WAL-aware EXPLAIN output
+SELECT jsonb_path_exists(plan, '$.**."Task Count"') AS wal_explain_distributed_plan
+FROM wal_explain_plan;
+ wal_explain_distributed_plan
+---------------------------------------------------------------------
+ t
+(1 row)
+
+-- New PG18 field: wal_buffers_full must survive the distributed EXPLAIN path
+SELECT jsonb_path_exists(plan, '$.**."WAL Buffers Full"') AS wal_buffers_full_present
+FROM wal_explain_plan;
+ wal_buffers_full_present
+---------------------------------------------------------------------
+ t
+(1 row)
+
+SELECT jsonb_path_query_first(plan, '$.**."WAL Buffers Full"') AS wal_buffers_full_value
+FROM wal_explain_plan;
+ wal_buffers_full_value
+---------------------------------------------------------------------
+ 0
+(1 row)
+
+DROP TABLE wal_explain_plan;
+SET citus.explain_all_tasks TO default;
 -- cleanup with minimum verbosity
 SET client_min_messages TO ERROR;
 RESET search_path;

src/test/regress/sql/pg18.sql

@@ -1849,6 +1849,48 @@ SELECT result FROM run_command_on_all_nodes(
   $$
 );
 
+-- PG18 Feature: EXPLAIN (WAL) reports WAL buffers becoming full.
+-- PG18 commit: https://github.com/postgres/postgres/commit/320545bfc
+SET search_path TO pg18_nn;
+SET citus.explain_all_tasks TO true;
+
+CREATE TABLE wal_explain_dist(id int, payload text);
+SELECT create_distributed_table('wal_explain_dist', 'id');
+
+INSERT INTO wal_explain_dist VALUES
+  (1, 'one'), (2, 'two'), (3, 'three');
+
+-- Helper to capture distributed EXPLAIN ANALYZE (WAL) as JSON
+CREATE OR REPLACE FUNCTION explain_analyze_wal_json(query text)
+RETURNS jsonb
+AS $$
+DECLARE
+  result jsonb;
+BEGIN
+  EXECUTE format('EXPLAIN (ANALYZE true, WAL true, FORMAT JSON) %s', query)
+    INTO result;
+  RETURN result;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TEMP TABLE wal_explain_plan(plan jsonb);
+INSERT INTO wal_explain_plan(plan)
+SELECT explain_analyze_wal_json($$SELECT count(*) FROM wal_explain_dist$$);
+
+-- Ensure Citus keeps distributed-task context in the WAL-aware EXPLAIN output
+SELECT jsonb_path_exists(plan, '$.**."Task Count"') AS wal_explain_distributed_plan
+FROM wal_explain_plan;
+
+-- New PG18 field: wal_buffers_full must survive the distributed EXPLAIN path
+SELECT jsonb_path_exists(plan, '$.**."WAL Buffers Full"') AS wal_buffers_full_present
+FROM wal_explain_plan;
+
+SELECT jsonb_path_query_first(plan, '$.**."WAL Buffers Full"') AS wal_buffers_full_value
+FROM wal_explain_plan;
+
+DROP TABLE wal_explain_plan;
+SET citus.explain_all_tasks TO default;
+
 -- cleanup with minimum verbosity
 SET client_min_messages TO ERROR;
 RESET search_path;