Pgpool closes the frontend for extended-protocol `DISCARD ALL`

[apkipa]

Pgpool-II version

4.8devel (1a873669), backend_clustering_mode = raw, connection_cache = on, one configured PostgreSQL node.

Description

In raw mode, an extended-protocol DISCARD ALL succeeds on direct PostgreSQL but causes Pgpool to close the frontend connection.

The same failure is reachable from an ordinary PgJDBC 42.7.7 Statement.execute("DISCARD ALL") call: its default preferQueryMode=extended selects this path. No PreparedStatement or pipeline is required; preferQueryMode=simple avoids the failure.

CommandComplete() processes the command before forwarding its completion. Its DISCARD ALL branch calls pool_clear_sent_message_list(). Clearing the final sent message destroys its query context through pool_sent_message_destroy(), and pool_query_context_destroy() sets the session query context to NULL. CommandComplete() then calls pool_set_query_state(), whose null-context check (CHECK_QUERY_CONTEXT_IS_VALID) raises the local error.

Reproduce

Start PostgreSQL 16 and Pgpool-II in the configuration above, download postgresql-42.7.7.jar, and run:

javac -cp postgresql-42.7.7.jar reproduce.java
java -cp .:postgresql-42.7.7.jar Reproduce <direct-jdbc-url> <pgpool-jdbc-url>

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

class Reproduce {
  static void run(String label, String url) {
    Connection c = null;
    try {
      c = DriverManager.getConnection(url);
      try (Statement s = c.createStatement()) {
        s.execute("DISCARD ALL");
        try (ResultSet r = s.executeQuery("SELECT 99")) {
          r.next();
          System.out.println(label + ": " + r.getInt(1));
        }
      }
    } catch (SQLException e) {
      boolean closed = false;
      if (c != null) {
        try { closed = c.isClosed(); } catch (SQLException ignored) { closed = true; }
      }
      System.out.println(label + ": " + e.getClass().getSimpleName() + ": "
          + e.getMessage().split("\\R", 2)[0] + "; closed=" + closed);
    } finally {
      if (c != null) {
        try { c.close(); } catch (SQLException ignored) { }
      }
    }
  }

  public static void main(String[] args) {
    run("direct", args[0]);
    run("pgpool", args[1]);
  }
}

Expected behavior

Both endpoints should complete DISCARD ALL, remain usable, and print:

direct: 99
pgpool: 99

Actual behavior

The reproducer prints:

direct: 99
pgpool: PSQLException: ERROR: setting db node for query to be sent, no query context; closed=true