Prohibit transaction commands in security definer procedures

Starting and aborting transactions in security definer procedures
doesn't work.  StartTransaction() insists that the security context
stack is empty, so this would currently cause a crash, and
AbortTransaction() resets it.  This could be made to work by
reorganizing the code, but right now we just prohibit it.

Reported-by: amul sul <sulamul@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b96Gupt_LFL7uNyy3c50-wbhA68NUjiK5%3DrF6_w%3Dpq_T%3DQ%40mail.gmail.com

doc/src/sgml/ref/create_procedure.sgml

@@ -203,6 +203,12 @@ CREATE [ OR REPLACE ] PROCEDURE
       conformance, but it is optional since, unlike in SQL, this feature
       applies to all procedures not only external ones.
      </para>
+
+     <para>
+      A <literal>SECURITY DEFINER</literal> procedure cannot execute
+      transaction control statements (for example, <command>COMMIT</command>
+      and <command>ROLLBACK</command>, depending on the language).
+     </para>
     </listitem>
    </varlistentry>
 

src/backend/commands/functioncmds.c

@@ -2245,6 +2245,15 @@ ExecuteCallStmt(CallStmt *stmt, ParamListInfo params, bool atomic, DestReceiver
 	if (!heap_attisnull(tp, Anum_pg_proc_proconfig, NULL))
 		callcontext->atomic = true;
 
+	/*
+	 * In security definer procedures, we can't allow transaction commands.
+	 * StartTransaction() insists that the security context stack is empty,
+	 * and AbortTransaction() resets the security context.  This could be
+	 * reorganized, but right now it doesn't work.
+	 */
+	if (((Form_pg_proc )GETSTRUCT(tp))->prosecdef)
+		callcontext->atomic = true;
+
 	/*
 	 * Expand named arguments, defaults, etc.
 	 */

src/pl/plpgsql/src/expected/plpgsql_transaction.out

@@ -130,6 +130,18 @@ $$;
 CALL transaction_test5();
 ERROR:  invalid transaction termination
 CONTEXT:  PL/pgSQL function transaction_test5() line 3 at COMMIT
+-- SECURITY DEFINER currently disallow transaction statements
+CREATE PROCEDURE transaction_test5b()
+LANGUAGE plpgsql
+SECURITY DEFINER
+AS $$
+BEGIN
+    COMMIT;
+END;
+$$;
+CALL transaction_test5b();
+ERROR:  invalid transaction termination
+CONTEXT:  PL/pgSQL function transaction_test5b() line 3 at COMMIT
 TRUNCATE test1;
 -- nested procedure calls
 CREATE PROCEDURE transaction_test6(c text)

src/pl/plpgsql/src/sql/plpgsql_transaction.sql

@@ -116,6 +116,19 @@ $$;
 CALL transaction_test5();
 
 
+-- SECURITY DEFINER currently disallow transaction statements
+CREATE PROCEDURE transaction_test5b()
+LANGUAGE plpgsql
+SECURITY DEFINER
+AS $$
+BEGIN
+    COMMIT;
+END;
+$$;
+
+CALL transaction_test5b();
+
+
 TRUNCATE test1;
 
 -- nested procedure calls