Fix bugs in plpgsql's handling of CALL argument lists.

exec_stmt_call() tried to extract information out of a CALL statement's
argument list without using expand_function_arguments(), apparently in
the hope of saving a few nanoseconds by not processing defaulted
arguments.  It got that quite wrong though, leading to crashes with
named arguments, as well as failure to enforce writability of the
argument for a defaulted INOUT parameter.  Fix and simplify the logic
by using expand_function_arguments() before examining the list.

Also, move the argument-examination to just after producing the CALL
command's plan, before invoking the called procedure.  This ensures
that we'll track possible changes in the procedure's argument list
correctly, and avoids a hazard of the plan cache being flushed while
the procedure executes.

Also fix assorted falsehoods and omissions in associated documentation.

Per bug #15477 from Alexey Stepanov.

Patch by me, with some help from Pavel Stehule.  Back-patch to v11.

Discussion: https://postgr.es/m/15477-86075b1d1d319e0a@postgresql.org
Discussion: https://postgr.es/m/CAFj8pRA6UsujpTs9Sdwmk-R6yQykPx46wgjj+YZ7zxm4onrDyw@mail.gmail.com

doc/src/sgml/plpgsql.sgml

@@ -1864,15 +1864,29 @@ SELECT * FROM get_available_flightid(CURRENT_DATE);
 
     <para>
      A procedure does not have a return value.  A procedure can therefore end
-     without a <command>RETURN</command> statement.  If
-     a <command>RETURN</command> statement is desired to exit the code early,
-     then <symbol>NULL</symbol> must be returned.  Returning any other value
-     will result in an error.
+     without a <command>RETURN</command> statement.  If you wish to use
+     a <command>RETURN</command> statement to exit the code early, write
+     just <command>RETURN</command> with no expression.
     </para>
 
     <para>
-     If a procedure has output parameters, then the output values can be
-     assigned to the parameters as if they were variables.  For example:
+     If the procedure has output parameters, the final values of the output
+     parameter variables will be returned to the caller.
+    </para>
+   </sect2>
+
+   <sect2 id="plpgsql-statements-calling-procedure">
+    <title>Calling a Procedure</title>
+
+    <para>
+     A <application>PL/pgSQL</application> function, procedure,
+     or <command>DO</command> block can call a procedure
+     using <command>CALL</command>.  Output parameters are handled
+     differently from the way that <command>CALL</command> works in plain
+     SQL.  Each <literal>INOUT</literal> parameter of the procedure must
+     correspond to a variable in the <command>CALL</command> statement, and
+     whatever the procedure returns is assigned back to that variable after
+     it returns.  For example:
 <programlisting>
 CREATE PROCEDURE triple(INOUT x int)
 LANGUAGE plpgsql
@@ -1882,7 +1896,13 @@ BEGIN
 END;
 $$;
 
-CALL triple(5);
+DO $$
+DECLARE myvar int := 5;
+BEGIN
+  CALL triple(myvar);
+  RAISE NOTICE 'myvar = %', myvar;  -- prints 15
+END
+$$;
 </programlisting>
     </para>
    </sect2>

doc/src/sgml/ref/call.sgml

@@ -33,7 +33,8 @@ CALL <replaceable class="parameter">name</replaceable> ( [ <replaceable class="p
   </para>
 
   <para>
-   If the procedure has output arguments, then a result row will be returned.
+   If the procedure has any output parameters, then a result row will be
+   returned, containing the values of those parameters.
   </para>
  </refsect1>
 
@@ -54,7 +55,7 @@ CALL <replaceable class="parameter">name</replaceable> ( [ <replaceable class="p
     <term><replaceable class="parameter">argument</replaceable></term>
     <listitem>
      <para>
-      An argument for the procedure call.
+      An input argument for the procedure call.
       See <xref linkend="sql-syntax-calling-funcs"/> for the full details on
       function and procedure call syntax, including use of named parameters.
      </para>
@@ -81,6 +82,12 @@ CALL <replaceable class="parameter">name</replaceable> ( [ <replaceable class="p
    Transaction control statements are only allowed if <command>CALL</command>
    is executed in its own transaction.
   </para>
+
+  <para>
+   <application>PL/pgSQL</application> handles output parameters
+   in <command>CALL</command> commands differently;
+   see <xref linkend="plpgsql-statements-calling-procedure"/>.
+  </para>
  </refsect1>
 
  <refsect1>