Add missing PGDLLEXPORT markings in contrib/pg_prewarm.

After commit 089480c07, it's necessary for background worker entry
points to be marked PGDLLEXPORT, else they aren't findable by
LookupBackgroundWorkerFunction().  Since pg_prewarm lacks any
regression tests, it's not surprising its worker entry points were
overlooked.  (A quick search turned up no other such oversights.)

I added some documentation pointing out the need for this, too.

Robins Tharakan and Tom Lane

CAEP4nAzndnQv3-1QKb=D-hLoK3Rko12HHMFHHtdj2GQAUXO3gw@mail.gmail.com

contrib/pg_prewarm/autoprewarm.c

@@ -82,8 +82,8 @@ typedef struct AutoPrewarmSharedState
 	int			prewarmed_blocks;
 } AutoPrewarmSharedState;
 
-void		autoprewarm_main(Datum main_arg);
-void		autoprewarm_database_main(Datum main_arg);
+PGDLLEXPORT void autoprewarm_main(Datum main_arg);
+PGDLLEXPORT void autoprewarm_database_main(Datum main_arg);
 
 PG_FUNCTION_INFO_V1(autoprewarm_start_worker);
 PG_FUNCTION_INFO_V1(autoprewarm_dump_now);

doc/src/sgml/bgworker.sgml

@@ -141,14 +141,15 @@ typedef struct BackgroundWorker
    which the initial entry point for the background worker should be sought.
    The named library will be dynamically loaded by the worker process and
    <structfield>bgw_function_name</structfield> will be used to identify the
-   function to be called.  If loading a function from the core code, this must
-   be set to "postgres".
+   function to be called.  If calling a function in the core code, this must
+   be set to <literal>"postgres"</literal>.
   </para>
 
   <para>
-   <structfield>bgw_function_name</structfield> is the name of a function in
-   a dynamically loaded library which should be used as the initial entry point
-   for a new background worker.
+   <structfield>bgw_function_name</structfield> is the name of the function
+   to use as the initial entry point for the new background worker.  If
+   this function is in a dynamically loaded library, it must be marked
+   <literal>PGDLLEXPORT</literal> (and not <literal>static</literal>).
   </para>
 
   <para>