Allow background workers to be started dynamically.

There is a new API, RegisterDynamicBackgroundWorker, which allows
an ordinary user backend to register a new background writer during
normal running.  This means that it's no longer necessary for all
background workers to be registered during processing of
shared_preload_libraries, although the option of registering workers
at that time remains available.

When a background worker exits and will not be restarted, the
slot previously used by that background worker is automatically
released and becomes available for reuse.  Slots used by background
workers that are configured for automatic restart can't (yet) be
released without shutting down the system.

This commit adds a new source file, bgworker.c, and moves some
of the existing control logic for background workers there.
Previously, there was little enough logic that it made sense to
keep everything in postmaster.c, but not any more.

This commit also makes the worker_spi contrib module into an
extension and adds a new function, worker_spi_launch, which can
be used to demonstrate the new facility.

1 files changed, 46 insertions, 9 deletions

diff --git a/doc/src/sgml/bgworker.sgml b/doc/src/sgml/bgworker.sgml
index f7126388af1..9d9b631ac1a 100644
--- a/doc/src/sgml/bgworker.sgml
+++ b/doc/src/sgml/bgworker.sgml
@@ -30,23 +30,35 @@
  </warning>
 
  <para>
-  Only modules listed in <varname>shared_preload_libraries</> can run
-  background workers.  A module wishing to run a background worker needs
-  to register it by calling
+  Background workers can be initialized at the time that
+  <productname>PostgreSQL</> is started including the module name in
+  <varname>shared_preload_libraries</>.  A module wishing to run a background
+  worker can register it by calling
   <function>RegisterBackgroundWorker(<type>BackgroundWorker *worker</type>)</function>
-  from its <function>_PG_init()</>.
+  from its <function>_PG_init()</>.  Background workers can also be started
+  after the system is up and running by calling the function
+  <function>RegisterDynamicBackgroundWorker</function>(<type>BackgroundWorker
+  *worker</type>).  Unlike <function>RegisterBackgroundWorker</>, which can
+  only be called from within the postmaster,
+  <function>RegisterDynamicBackgroundWorker</function> must be called from
+  a regular backend.
+ </para>
+
+ <para>
   The structure <structname>BackgroundWorker</structname> is defined thus:
 <programlisting>
 typedef void (*bgworker_main_type)(void *main_arg);
 typedef void (*bgworker_sighdlr_type)(SIGNAL_ARGS);
 typedef struct BackgroundWorker
 {
-    char       *bgw_name;
+    char        bgw_name[BGW_MAXLEN];
     int         bgw_flags;
     BgWorkerStartTime bgw_start_time;
     int         bgw_restart_time;       /* in seconds, or BGW_NEVER_RESTART */
-    bgworker_main_type  bgw_main;
-    void       *bgw_main_arg;
+    bgworker_main_type bgw_main;
+    char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
+    char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
+    Datum       bgw_main_arg;
     bgworker_sighdlr_type bgw_sighup;
     bgworker_sighdlr_type bgw_sigterm;
 } BackgroundWorker;
@@ -101,7 +113,29 @@ typedef struct BackgroundWorker
    <structfield>bgw_main_arg</structfield> will be passed to it as its only
    argument.  Note that the global variable <literal>MyBgworkerEntry</literal>
    points to a copy of the <structname>BackgroundWorker</structname> structure
-   passed at registration time.
+   passed at registration time. <structfield>bgw_main</structfield> may be
+   NULL; in that case, <structfield>bgw_library_name</structfield> and
+   <structfield>bgw_function_name</structfield> will be used to determine
+   the entrypoint.  This is useful for background workers launched after
+   postmaster startup, where the postmaster does not have the requisite
+   library loaded.
+  </para>
+
+  <para>
+   <structfield>bgw_library_name</structfield> is the name of a library in
+   which the initial entrypoint for the background worker should be sought.
+   It is ignored unless <structfield>bgw_main</structfield> is NULL.
+   But if <structfield>bgw_main</structfield> is NULL, then 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.
+  </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 entrypoint
+   for a new background worker.  It is ignored unless
+   <structfield>bgw_main</structfield> is NULL.
   </para>
 
   <para>
@@ -109,7 +143,10 @@ typedef struct BackgroundWorker
    pointers to functions that will be installed as signal handlers for the new
    process.  If <structfield>bgw_sighup</> is NULL, then <literal>SIG_IGN</>
    is used; if <structfield>bgw_sigterm</> is NULL, a handler is installed that
-   will terminate the process after logging a suitable message.
+   will terminate the process after logging a suitable message.  These
+   fields should not be used if <structfield>bgw_main</> is NULL; instead,
+   the worker process should set its own signal handlers before calling
+   <function>BackgroundWorkerUnblockSignals()</function>.
   </para>
 
   <para>Once running, the process can connect to a database by calling