Allow archiving via loadable modules.

Running a shell command for each file to be archived has a lot of
overhead and may not offer as much error checking as you want, or the
exact semantics that you want. So, offer the option to call a loadable
module for each file to be archived, rather than running a shell command.

Also, add a 'basic_archive' contrib module as an example implementation
that archives to a local directory.

Nathan Bossart, with a little bit of kibitzing by me.

Discussion: http://postgr.es/m/20220202224433.GA1036711@nathanxps13

src/backend/utils/misc/guc.c

@@ -3881,13 +3881,23 @@ static struct config_string ConfigureNamesString[] =
 	{
 		{"archive_command", PGC_SIGHUP, WAL_ARCHIVING,
 			gettext_noop("Sets the shell command that will be called to archive a WAL file."),
-			NULL
+			gettext_noop("This is used only if \"archive_library\" is not set.")
 		},
 		&XLogArchiveCommand,
 		"",
 		NULL, NULL, show_archive_command
 	},
 
+	{
+		{"archive_library", PGC_SIGHUP, WAL_ARCHIVING,
+			gettext_noop("Sets the library that will be called to archive a WAL file."),
+			gettext_noop("An empty string indicates that \"archive_command\" should be used.")
+		},
+		&XLogArchiveLibrary,
+		"",
+		NULL, NULL, NULL
+	},
+
 	{
 		{"restore_command", PGC_SIGHUP, WAL_ARCHIVE_RECOVERY,
 			gettext_noop("Sets the shell command that will be called to retrieve an archived WAL file."),

src/backend/utils/misc/postgresql.conf.sample

@@ -245,6 +245,9 @@
 
 #archive_mode = off		# enables archiving; off, on, or always
 				# (change requires restart)
+#archive_library = ''		# library to use to archive a logfile segment
+				# (empty string indicates archive_command should
+				# be used)
 #archive_command = ''		# command to use to archive a logfile segment
 				# placeholders: %p = path of file to archive
 				#               %f = file name only