Add macro for customizing an archiving WARNING message.

Presently, if an archive module's check_configured_cb callback
returns false, a generic WARNING message is emitted, which
unfortunately provides no actionable details about the reason why
the module is not configured.  This commit introduces a macro that
archive module authors can use to add a DETAIL line to this WARNING
message.

Co-authored-by: Tung Nguyen
Reviewed-by: Daniel Gustafsson, Álvaro Herrera
Discussion: https://postgr.es/m/4109578306242a7cd5661171647e11b2%40oss.nttdata.com

contrib/basic_archive/basic_archive.c

@@ -161,7 +161,12 @@ check_archive_directory(char **newval, void **extra, GucSource source)
 static bool
 basic_archive_configured(ArchiveModuleState *state)
 {
-	return archive_directory != NULL && archive_directory[0] != '\0';
+	if (archive_directory != NULL && archive_directory[0] != '\0')
+		return true;
+
+	arch_module_check_errdetail("%s is not set.",
+								"basic_archive.archive_directory");
+	return false;
 }
 
 /*

doc/src/sgml/archive-modules.sgml

@@ -114,6 +114,18 @@ WARNING:  archive_mode enabled, yet archiving is not configured
     In the latter case, the server will periodically call this function, and
     archiving will proceed only when it returns <literal>true</literal>.
    </para>
+
+   <note>
+    <para>
+     When returning <literal>false</literal>, it may be useful to append some
+     additional information to the generic warning message.  To do that, provide
+     a message to the <function>arch_module_check_errdetail</function> macro
+     before returning <literal>false</literal>.  Like
+     <function>errdetail()</function>, this macro accepts a format string
+     followed by an optional list of arguments.  The resulting string will be
+     emitted as the <literal>DETAIL</literal> line of the warning message.
+    </para>
+   </note>
   </sect2>
 
   <sect2 id="archive-module-archive">

src/backend/archive/shell_archive.c

@@ -45,7 +45,12 @@ shell_archive_init(void)
 static bool
 shell_archive_configured(ArchiveModuleState *state)
 {
-	return XLogArchiveCommand[0] != '\0';
+	if (XLogArchiveCommand[0] != '\0')
+		return true;
+
+	arch_module_check_errdetail("%s is not set.",
+								"archive_command");
+	return false;
 }
 
 static bool

src/backend/postmaster/pgarch.c

@@ -88,6 +88,7 @@ typedef struct PgArchData
 } PgArchData;
 
 char	   *XLogArchiveLibrary = "";
+char	   *arch_module_check_errdetail_string;
 
 
 /* ----------
@@ -401,12 +402,17 @@ pgarch_ArchiverCopyLoop(void)
 			 */
 			HandlePgArchInterrupts();
 
+			/* Reset variables that might be set by the callback */
+			arch_module_check_errdetail_string = NULL;
+
 			/* can't do anything if not configured ... */
 			if (ArchiveCallbacks->check_configured_cb != NULL &&
 				!ArchiveCallbacks->check_configured_cb(archive_module_state))
 			{
 				ereport(WARNING,
-						(errmsg("archive_mode enabled, yet archiving is not configured")));
+						(errmsg("archive_mode enabled, yet archiving is not configured"),
+						 arch_module_check_errdetail_string ?
+						 errdetail_internal("%s", arch_module_check_errdetail_string) : 0));
 				return;
 			}
 

src/include/archive/archive_module.h

@@ -56,4 +56,12 @@ typedef const ArchiveModuleCallbacks *(*ArchiveModuleInit) (void);
 
 extern PGDLLEXPORT const ArchiveModuleCallbacks *_PG_archive_module_init(void);
 
+/* Support for messages reported from archive module callbacks. */
+
+extern PGDLLIMPORT char *arch_module_check_errdetail_string;
+
+#define arch_module_check_errdetail \
+	pre_format_elog_string(errno, TEXTDOMAIN), \
+	arch_module_check_errdetail_string = format_elog_string
+
 #endif							/* _ARCHIVE_MODULE_H */