gitweb: Add a per-repository authorization hook.

Add a configuration variable that can be used to specify an
arbitrary subroutine that will be called in the same situations
where $export_ok is checked, and its return value used
to decide whether the repository is to be shown.

This allows the user to implement custom authentication
schemes, for example by issuing a subrequest through mod_perl
and checking if Apache will authorize it.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/gitweb/gitweb.perl b/gitweb/gitweb.perl
index 68bdf62..74672bf 100755
--- a/gitweb/gitweb.perl
+++ b/gitweb/gitweb.perl
@@ -95,6 +95,11 @@
 # (only effective if this variable evaluates to true)
 our $export_ok = "++GITWEB_EXPORT_OK++";
 
+# show repository only if this subroutine returns true
+# when given the path to the project, for example:
+#    sub { return -e "$_[0]/git-daemon-export-ok"; }
+our $export_auth_hook = undef;
+
 # only allow viewing of repositories also shown on the overview page
 our $strict_export = "++GITWEB_STRICT_EXPORT++";
 
@@ -400,7 +405,8 @@
 sub check_export_ok {
 	my ($dir) = @_;
 	return (check_head_link($dir) &&
-		(!$export_ok || -e "$dir/$export_ok"));
+		(!$export_ok || -e "$dir/$export_ok") &&
+		(!$export_auth_hook || $export_auth_hook->($dir)));
 }
 
 # process alternate names for backward compatibility