Allow httpd subservices to set the document root

Only the main service OR one of the subservices can set the document
root. This is used by mediawiki when it is hosted at the root of the
vhost.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -63,6 +63,7 @@ let
           enablePHP = false;
           phpOptions = "";
           options = {};
+          documentRoot = null;
         };
         res = defaults // svcFunction { inherit config pkgs serverInfo php; };
       in res;
@@ -188,7 +189,11 @@ let
 
     subservices = callSubservices serverInfo cfg.extraSubservices;
 
-    documentRoot = if cfg.documentRoot != null then cfg.documentRoot else
+    maybeDocumentRoot = fold (svc: acc:
+      if acc == null then svc.documentRoot else assert svc.documentRoot == null; acc
+    ) null ([ cfg ] ++ subservices);
+
+    documentRoot = if maybeDocumentRoot != null then maybeDocumentRoot else
       pkgs.runCommand "empty" {} "ensureDir $out";
 
     documentRootConf = ''
@@ -240,7 +245,7 @@ let
 
     ${robotsConf}
 
-    ${if isMainServer || cfg.documentRoot != null then documentRootConf else ""}
+    ${if isMainServer || maybeDocumentRoot != null then documentRootConf else ""}
 
     ${if cfg.enableUserDir then ''
 

nixos/modules/services/web-servers/apache-httpd/mediawiki.nix

@@ -126,7 +126,7 @@ in
         </Directory>
       ''}
 
-      Alias ${config.urlPrefix} ${mediawikiRoot}
+      ${optionalString (config.urlPrefix != "") "Alias ${config.urlPrefix} ${mediawikiRoot}"}
 
       <Directory ${mediawikiRoot}>
           Order allow,deny
@@ -139,6 +139,8 @@ in
       ''}
     '';
 
+  documentRoot = if config.urlPrefix == "" then mediawikiRoot else null;
+
   enablePHP = true;
 
   options = {