implemented proposal by niksnut.

Now you have to use either date= or tag= when specifying cvs revision

svn path=/nixpkgs/trunk/; revision=9661

pkgs/applications/misc/synergy/default.nix

@@ -5,7 +5,7 @@ args.stdenv.mkDerivation {
   src = args.fetchcvs {
     url = ":pserver:anonymous@synergy2.cvs.sourceforge.net:/cvsroot/synergy2";
     module = "synergy";
-    tag = "-DNOW";
+    date = "NOW";
     sha256 = "ef8e2ebfda6e43240051a7af9417092b2af50ece8b5c6c3fbd908ba91c4fe068";
   };
 

pkgs/build-support/fetchcvs/builder.sh

@@ -1,8 +1,5 @@
 source $stdenv/setup
 
-if test -z "$tag"; then
-  tag="-DNOW"
-fi
 # creating the export drictory and checking out there only to be able to
 # move the content without the root directory into $out ...
 # cvs -f -d "$url" export $tag -d "$out" "$module"
@@ -10,6 +7,16 @@ fi
 # See als man Page for those options
 
 ensureDir $out export
+set -x
+if [ -n "$tag" ]; then
+  tag="-r $tag"
+else
+  if [ -n "$date" ]; then
+    tag="-D $date"
+  else
+    tag="-D NOW"
+  fi
+fi
 cd export; cvs -f -d "$url" export $tag "$module"
 mv */* $out
 

pkgs/build-support/fetchcvs/default.nix

@@ -1,13 +1,13 @@
 # example tags:
-# "-DNOW" (get current version)
-# "-D2007-20-10" (get the last version before given date)
-# "-r <tagname>" (get version by tag name)
-{stdenv, cvs, nix}: {url, module, tag, sha256}:
+# date="2007-20-10"; (get the last version before given date)
+# tag="<tagname>" (get version by tag name)
+# If you don't specify neither one date="NOW" will be used (get latest)
+
+{stdenv, cvs, nix}: {url, module, tag ? null, date ? null, sha256}:
 
 stdenv.mkDerivation {
   name = "cvs-export";
   builder = ./builder.sh;
   buildInputs = [cvs nix];
-
-  inherit url module tag sha256;
+  inherit url module sha256 tag date;
 }