depot: add version attribute which is the current changeset

default.nix

@@ -21,6 +21,8 @@ in fix (self:
     web = import ./web ch;
     go = import ./go ch;
 
+    version = import ./version.nix ch;
+
     lib = self.third_party.nixpkgs.lib;
 
     pkgs = self.third_party.nixpkgs // self.nix.pkgs;

version.nix

@@ -0,0 +1,17 @@
+{ pkgs, ... }:
+
+let
+  hgRepo = builtins.path { path = ./.hg; name = "depot-hg"; };
+  changesetDeriv = pkgs.runCommandLocal "depot-version" {
+    HG_REPO = hgRepo;
+    MERCURIAL = pkgs.mercurial;
+  } ''
+    mytmp=$(mktemp -d)
+    pushd $mytmp
+    ln -s $HG_REPO .hg
+    echo -n "depot-$($MERCURIAL/bin/hg id --id | tr -d '+')" > $out
+    popd
+    rm -rf $mytmp
+  '';
+in
+  builtins.readFile changesetDeriv