build-support: Add fetchfossil function.

This is a dead-simple fetcher which clones a Fossil repository, opens it
directly into $out, and then nicks out the single Fossil checkout marker.

pkgs/build-support/fetchfossil/builder.sh

@@ -0,0 +1,21 @@
+source $stdenv/setup
+header "Cloning Fossil $url into $out"
+
+# Fossil, bless its adorable little heart, wants to write global configuration
+# to $HOME/.fossil. AFAICT, there is no way to disable this functionality.
+export HOME=.
+
+# We must explicitly set the admin user for the clone to something reasonable.
+fossil clone -A nobody "$url" fossil-clone.fossil
+
+mkdir fossil-clone
+WORKDIR=$(pwd)
+mkdir $out
+pushd $out
+fossil open "$WORKDIR/fossil-clone.fossil"
+popd
+
+# Just nuke the checkout file.
+rm $out/.fslckout
+
+stopNest

pkgs/build-support/fetchfossil/default.nix

@@ -0,0 +1,21 @@
+{stdenv, fossil}:
+
+{name ? null, url, rev ? null, md5 ? null, sha256 ? null}:
+
+# TODO: statically check if mercurial as the https support if the url starts woth https.
+stdenv.mkDerivation {
+  name = "fossil-archive" + (if name != null then "-${name}" else "");
+  builder = ./builder.sh;
+  buildInputs = [fossil];
+
+  impureEnvVars = [
+    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+  ];
+
+  outputHashAlgo = if md5 != null then "md5" else "sha256";
+  outputHashMode = "recursive";
+  outputHash = if md5 != null then md5 else sha256;
+
+  inherit url rev;
+  preferLocalBuild = true;
+}

pkgs/top-level/all-packages.nix

@@ -162,6 +162,8 @@ in
 
   fetchdarcs = callPackage ../build-support/fetchdarcs { };
 
+  fetchfossil = callPackage ../build-support/fetchfossil { };
+
   fetchgit = callPackage ../build-support/fetchgit {
     git = gitMinimal;
   };