Add fetchGitMirror function

pkgs/lib.nix

@@ -0,0 +1,30 @@
+{ lib }:
+
+let
+  # If not supported, fall back to tryEval, which will fail in the first case.
+  safeCatchAll = if (builtins ? catchAll)
+    then builtins.catchAll
+    else e: (builtins.tryEval e) // { msg = ""; };
+in lib.extend (_: lib: {
+  # Same as fetchGit but accepts a list of mirror urls
+  fetchGitMirror = { urls, ... } @ args:
+  let
+    cleanArgs = lib.removeAttrs args [ "urls" ];
+    fetchUrl = url: builtins.fetchGit (cleanArgs // { inherit url; });
+    safeFetch = url: safeCatchAll (fetchUrl url);
+    complain = url:
+    let
+      r = safeFetch url;
+    in
+      if (r.success) then r
+      else lib.warn "cannot fetch ${url}, trying next
+      mirror:${builtins.replaceStrings ["\n" ] ["\n> "] ("\n"+r.msg)}" r;
+    fetchList = lib.map (url: complain url) urls;
+    bad = throw "cannot fetch from any mirror";
+    good = lib.findFirst (e: e.success) bad fetchList;
+  in good.value;
+
+  maintainers = lib.maintainers // {
+    bsc = import ./maintainers.nix;
+  };
+})