Make the search path lazier with non-fatal errors

Thus, -I / $NIX_PATH entries are now downloaded only when they are
needed for evaluation. An error to download an entry is a non-fatal
warning (just like non-existant paths).

This does change the semantics of builtins.nixPath, which now returns
the original, rather than resulting path. E.g., before we had

  [ { path = "/nix/store/hgm3yxf1lrrwa3z14zpqaj5p9vs0qklk-nixexprs.tar.xz"; prefix = "nixpkgs"; } ... ]

but now

  [ { path = "https://nixos.org/channels/nixos-16.03/nixexprs.tar.xz"; prefix = "nixpkgs"; } ... ]

Fixes #792.

1 files changed, 7 insertions, 3 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 8ce2f3dfa..03d359725 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -278,7 +278,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
 
     /* Initialise the Nix expression search path. */
     Strings paths = parseNixPath(getEnv("NIX_PATH", ""));
-    for (auto & i : _searchPath) addToSearchPath(i, true);
+    for (auto & i : _searchPath) addToSearchPath(i);
     for (auto & i : paths) addToSearchPath(i);
     addToSearchPath("nix=" + settings.nixDataDir + "/nix/corepkgs");
 
@@ -301,11 +301,15 @@ Path EvalState::checkSourcePath(const Path & path_)
     if (!restricted) return path_;
 
     /* Resolve symlinks. */
+    debug(format("checking access to ‘%s’") % path_);
     Path path = canonPath(path_, true);
 
-    for (auto & i : searchPath)
-        if (path == i.second || isInDir(path, i.second))
+    for (auto & i : searchPath) {
+        auto r = resolveSearchPathElem(i);
+        if (!r.first) continue;
+        if (path == r.second || isInDir(path, r.second))
             return path;
+    }
 
     /* To support import-from-derivation, allow access to anything in
        the store. FIXME: only allow access to paths that have been