* Get rid of the substitutes database table (NIX-47). Instead, if we

need any info on substitutable paths, we just call the substituters
  (such as download-using-manifests.pl) directly.  This means that
  it's no longer necessary for nix-pull to register substitutes or for
  nix-channel to clear them, which makes those operations much faster
  (NIX-95).  Also, we don't have to worry about keeping nix-pull
  manifests (in /nix/var/nix/manifests) and the database in sync with
  each other.

  The downside is that there is some overhead in calling an external
  program to get the substitutes info.  For instance, "nix-env -qas"
  takes a bit longer.

  Abolishing the substitutes table also makes the logic in
  local-store.cc simpler, as we don't need to store info for invalid
  paths.  On the downside, you cannot do things like "nix-store -qR"
  on a substitutable but invalid path (but nobody did that anyway).

* Never catch interrupts (the Interrupted exception).

src/libstore/store-api.cc

@@ -8,7 +8,8 @@ namespace nix {
 
 bool StoreAPI::hasSubstitutes(const Path & path)
 {
-    return !querySubstitutes(path).empty();
+    PathSet paths = querySubstitutablePaths();
+    return paths.find(path) != paths.end();
 }
 
 
@@ -130,6 +131,24 @@ Path computeStorePathForText(const string & suffix, const string & s,
 }
 
 
+ValidPathInfo decodeValidPathInfo(std::istream & str)
+{
+    ValidPathInfo info;
+    getline(str, info.path);
+    if (str.eof()) { info.path = ""; return info; }
+    getline(str, info.deriver);
+    string s; int n;
+    getline(str, s);
+    if (!string2Int(s, n)) throw Error("number expected");
+    while (n--) {
+        getline(str, s);
+        info.references.insert(s);
+    }
+    if (!str || str.eof()) throw Error("missing input");
+    return info;
+}
+
+
 }