* Substitutes now should produce a path with the same id as they are

  substituting for (obvious, really).

* For greater efficiency, nix-pull/unnar will place the output in a
  path that is probably the same as what is actually needed, thus
  preventing a path copy.

* Even if a output id is given in a Fix package expression, ensure
  that the resulting Nix derive expression has a different id.  This
  is because Nix expressions that are semantically equivalent (i.e.,
  build the same result) might be different w.r.t. efficiency or
  divergence.  It is absolutely vital for the substitute mechanism
  that such expressions are not used interchangeably.

1 files changed, 15 insertions, 18 deletions

diff --git a/src/store.cc b/src/store.cc
index 65c44ca37f..013bd2e2a7 100644
--- a/src/store.cc
+++ b/src/store.cc
@@ -165,8 +165,11 @@ bool isInPrefix(const string & path, const string & _prefix)
 
 
 string expandId(const FSId & id, const string & target,
-    const string & prefix)
+    const string & prefix, FSIdSet pending)
 {
+    debug(format("expanding %1%") % (string) id);
+    Nest nest(true);
+
     Strings paths;
 
     if (!target.empty() && !isInPrefix(target, prefix))
@@ -203,30 +206,24 @@ string expandId(const FSId & id, const string & target,
         }
     }
 
-    /* Try to realise the substitutes. */
+    if (pending.find(id) != pending.end())
+        throw Error(format("id %1% already being expanded") % (string) id);
+    pending.insert(id);
 
+    /* Try to realise the substitutes, but only if this id is not
+       already being realised by a substitute. */
     Strings subs;
     queryListDB(nixDB, dbSubstitutes, id, subs); /* non-existence = ok */
 
     for (Strings::iterator it = subs.begin(); it != subs.end(); it++) {
         FSId subId = parseHash(*it);
-        Slice slice = normaliseFState(subId);
-        realiseSlice(slice);
-        
-        Strings paths = fstatePaths(subId, true);
-        if (paths.size() != 1) 
-            throw Error("substitute created more than 1 path");
-        string path = *(paths.begin());
 
-        if (target.empty())
-            return path; /* !!! prefix */
-        else {
-            if (path != target) {
-                copyPath(path, target);
-                registerPath(target, id);
-            }
-            return target;
-        }
+        debug(format("trying substitute %1%") % (string) subId);
+
+        Slice slice = normaliseFState(subId, pending);
+        realiseSlice(slice, pending);
+
+        return expandId(id, target, prefix, pending);
     }
     
     throw Error(format("cannot expand id `%1%'") % (string) id);