* `nix-store -qb' to query derivation environment bindings. Useful

for finding build-time dependencies (possibly after a build).  E.g.,

    $ nix-store -qb aterm $(nix-store -qd $(which strc))
    /nix/store/jw7c7s65n1gwhxpn35j9rgcci6ilzxym-aterm-2.3.1

* Arguments to nix-store can be files within store objects, e.g.,
  /nix/store/jw7c...-aterm-2.3.1/bin/baffle.

* Idem for garbage collector roots.

src/libstore/gc.cc

@@ -60,7 +60,7 @@ void createSymlink(const Path & link, const Path & target, bool careful)
 
     /* Remove the old symlink. */
     if (pathExists(link)) {
-        if (careful && (!isLink(link) || !isStorePath(readLink(link))))
+        if (careful && (!isLink(link) || !isInStore(readLink(link))))
             throw Error(format("cannot create symlink `%1%'; already exists") % link);
         unlink(link.c_str());
     }
@@ -250,10 +250,10 @@ static void findRoots(const Path & path, bool recurseSymlinks,
         string target = readLink(path);
         Path target2 = absPath(target, dirOf(path));
 
-        if (isStorePath(target2)) {
+        if (isInStore(target2)) {
             debug(format("found root `%1%' in `%2%'")
                 % target2 % path);
-            roots.insert(target2);
+            roots.insert(toStorePath(target2));
         }
 
         else if (recurseSymlinks) {

src/libstore/store.cc

@@ -176,12 +176,18 @@ void copyPath(const Path & src, const Path & dst)
 }
 
 
-bool isStorePath(const Path & path)
+bool isInStore(const Path & path)
 {
     return path[0] == '/'
         && path.compare(0, nixStore.size(), nixStore) == 0
         && path.size() >= nixStore.size() + 2
-        && path[nixStore.size()] == '/'
+        && path[nixStore.size()] == '/';
+}
+
+
+bool isStorePath(const Path & path)
+{
+    return isInStore(path)
         && path.find('/', nixStore.size() + 1) == Path::npos;
 }
 
@@ -193,6 +199,18 @@ void assertStorePath(const Path & path)
 }
 
 
+Path toStorePath(const Path & path)
+{
+    if (!isInStore(path))
+        throw Error(format("path `%1%' is not in the Nix store") % path);
+    unsigned int slash = path.find('/', nixStore.size() + 1);
+    if (slash == Path::npos)
+        return path;
+    else
+        return Path(path, 0, slash);
+}
+
+
 void canonicalisePathMetaData(const Path & path)
 {
     checkInterrupt();

src/libstore/store.hh

@@ -63,8 +63,13 @@ void registerValidPath(const Transaction & txn,
 /* Throw an exception if `path' is not directly in the Nix store. */
 void assertStorePath(const Path & path);
 
+bool isInStore(const Path & path);
 bool isStorePath(const Path & path);
 
+/* Chop off the parts after the top-level store name, e.g.,
+   /nix/store/abcd-foo/bar => /nix/store/abcd-foo. */
+Path toStorePath(const Path & path);
+
 /* "Fix", or canonicalise, the meta-data of the files in a store path
    after it has been built.  In particular:
    - the last modification date on each file is set to 0 (i.e.,