* Added a switch `--fallback'. From the manual:

  Whenever Nix attempts to realise a derivation for which a closure is
  already known, but this closure cannot be realised, fall back on
  normalising the derivation.

  The most common scenario in which this is useful is when we have
  registered substitutes in order to perform binary distribution from,
  say, a network repository.  If the repository is down, the
  realisation of the derivation will fail.  When this option is
  specified, Nix will build the derivation instead.  Thus, binary
  installation falls back on a source installation.  This option is
  not the default since it is generally not desirable for a transient
  failure in obtaining the substitutes to lead to a full build from
  source (with the related consumption of resources).

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstore/store.hh b/src/libstore/store.hh
index 40d1859e5..68f7d6190 100644
--- a/src/libstore/store.hh
+++ b/src/libstore/store.hh
@@ -54,6 +54,9 @@ void copyPath(const Path & src, const Path & dst);
 void registerSuccessor(const Transaction & txn,
     const Path & srcPath, const Path & sucPath);
 
+/* Remove a successor mapping. */
+void unregisterSuccessor(const Path & srcPath);
+
 /* Return the predecessors of the Nix expression stored at the given
    path. */
 bool querySuccessor(const Path & srcPath, Path & sucPath);