* 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, 7 insertions, 7 deletions

diff --git a/src/libstore/normalise.hh b/src/libstore/normalise.hh
index bbde545c40ec..43be136e5b75 100644
--- a/src/libstore/normalise.hh
+++ b/src/libstore/normalise.hh
@@ -10,13 +10,13 @@
    successor is known. */
 Path normaliseStoreExpr(const Path & nePath);
 
-/* Realise a closure store expression in the file system. 
-
-   The pending paths are those that are already being realised.  This
-   prevents infinite recursion for paths realised through a substitute
-   (since when we build the substitute, we would first try to realise
-   its output paths through substitutes... kaboom!). */
-void realiseClosure(const Path & nePath);
+/* Realise a store expression.  If the expression is a derivation, it
+   is first normalised into a closure.  The closure is then realised
+   in the file system (i.e., it is ensured that each path in the
+   closure exists in the file system, if necessary by using the
+   substitute mechanism).  Returns the normal form of the expression
+   (i.e., its closure expression). */
+Path realiseStoreExpr(const Path & nePath);
 
 /* Ensure that a path exists, possibly by instantiating it by
    realising a substitute. */