* 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, 25 insertions, 4 deletions

diff --git a/src/libstore/store.cc b/src/libstore/store.cc
index 9677f84223..44b3a29e34 100644
--- a/src/libstore/store.cc
+++ b/src/libstore/store.cc
@@ -237,6 +237,30 @@ void registerSuccessor(const Transaction & txn,
 }
 
 
+void unregisterSuccessor(const Path & srcPath)
+{
+    assertStorePath(srcPath);
+
+    Transaction txn(nixDB);
+
+    Path sucPath;
+    if (!nixDB.queryString(txn, dbSuccessors, srcPath, sucPath)) {
+        txn.abort();
+        return;
+    }
+    nixDB.delPair(txn, dbSuccessors, srcPath);
+
+    Paths revs;
+    nixDB.queryStrings(txn, dbSuccessorsRev, sucPath, revs);
+    Paths::iterator i = find(revs.begin(), revs.end(), srcPath);
+    assert(i != revs.end());
+    revs.erase(i);
+    nixDB.setStrings(txn, dbSuccessorsRev, sucPath, revs);
+
+    txn.commit();
+}
+
+
 bool querySuccessor(const Path & srcPath, Path & sucPath)
 {
     return nixDB.queryString(noTxn, dbSuccessors, srcPath, sucPath);
@@ -294,10 +318,7 @@ static void writeSubstitutes(const Transaction & txn,
         ss.push_back(packStrings(ss2));
     }
 
-    if (ss.size() == 0)
-        nixDB.delPair(txn, dbSubstitutes, srcPath);
-    else
-        nixDB.setStrings(txn, dbSubstitutes, srcPath, ss);
+    nixDB.setStrings(txn, dbSubstitutes, srcPath, ss);
 }