* Added a flag --ignore-liveness' to nix-store --delete'. It

deletes a path even if it is reachable from a root.  However, it
  won't delete a path that still has referrers (since that would
  violate store invariants).

  Don't try this at home.  It's a useful hack for recovering from
  certain situations in a somewhat clean way (e.g., holes in closures
  due to disk corruption).

src/libstore/gc.cc

@@ -304,7 +304,7 @@ static Paths topoSort(const PathSet & paths)
 
 
 void collectGarbage(GCAction action, const PathSet & pathsToDelete,
-    PathSet & result, unsigned long long & bytesFreed)
+    bool ignoreLiveness, PathSet & result, unsigned long long & bytesFreed)
 {
     result.clear();
     bytesFreed = 0;
@@ -323,7 +323,8 @@ void collectGarbage(GCAction action, const PathSet & pathsToDelete,
        permanent roots cannot increase now. */
     Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
     PathSet roots;
-    findRoots(rootsDir, true, roots);
+    if (!ignoreLiveness)
+        findRoots(rootsDir, true, roots);
 
     if (action == gcReturnRoots) {
         result = roots;

src/libstore/gc.hh

@@ -21,7 +21,7 @@ typedef enum {
    set of paths not reachable from the roots.  If `action' is
    `gcDeleteDead', actually delete the latter set. */
 void collectGarbage(GCAction action, const PathSet & pathsToDelete,
-    PathSet & result, unsigned long long & bytesFreed);
+    bool ignoreLiveness, PathSet & result, unsigned long long & bytesFreed);
 
 /* Register a temporary GC root.  This root will automatically
    disappear when this process exits.  WARNING: this function should