Add operation ‘nix-store --repair-path’

This operation allows fixing corrupted or accidentally deleted store
paths by redownloading them using substituters, if available.

Since the corrupted path cannot be replaced atomically, there is a
very small time window (one system call) during which neither the old
(corrupted) nor the new (repaired) contents are available.  So
repairing should be used with some care on critical packages like
Glibc.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 8899873a72c6..80db10de10d0 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -197,6 +197,10 @@ public:
 
     void vacuumDB();
 
+    /* Repair the contents of the given path by redownloading it using
+       a substituter (if available). */
+    void repairPath(const Path & path);
+
 private:
 
     Path schemaPath;