Harmonise the Store::queryPathInfoUncached interface

This:
 - Consistently returns `nullptr` for a non-existent
   store path, instead of a mix of `nullptr` and
   throwing exceptions.

 - If a store returns "bad" store paths in response
   to a request (e.g. incorrect hash or name), don't
   cache this result. This removes some duplication
   of code at the cache-access layer of queryPathInfo()
   checking this, and ­allows us to provide more
   specific errors.

Part of #270.

Change-Id: I86612c6499b1a37ab872c712c2304d6a3ff19edb

1 files changed, 4 insertions, 0 deletions

diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 397ebe759..de4dc3f03 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -399,6 +399,10 @@ public:
 
 protected:
 
+    /**
+     * Queries the path info without caching.
+     * Note to implementors: should return `nullptr` when the path is not found.
+     */
     virtual std::shared_ptr<const ValidPathInfo> queryPathInfoUncached(const StorePath & path) = 0;
     virtual std::shared_ptr<const Realisation> queryRealisationUncached(const DrvOutput &) = 0;