Add function queryPathFromHashPart()

To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5").  (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.)  However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory.  queryPathFromHashPart() prevents this by doing a cheap
database lookup.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 5e5561a6ae..cbb70b2fd7 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -341,6 +341,18 @@ PathSet RemoteStore::queryDerivationOutputNames(const Path & path)
 }
 
 
+Path RemoteStore::queryPathFromHashPart(const string & hashPart)
+{
+    openConnection();
+    writeInt(wopQueryPathFromHashPart, to);
+    writeString(hashPart, to);
+    processStderr();
+    Path path = readString(from);
+    if (!path.empty()) assertStorePath(path);
+    return path;
+}
+
+
 Path RemoteStore::addToStore(const Path & _srcPath,
     bool recursive, HashType hashAlgo, PathFilter & filter)
 {