79/118: Allow regular files as GC roots

[Ludovic Courtès]

civodul pushed a commit to branch nix
in repository guix.

commit a98fa2d9e2b06e2561330c5ef845ffaf131e95ac
Author: Eelco Dolstra <address@hidden>
Date:   Fri Aug 1 16:46:01 2014 +0200

    Allow regular files as GC roots
    
    If a root is a regular file, then its name must denote a store
    path. For instance, the existence of the file
    
      
/nix/var/nix/gcroots/per-user/eelco/hydra-roots/wzc3cy1wwwd6d0dgxpa77ijr1yp50s6v-libxml2-2.7.7
    
    would cause
    
      /nix/store/wzc3cy1wwwd6d0dgxpa77ijr1yp50s6v-libxml2-2.7.7
    
    to be a root.
    
    This is useful because it involves less I/O (no need for a readlink()
    call) and takes up less disk space (the symlink target typically takes
    up a full disk block, while directory entries are packed more
    efficiently). This is particularly important for hydra.nixos.org,
    which has hundreds of thousands of roots, and where reading the roots
    can take 25 minutes.
---
 nix/libstore/gc.cc |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/nix/libstore/gc.cc b/nix/libstore/gc.cc
index f90edac..e7285fb 100644
--- a/nix/libstore/gc.cc
+++ b/nix/libstore/gc.cc
@@ -328,6 +328,12 @@ static void findRoots(StoreAPI & store, const Path & path, 
Roots & roots)
             }
         }
 
+        else if (S_ISREG(st.st_mode)) {
+            Path storePath = settings.nixStore + "/" + baseNameOf(path);
+            if (store.isValidPath(storePath))
+                roots[path] = storePath;
+        }
+
     }
 
     catch (SysError & e) {