* Put chroots in the Nix store. This ensures that we can create hard

links to the inputs.

src/libstore/build.cc

@@ -811,6 +811,11 @@ void DerivationGoal::haveDerivation()
         return;
     }
 
+    /* `drvPath' should already be a root, but let's be on the safe
+       side: if the user forgot to make it a root, we wouldn't want
+       things being garbage collected while we're busy. */
+    worker.store.addTempRoot(drvPath);
+    
     assert(worker.store.isValidPath(drvPath));
 
     /* Get the derivation. */
@@ -1669,8 +1674,10 @@ void DerivationGoal::startBuilder()
     if (useChroot) {
 #if CHROOT_ENABLED
         /* Create a temporary directory in which we set up the chroot
-           environment using bind-mounts. */
+           environment using bind-mounts.  We put it in the Nix store
-        chrootRootDir = createTempDir("", "nix-chroot");
+           to ensure that we can create hard-links to non-directory
+           inputs in the fake Nix store in the chroot (see below). */
+        chrootRootDir = drvPath + ".chroot";
 
         /* Clean up the chroot directory automatically. */
         autoDelChroot = boost::shared_ptr<AutoDelete>(new AutoDelete(chrootRootDir));

src/libstore/gc.cc

@@ -565,6 +565,11 @@ static bool isLive(const Path & path, const PathSet & livePaths,
     if (hasSuffix(path, ".lock") && tempRoots.find(string(path, 0, path.size() - 5)) != tempRoots.end())
         return true;
 
+    /* Don't delete .chroot directories for derivations that are
+       currently being built. */
+    if (hasSuffix(path, ".chroot") && tempRoots.find(string(path, 0, path.size() - 7)) != tempRoots.end())
+        return true;
+
     return false;
 }