Prevent inputs from being garbage collected during evaluation

There is still a tiny window between the calls to nix-prefetch-* and
addTempRoot. This could be eliminated by adding a "-o" option to
nix-prefetch-*, or by not using those scripts at all (and use
addToStore directly).

src/lib/Hydra/Plugin/BazaarInput.pm

@@ -36,6 +36,8 @@ sub fetchInput {
     (my $cachedInput) = $self->{db}->resultset('CachedBazaarInputs')->search(
         {uri => $uri, revision => $revision});
 
+    addTempRoot($cachedInput->storepath) if defined $cachedInput;
+
     if (defined $cachedInput && isValidPath($cachedInput->storepath)) {
         $storePath = $cachedInput->storepath;
         $sha256 = $cachedInput->sha256hash;
@@ -53,6 +55,9 @@ sub fetchInput {
 
         ($sha256, $storePath) = split ' ', $stdout;
 
+        # FIXME: time window between nix-prefetch-bzr and addTempRoot.
+        addTempRoot($storePath);
+
         txn_do($self->{db}, sub {
             $self->{db}->resultset('CachedBazaarInputs')->create(
                 { uri => $uri