Stream files from store instead of buffering them

When an artifact is requested from hydra the output is first copied
from the nix store into memory and then sent as a response, delaying
the download and taking up significant amounts of memory.

As reported in NixOS#1357

Instead of calling a command and blocking while reading in the entire
output, this adds read_into_socket(). the function takes a
command, starting a subprocess with that command, returning a file
descriptor attached to stdout.
This file descriptor is then by responsebuilder of Catalyst to steam
the output directly

src/lib/Hydra/Controller/Build.pm

@@ -240,7 +240,7 @@ sub serveFile {
         # XSS hole.
         $c->response->header('Content-Security-Policy' => 'sandbox allow-scripts');
 
-        $c->stash->{'plain'} = { data => grab(cmd => ["nix", "--experimental-features", "nix-command",
+        $c->stash->{'plain'} = { data => readIntoSocket(cmd => ["nix", "--experimental-features", "nix-command",
                                                       "store", "cat", "--store", getStoreUri(), "$path"]) };
 
         # Detect MIME type.

src/lib/Hydra/Helper/Nix.pm

@@ -36,6 +36,7 @@ our @EXPORT = qw(
     jobsetOverview
     jobsetOverview_
     pathIsInsidePrefix
+    readIntoSocket
     readNixFile
     registerRoot
     restartBuilds
@@ -406,6 +407,17 @@ sub pathIsInsidePrefix {
     return $cur;
 }
 
+sub readIntoSocket{
+    my (%args) = @_;
+    my $sock;
+
+    eval {
+        my $x= join(" ", @{$args{cmd}});
+        open($sock, "-|", $x) or die q(failed to open socket from command:\n $x);
+    };
+
+    return $sock;
+}