nix-push: Add ‘--link’ flag

If ‘--link’ is given, nix-push will create hard links to the NAR files
in the store, rather than copying them.  This is faster and requires
less disk space.  However, it doesn't work if the store is on a
different file system.

scripts/nix-push.in

@@ -23,6 +23,7 @@ my $force = 0;
 my $destDir;
 my $writeManifest = 0;
 my $archivesURL;
+my $link = 0;
 my @roots;
 
 for (my $n = 0; $n < scalar @ARGV; $n++) {
@@ -45,6 +46,8 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
         $n++;
         die "$0: `$arg' requires an argument\n" unless $n < scalar @ARGV;
         $archivesURL = $ARGV[$n];
+    } elsif ($arg eq "--link") {
+        $link = 1;
     } elsif (substr($arg, 0, 1) eq "-") {
         die "$0: unknown flag `$arg'\n";
     } else {
@@ -209,7 +212,11 @@ for (my $n = 0; $n < scalar @storePaths2; $n++) {
     my $dst = "$destDir/$narName";
     if (! -f $dst) {
         my $tmp = "$destDir/.tmp.$$.$narName";
-        copy($narFile, $tmp) or die "cannot copy $narFile to $tmp: $!\n";
+        if ($link) {
+            link($narFile, $tmp) or die "cannot link $tmp to $narFile: $!\n";
+        } else {
+            copy($narFile, $tmp) or die "cannot copy $narFile to $tmp: $!\n";
+        }
         rename($tmp, $dst) or die "cannot rename $tmp to $dst: $!\n";
     }