Lock paths in the scm cache

This is necessary now that hydra-evaluator runs multiple evaluations
in parallel, to prevent corruption of Git/Mercurial clones.

src/lib/Hydra/Plugin/MercurialInput.pm

@@ -6,6 +6,7 @@ use Digest::SHA qw(sha256_hex);
 use File::Path;
 use Hydra::Helper::Nix;
 use Nix::Store;
+use Fcntl qw(:flock);
 
 sub supportedInputTypes {
     my ($self, $inputTypes) = @_;
@@ -40,6 +41,9 @@ sub fetchInput {
 
     my $clonePath = _clonePath($uri);
 
+    open(my $lock, ">", "$clonePath.lock") or die;
+    flock($lock, LOCK_EX) or die;
+
     if (! -d $clonePath) {
         (my $res, $stdout, $stderr) = captureStdoutStderr(600,
             "hg", "clone", $uri, $clonePath);