Use BINARY_CACHE_STORE where appropriate

Replaces / Closes NixOS#1352

Consider the following setup:

* `store_uri` points to a "special" store, e.g. `s3://` or `file://`.
* Hydra runs inside e.g. an nspawn container and exclusively builds
  on remote systems.

Then, build products are never in the nspawn's `/nix/store`. This in
turn means that `$MACHINE_LOCAL_STORE->isValidPath` is always `false`
(or `0` 🤷) for output paths and thus Hydra wrongly claims that the
output got garbage collected.

Instead, use the BINARY_CACHE_STORE to look for the availability in the
correct store.

Everything touching the `drv` rather than the output paths still uses
MACHINE_LOCAL_STORE: this is because `hydra-eval-jobs` does `openStore()`
on `auto`, so the derivations end up there rather than on the
BINARY_CACHE_STORE.

src/lib/Hydra/Base/Controller/NixChannel.pm

@@ -29,7 +29,7 @@ sub getChannelData {
         my $outputs = {};
         foreach my $output (@outputs) {
             my $outPath = $output->get_column("outpath");
-            next if $checkValidity && !$MACHINE_LOCAL_STORE->isValidPath($outPath);
+            next if $checkValidity && !$BINARY_CACHE_STORE->isValidPath($outPath);
             $outputs->{$output->get_column("outname")} = $outPath;
             push @storePaths, $outPath;
             # Put the system type in the manifest (for top-level

src/lib/Hydra/Controller/Build.pm

@@ -80,7 +80,7 @@ sub build_GET {
     # false because `$_->path` will be empty
     $c->stash->{available} =
         $c->stash->{isLocalStore}
-        ? all { $_->path && $MACHINE_LOCAL_STORE->isValidPath($_->path) } $build->buildoutputs->all
+        ? all { $_->path && $BINARY_CACHE_STORE->isValidPath($_->path) } $build->buildoutputs->all
         : 1;
     $c->stash->{drvAvailable} = $MACHINE_LOCAL_STORE->isValidPath($build->drvpath);
 
@@ -306,7 +306,7 @@ sub output : Chained('buildChain') PathPart Args(1) {
     error($c, "This build is not finished yet.") unless $build->finished;
     my $output = $build->buildoutputs->find({name => $outputName});
     notFound($c, "This build has no output named ‘$outputName’") unless defined $output;
-    gone($c, "Output is no longer available.") unless $MACHINE_LOCAL_STORE->isValidPath($output->path);
+    gone($c, "Output is no longer available.") unless $BINARY_CACHE_STORE->isValidPath($output->path);
 
     $c->response->header('Content-Disposition', "attachment; filename=\"build-${\$build->id}-${\$outputName}.nar.bz2\"");
     $c->stash->{current_view} = 'NixNAR';
@@ -423,15 +423,15 @@ sub getDependencyGraph {
             };
         $$done{$path} = $node;
         my @refs;
-        foreach my $ref ($MACHINE_LOCAL_STORE->queryReferences($path)) {
+        foreach my $ref ($BINARY_CACHE_STORE->queryReferences($path)) {
             next if $ref eq $path;
             next unless $runtime || $ref =~ /\.drv$/;
             getDependencyGraph($self, $c, $runtime, $done, $ref);
             push @refs, $ref;
         }
         # Show in reverse topological order to flatten the graph.
         # Should probably do a proper BFS.
-        my @sorted = reverse $MACHINE_LOCAL_STORE->topoSortPaths(@refs);
+        my @sorted = reverse $BINARY_CACHE_STORE->topoSortPaths(@refs);
         $node->{refs} = [map { $$done{$_} } @sorted];
     }
 
@@ -459,7 +459,7 @@ sub runtime_deps : Chained('buildChain') PathPart('runtime-deps') {
 
     requireLocalStore($c);
 
-    error($c, "Build outputs no longer available.") unless all { $MACHINE_LOCAL_STORE->isValidPath($_) } @outPaths;
+    error($c, "Build outputs no longer available.") unless all { $BINARY_CACHE_STORE->isValidPath($_) } @outPaths;
 
     my $done = {};
     $c->stash->{runtimeGraph} = [ map { getDependencyGraph($self, $c, 1, $done, $_) } @outPaths ];
@@ -479,7 +479,7 @@ sub nix : Chained('buildChain') PathPart('nix') CaptureArgs(0) {
     if (isLocalStore) {
         foreach my $out ($build->buildoutputs) {
             notFound($c, "Path " . $out->path . " is no longer available.")
-                unless $MACHINE_LOCAL_STORE->isValidPath($out->path);
+                unless $BINARY_CACHE_STORE->isValidPath($out->path);
         }
     }
 

src/lib/Hydra/Controller/Root.pm

@@ -395,7 +395,7 @@ sub narinfo :Path :Args(StrMatch[NARINFO_REGEX]) {
         my ($hash) = $narinfo =~ NARINFO_REGEX;
 
         die("Hash length was not 32") if length($hash) != 32;
-        my $path = $MACHINE_LOCAL_STORE->queryPathFromHashPart($hash);
+        my $path = $BINARY_CACHE_STORE->queryPathFromHashPart($hash);
 
         if (!$path) {
             $c->response->status(404);

src/lib/Hydra/Helper/Nix.pm

@@ -41,9 +41,11 @@ our @EXPORT = qw(
     restartBuilds
     run
     $MACHINE_LOCAL_STORE
+    $BINARY_CACHE_STORE
     );
 
 our $MACHINE_LOCAL_STORE = Nix::Store->new();
+our $BINARY_CACHE_STORE = Nix::Store->new(getStoreUri());
 
 
 sub getHydraHome {