Test nix-build with non-local-store --store

Just a few small things needed fixing!

src/libstore/build/derivation-goal.cc

@@ -592,6 +592,14 @@ void DerivationGoal::tryToBuild()
     PathSet lockFiles;
     /* FIXME: Should lock something like the drv itself so we don't build same
        CA drv concurrently */
+    if (dynamic_cast<LocalStore *>(&worker.store))
+        /* If we aren't a local store, we might need to use the local store as
+           a build remote, but that would cause a deadlock. */
+        /* FIXME: Make it so we can use ourselves as a build remote even if we
+           are the local store (separate locking for building vs scheduling? */
+        /* FIXME: find some way to lock for scheduling for the other stores so
+           a forking daemon with --store still won't farm out redundant builds.
+           */
         for (auto & i : drv->outputsAndOptPaths(worker.store))
             if (i.second.second)
                 lockFiles.insert(worker.store.Store::toRealPath(*i.second.second));
@@ -680,6 +688,12 @@ void DerivationGoal::tryLocalBuild() {
     /* Make sure that we are allowed to start a build.  If this
        derivation prefers to be done locally, do it even if
        maxBuildJobs is 0. */
+    if (!dynamic_cast<LocalStore *>(&worker.store)) {
+        throw Error(
+            "unable to build with a primary store that isn't a local store; "
+            "either pass a different '--store' or enable remote builds."
+            "\nhttps://nixos.org/nix/manual/#chap-distributed-builds");
+    }
     unsigned int curBuilds = worker.getNrLocalBuilds();
     if (curBuilds >= settings.maxBuildJobs && !(buildLocally && curBuilds == 0)) {
         worker.waitForBuildSlot(shared_from_this());

tests/binary-cache-build-remote.sh

@@ -0,0 +1,13 @@
+source common.sh
+
+clearStore
+clearCacheCache
+
+# Fails without remote builders
+(! nix-build --store "file://$cacheDir" dependencies.nix)
+
+# Succeeds with default store as build remote.
+nix-build --store "file://$cacheDir" --builders 'auto - - 1 1' -j0 dependencies.nix
+
+# Succeeds without any build capability because no-op
+nix-build --store "file://$cacheDir" -j0 dependencies.nix

tests/local.mk

@@ -9,7 +9,9 @@ nix_tests = \
   local-store.sh remote-store.sh export.sh export-graph.sh \
   timeout.sh secure-drv-outputs.sh nix-channel.sh \
   multiple-outputs.sh import-derivation.sh fetchurl.sh optimise-store.sh \
-  binary-cache.sh nix-profile.sh repair.sh dump-db.sh case-hack.sh \
+  binary-cache.sh \
+  binary-cache-build-remote.sh \
+  nix-profile.sh repair.sh dump-db.sh case-hack.sh \
   check-reqs.sh pass-as-file.sh tarball.sh restricted.sh \
   placeholders.sh nix-shell.sh \
   linux-sandbox.sh \