From b56dd21c311b1ad1e19bfb1180a0b5f94834b85d Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Fri, 10 Oct 2025 17:18:40 -0400
Subject: [PATCH] `Settings::ExternalBuilder::systems` make set

Nothing cares about the order, actually.
---
 src/libstore/include/nix/store/globals.hh              | 2 +-
 src/libstore/unix/build/external-derivation-builder.cc | 7 +++----
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/libstore/include/nix/store/globals.hh b/src/libstore/include/nix/store/globals.hh
index 1b59bd6fc..be3561848 100644
--- a/src/libstore/include/nix/store/globals.hh
+++ b/src/libstore/include/nix/store/globals.hh
@@ -1375,7 +1375,7 @@ public:
 
     struct ExternalBuilder
     {
-        std::vector<std::string> systems;
+        StringSet systems;
         Path program;
         std::vector<std::string> args;
     };
diff --git a/src/libstore/unix/build/external-derivation-builder.cc b/src/libstore/unix/build/external-derivation-builder.cc
index 71cfd1a62..f20badb85 100644
--- a/src/libstore/unix/build/external-derivation-builder.cc
+++ b/src/libstore/unix/build/external-derivation-builder.cc
@@ -19,10 +19,9 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
         LocalStore & store, std::unique_ptr<DerivationBuilderCallbacks> & miscMethods, DerivationBuilderParams & params)
     {
         for (auto & handler : settings.externalBuilders.get()) {
-            for (auto & system : handler.systems)
-                if (params.drv.platform == system)
-                    return std::make_unique<ExternalDerivationBuilder>(
-                        store, std::move(miscMethods), std::move(params), handler);
+            if (handler.systems.contains(params.drv.platform))
+                return std::make_unique<ExternalDerivationBuilder>(
+                    store, std::move(miscMethods), std::move(params), handler);
         }
         return {};
     }