Add external builders

These are helper programs that execute derivations for specified
system types (e.g. using QEMU to emulate another system type).

To use, set `external-builders`:

  external-builders = [{"systems": ["aarch64-linux"], "program": "/path/to/external-builder.py"}]

The external builder gets one command line argument, the path to a JSON file containing all necessary information about the derivation:

  {
    "args": [...],
    "builder": "/nix/store/kwcyvgdg98n98hqapaz8sw92pc2s78x6-bash-5.2p37/bin/bash",
    "env": {
      "HOME": "/homeless-shelter",
      ...
    },
    "realStoreDir": "/tmp/nix/nix/store",
    "storeDir": "/nix/store",
    "tmpDir": "/tmp/nix-shell.dzQ2hE/nix-build-patchelf-0.14.3.drv-46/build",
    "tmpDirInSandbox": "/build"
  }

Co-authored-by: Cole Helbling <cole.helbling@determinate.systems>

src/libstore/unix/build/external-derivation-builder.cc

@@ -0,0 +1,110 @@
+namespace nix {
+
+struct ExternalDerivationBuilder : DerivationBuilderImpl
+{
+    Settings::ExternalBuilder externalBuilder;
+
+    ExternalDerivationBuilder(
+        LocalStore & store,
+        std::unique_ptr<DerivationBuilderCallbacks> miscMethods,
+        DerivationBuilderParams params,
+        Settings::ExternalBuilder externalBuilder)
+        : DerivationBuilderImpl(store, std::move(miscMethods), std::move(params))
+        , externalBuilder(std::move(externalBuilder))
+    {
+        experimentalFeatureSettings.require(Xp::ExternalBuilders);
+    }
+
+    static std::unique_ptr<ExternalDerivationBuilder> newIfSupported(
+        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);
+        }
+        return {};
+    }
+
+    Path tmpDirInSandbox() override
+    {
+        /* In a sandbox, for determinism, always use the same temporary
+           directory. */
+        return "/build";
+    }
+
+    void setBuildTmpDir() override
+    {
+        tmpDir = topTmpDir + "/build";
+        createDir(tmpDir, 0700);
+    }
+
+    void checkSystem() override {}
+
+    void startChild() override
+    {
+        if (drvOptions.getRequiredSystemFeatures(drv).count("recursive-nix"))
+            throw Error("'recursive-nix' is not supported yet by external derivation builders");
+
+        auto json = nlohmann::json::object();
+
+        json.emplace("builder", drv.builder);
+        {
+            auto l = nlohmann::json::array();
+            for (auto & i : drv.args)
+                l.push_back(rewriteStrings(i, inputRewrites));
+            json.emplace("args", std::move(l));
+        }
+        {
+            auto j = nlohmann::json::object();
+            for (auto & [name, value] : env)
+                j.emplace(name, rewriteStrings(value, inputRewrites));
+            json.emplace("env", std::move(j));
+        }
+        json.emplace("topTmpDir", topTmpDir);
+        json.emplace("tmpDir", tmpDir);
+        json.emplace("tmpDirInSandbox", tmpDirInSandbox());
+        json.emplace("storeDir", store.storeDir);
+        json.emplace("realStoreDir", store.config->realStoreDir.get());
+        json.emplace("system", drv.platform);
+
+        // TODO(cole-h): writing this to stdin is too much effort right now, if we want to revisit
+        // that, see this comment by Eelco about how to make it not suck:
+        // https://github.com/DeterminateSystems/nix-src/pull/141#discussion_r2205493257
+        auto jsonFile = std::filesystem::path{topTmpDir} / "build.json";
+        writeFile(jsonFile, json.dump());
+
+        pid = startProcess([&]() {
+            openSlave();
+            try {
+                commonChildInit();
+
+                Strings args = {externalBuilder.program};
+
+                if (!externalBuilder.args.empty()) {
+                    args.insert(args.end(), externalBuilder.args.begin(), externalBuilder.args.end());
+                }
+
+                args.insert(args.end(), jsonFile);
+
+                if (chdir(tmpDir.c_str()) == -1)
+                    throw SysError("changing into '%1%'", tmpDir);
+
+                chownToBuilder(topTmpDir);
+
+                setUser();
+
+                debug("executing external builder: %s", concatStringsSep(" ", args));
+                execv(externalBuilder.program.c_str(), stringsToCharPtrs(args).data());
+
+                throw SysError("executing '%s'", externalBuilder.program);
+            } catch (...) {
+                handleChildException(true);
+                _exit(1);
+            }
+        });
+    }
+};
+
+} // namespace nix