Allow specifying args to external builder program

src/libstore/globals.cc

@@ -309,7 +309,7 @@ unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
     }
 }
 
-NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Settings::ExternalBuilder, systems, program);
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Settings::ExternalBuilder, systems, program, args);
 
 template<> Settings::ExternalBuilders BaseSetting<Settings::ExternalBuilders>::parse(const std::string & str) const
 {

src/libstore/include/nix/store/globals.hh

@@ -1241,6 +1241,7 @@ public:
     {
         std::vector<std::string> systems;
         Path program;
+        std::optional<std::vector<std::string>> args;
     };
 
     using ExternalBuilders = std::vector<ExternalBuilder>;
@@ -1251,6 +1252,68 @@ public:
         "external-builders",
         R"(
           Helper programs that execute derivations.
+
+          The program is passed a JSON document that describes the build environment as the final argument.
+          The JSON document looks like this:
+
+            {
+              "args": [
+                "-e",
+                "/nix/store/vj1c3wf9c11a0qs6p3ymfvrnsdgsdcbq-source-stdenv.sh",
+                "/nix/store/shkw4qm9qcw5sc5n1k5jznc83ny02r39-default-builder.sh"
+              ],
+              "builder": "/nix/store/s1qkj0ph0ma64a6743mvkwnabrbw1hsc-bash-5.2p37/bin/bash",
+              "env": {
+                "HOME": "/homeless-shelter",
+                "NIX_BUILD_CORES": "14",
+                "NIX_BUILD_TOP": "/build",
+                "NIX_LOG_FD": "2",
+                "NIX_STORE": "/nix/store",
+                "PATH": "/path-not-set",
+                "PWD": "/build",
+                "TEMP": "/build",
+                "TEMPDIR": "/build",
+                "TERM": "xterm-256color",
+                "TMP": "/build",
+                "TMPDIR": "/build",
+                "__structuredAttrs": "",
+                "buildInputs": "",
+                "builder": "/nix/store/s1qkj0ph0ma64a6743mvkwnabrbw1hsc-bash-5.2p37/bin/bash",
+                "cmakeFlags": "",
+                "configureFlags": "",
+                "depsBuildBuild": "",
+                "depsBuildBuildPropagated": "",
+                "depsBuildTarget": "",
+                "depsBuildTargetPropagated": "",
+                "depsHostHost": "",
+                "depsHostHostPropagated": "",
+                "depsTargetTarget": "",
+                "depsTargetTargetPropagated": "",
+                "doCheck": "1",
+                "doInstallCheck": "1",
+                "mesonFlags": "",
+                "name": "hello-2.12.2",
+                "nativeBuildInputs": "/nix/store/l31j72f1h33hsa4nq4iyhsmsqjyndq9f-version-check-hook",
+                "out": "/nix/store/2yx2prgxmzbkrnbb4liy6n4zkzb1cqai-hello-2.12.2",
+                "outputs": "out",
+                "patches": "",
+                "pname": "hello",
+                "postInstallCheck": "stat \"${!outputBin}/bin/hello\"\n",
+                "propagatedBuildInputs": "",
+                "propagatedNativeBuildInputs": "",
+                "src": "/nix/store/dw402azxjrgrzrk6j0p66wkqrab5mwgw-hello-2.12.2.tar.gz",
+                "stdenv": "/nix/store/i8bw5nqg1225m281zr6lgsz42bw04z7g-stdenv-linux",
+                "strictDeps": "",
+                "system": "aarch64-linux",
+                "version": "2.12.2"
+              },
+              "realStoreDir": "/nix/store",
+              "storeDir": "/nix/store",
+              "system": "aarch64-linux",
+              "tmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0/build",
+              "tmpDirInSandbox": "/build",
+              "topTmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0"
+            }
         )"
     };
 };

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

@@ -83,6 +83,7 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
         json.emplace("realStoreDir", getLocalStore(store).config->realStoreDir.get());
         json.emplace("system", drv.platform);
 
+        // FIXME: maybe write this JSON into the builder's stdin instead....?
         auto jsonFile = topTmpDir + "/build.json";
         writeFile(jsonFile, json.dump());
 
@@ -91,8 +92,15 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
             try {
                 commonChildInit();
 
-                Strings args = {externalBuilder.program, jsonFile};
+                Strings args = {externalBuilder.program};
 
+                if (externalBuilder.args) {
+                    args.insert(args.end(), externalBuilder.args->begin(), externalBuilder.args->end());
+                }
+
+                args.insert(args.end(), jsonFile);
+
+                debug("executing external builder: %s", concatStringsSep(" ", args));
                 execv(externalBuilder.program.c_str(), stringsToCharPtrs(args).data());
 
                 throw SysError("executing '%s'", externalBuilder.program);