add derivation parser benchmark

the current identified bottlenecks are parseString in derivations.cc and dirOf (because of std::filessystem creation).

src/libstore-tests/data/derivation/hello.drv

@@ -0,0 +1 @@
+Derive([("out","/nix/store/hhg83gh653wjw4ny49xn92f13v2j1za4-hello-2.12.2","","")],[("/nix/store/1xz4avqqrxqsxw7idz119vdzw837p1n1-version-check-hook.drv",["out"]),("/nix/store/bsv47sbqcar3205il55spxqacxp8j0fj-hello-2.12.2.tar.gz.drv",["out"]),("/nix/store/s4b8yadif84kiv8gyr9nxdi6zbg69b4g-bash-5.2p37.drv",["out"]),("/nix/store/sc2pgkzc1s6zp5dp8j7wsd4msilsnijn-stdenv-linux.drv",["out"])],["/nix/store/shkw4qm9qcw5sc5n1k5jznc83ny02r39-default-builder.sh","/nix/store/vj1c3wf9c11a0qs6p3ymfvrnsdgsdcbq-source-stdenv.sh"],"x86_64-linux","/nix/store/p79bgyzmmmddi554ckwzbqlavbkw07zh-bash-5.2p37/bin/bash",["-e","/nix/store/vj1c3wf9c11a0qs6p3ymfvrnsdgsdcbq-source-stdenv.sh","/nix/store/shkw4qm9qcw5sc5n1k5jznc83ny02r39-default-builder.sh"],[("NIX_MAIN_PROGRAM","hello"),("__structuredAttrs",""),("buildInputs",""),("builder","/nix/store/p79bgyzmmmddi554ckwzbqlavbkw07zh-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/fxzn6kr5anxn5jgh511x56wrg8b3a99a-version-check-hook"),("out","/nix/store/hhg83gh653wjw4ny49xn92f13v2j1za4-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/a13rl87yjhzqrbkc4gb0mrwz2mfkivcf-stdenv-linux"),("strictDeps",""),("system","x86_64-linux"),("version","2.12.2")])

src/libstore-tests/derivation-parser-bench.cc

@@ -0,0 +1,45 @@
+#include <benchmark/benchmark.h>
+#include "nix/store/derivations.hh"
+#include "nix/store/store-api.hh"
+#include "nix/util/experimental-features.hh"
+#include "nix/store/store-open.hh"
+#include "nix/store/globals.hh"
+#include <fstream>
+#include <sstream>
+
+using namespace nix;
+
+// Benchmark parsing real derivation files
+static void BM_ParseRealDerivationFile(benchmark::State & state, const std::string & filename)
+{
+    // Read the file once
+    std::ifstream file(filename);
+    std::stringstream buffer;
+    buffer << file.rdbuf();
+    std::string content = buffer.str();
+
+    auto store = openStore("dummy://");
+    ExperimentalFeatureSettings xpSettings;
+
+    for (auto _ : state) {
+        auto drv = parseDerivation(*store, std::string(content), "test", xpSettings);
+        benchmark::DoNotOptimize(drv);
+    }
+    state.SetBytesProcessed(state.iterations() * content.size());
+}
+
+// Register benchmarks for actual test derivation files if they exist
+BENCHMARK_CAPTURE(BM_ParseRealDerivationFile, hello, std::string(NIX_UNIT_TEST_DATA) + "/derivation/hello.drv");
+BENCHMARK_CAPTURE(BM_ParseRealDerivationFile, firefox, std::string(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");
+
+// Custom main to initialize Nix before running benchmarks
+int main(int argc, char ** argv)
+{
+    // Initialize libstore
+    nix::initLibStore(false);
+
+    // Initialize and run benchmarks
+    ::benchmark::Initialize(&argc, argv);
+    ::benchmark::RunSpecifiedBenchmarks();
+    return 0;
+}

src/libstore-tests/meson.build

@@ -105,3 +105,19 @@ test(
   },
   protocol : 'gtest',
 )
+
+# Build benchmarks if enabled
+if get_option('benchmarks')
+  gbenchmark = dependency('benchmark', required : true)
+
+  benchmark_exe = executable(
+    'nix-store-benchmarks',
+    'derivation-parser-bench.cc',
+    config_priv_h,
+    dependencies : deps_private_subproject + deps_private + deps_other + [gbenchmark],
+    include_directories : include_dirs,
+    link_args: linker_export_flags,
+    install : false,
+    cpp_args : ['-DNIX_UNIT_TEST_DATA="' + meson.current_source_dir() + '/data"'],
+  )
+endif

src/libstore-tests/meson.options

@@ -0,0 +1,9 @@
+# vim: filetype=meson
+
+option(
+  'benchmarks',
+  type : 'boolean',
+  value : false,
+  description : 'Build benchmarks (requires gbenchmark)',
+  yield : true,
+)

src/libstore-tests/package.nix

@@ -35,7 +35,7 @@ mkMesonExecutable (finalAttrs: {
     ../../.version
     ./.version
     ./meson.build
-    # ./meson.options
+    ./meson.options
     (fileset.fileFilter (file: file.hasExt "cc") ./.)
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
   ];