[M5+] add resolver::nixpkgs_git_resolve fallback

tests/nixpkgs_git_resolve.cpp

@@ -0,0 +1,115 @@
+// Unit + fixture-based test for resolver::nixpkgs_git_resolve. Avoids
+// the multi-GB nixos/nixpkgs clone by building a tiny throwaway repo
+// that mirrors the relevant slice of the nixpkgs layout.
+
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.resolver;
+import cargoxx.exec;
+import cargoxx.util;
+import std;
+
+using cargoxx::resolver::nixpkgs_git_resolve;
+using cargoxx::resolver::pick_youngest_commit;
+
+namespace {
+
+auto run_git(const std::filesystem::path& cwd, std::vector<std::string> args)
+    -> int {
+    auto r = cargoxx::exec::run(
+        "git", args,
+        cargoxx::exec::ExecOptions{
+            .cwd = cwd,
+            .env_overrides = {{"GIT_COMMITTER_NAME", "t"},
+                              {"GIT_COMMITTER_EMAIL", "t@t"},
+                              {"GIT_AUTHOR_NAME", "t"},
+                              {"GIT_AUTHOR_EMAIL", "t@t"},
+                              {"GIT_COMMITTER_DATE", "1700000000 +0000"},
+                              {"GIT_AUTHOR_DATE", "1700000000 +0000"}},
+            .timeout = std::chrono::seconds{30},
+            .inherit_stdio = false,
+        });
+    REQUIRE(r.has_value());
+    return r->exit_code;
+}
+
+auto write_text(const std::filesystem::path& p, std::string_view content) {
+    std::filesystem::create_directories(p.parent_path());
+    std::ofstream{p} << content;
+}
+
+auto fixture_repo(std::string_view pkg, std::string_view v1,
+                  std::string_view v2) -> std::filesystem::path {
+    auto root = std::filesystem::temp_directory_path() /
+                std::format("cargoxx-git-fixture-{}", std::random_device{}());
+    std::filesystem::create_directories(root);
+    REQUIRE(run_git(root, {"init", "-q", "-b", "main"}) == 0);
+
+    auto nix_path =
+        root / "pkgs" / "development" / "libraries" / std::string{pkg} / "default.nix";
+    write_text(nix_path, std::format("{{ stdenv }}: stdenv.mkDerivation {{ "
+                                     "version = \"{}\"; }}\n",
+                                     v1));
+    REQUIRE(run_git(root, {"add", "."}) == 0);
+    REQUIRE(run_git(root, {"commit", "-q", "-m", std::format("init {}", v1)}) == 0);
+
+    write_text(nix_path, std::format("{{ stdenv }}: stdenv.mkDerivation {{ "
+                                     "version = \"{}\"; }}\n",
+                                     v2));
+    REQUIRE(run_git(root, {"add", "."}) == 0);
+    REQUIRE(run_git(root, {"commit", "-q", "-m", std::format("bump {}", v2)}) == 0);
+
+    return root;
+}
+
+} // namespace
+
+TEST_CASE("pick_youngest_commit picks the highest-timestamp SHA",
+          "[resolver][git]") {
+    constexpr std::string_view input =
+        "abc111 1700000000\n"
+        "def222 1800000000\n"
+        "ghi333 1750000000\n";
+    auto r = pick_youngest_commit(input);
+    REQUIRE(r == std::string{"def222"});
+}
+
+TEST_CASE("pick_youngest_commit returns nullopt on empty input",
+          "[resolver][git]") {
+    REQUIRE_FALSE(pick_youngest_commit("").has_value());
+    REQUIRE_FALSE(pick_youngest_commit("\n\n").has_value());
+}
+
+TEST_CASE("pick_youngest_commit skips malformed lines", "[resolver][git]") {
+    constexpr std::string_view input =
+        "garbage\n"
+        "abc111 1700000000\n"
+        "no-timestamp\n";
+    auto r = pick_youngest_commit(input);
+    REQUIRE(r == std::string{"abc111"});
+}
+
+TEST_CASE("nixpkgs_git_resolve finds the introducing commit",
+          "[resolver][git]") {
+    auto repo = fixture_repo("fmt", "10.2.0", "10.3.0");
+
+    auto r = nixpkgs_git_resolve("fmt", "10.2.0", repo);
+    REQUIRE(r.has_value());
+    REQUIRE(r->size() == 40); // git sha
+
+    std::error_code ec;
+    std::filesystem::remove_all(repo, ec);
+}
+
+TEST_CASE("nixpkgs_git_resolve returns ResolutionVersionNotFound for an absent version",
+          "[resolver][git]") {
+    auto repo = fixture_repo("fmt", "10.2.0", "10.3.0");
+
+    auto r = nixpkgs_git_resolve("fmt", "99.99.99", repo);
+    REQUIRE_FALSE(r.has_value());
+    REQUIRE(r.error().code ==
+            cargoxx::util::ErrorCode::ResolutionVersionNotFound);
+
+    std::error_code ec;
+    std::filesystem::remove_all(repo, ec);
+}