fix: a Luban system's root is declared and its /xlings upstream is its store (2026.10.22.1) (#831)

In a Luban system (xlings design 2026-10-21 §3.7, J5, M1): the compiler's sysroot / is the system's own declared root (os-release ID=luban and /usr/lib/xlings-abi), and [xlings] upstream = ["/xlings"] reads the home the system's root record (/etc/xlings/usr) names when /xlings is not there -- a system made on a host keeps its home at the path of the home that made it. Both facts are the provider's layout (declared_system_root, system_home): mcpp names none of xlings' paths. Verified in a Luban KVM machine: mcpp self doctor without the sysroot or upstream warning; a first mcpp run of a new project in 4 s.

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: speak-agent

CHANGELOG.md

@@ -4,6 +4,19 @@
 > Each `## [<version>]` section is that release's notes. Entries are written in English
 > from 2026.9.28.3 on; earlier entries remain as written.
 
+## [2026.10.22.1] - 2026-10-22
+
+### Fixed
+
+- In a Luban system, the compiler's sysroot `/` is the system's own declared
+  root (`ID=luban` in `/etc/os-release` and `/usr/lib/xlings-abi`): `mcpp self
+  doctor` no longer reports it as belonging to neither the mcpp home nor the
+  project.
+- In a Luban system, `[xlings] upstream = ["/xlings"]` reads the store the
+  system's root record names (`/etc/xlings/usr`) when `/xlings` is not there --
+  a system made on a host keeps its home at the path of the home that made it.
+  Its gcc and glibc are reused rather than reported unreachable.
+
 ## [2026.10.21.1] - 2026-10-21
 
 ### Fixed

mcpp.toml

@@ -1,6 +1,6 @@
 [package]
 name        = "mcpp"
-version = "2026.10.21.1"
+version = "2026.10.22.1"
 description = "Modern C++ build & package management tool"
 license     = "Apache-2.0"
 authors     = ["mcpp-community"]

modules/versioning/src/version.cppm

@@ -31,6 +31,6 @@ import std;
 
 export namespace mcpp {
 
-inline constexpr std::string_view MCPP_VERSION = "2026.10.21.1";
+inline constexpr std::string_view MCPP_VERSION = "2026.10.22.1";
 
 } // namespace mcpp

src/config.cppm

@@ -838,6 +838,10 @@ std::expected<GlobalConfig, ConfigError> load_or_init(
                     dir = std::filesystem::path(h) / (entry.size() > 2 ? entry.substr(2) : std::string{});
             dir = dir.lexically_normal();
             std::error_code ec;
+            // A Luban system's /xlings is its store, wherever its xlings put it
+            // (the home that made it, for a system made on a host).
+            if (dir == "/xlings" && !std::filesystem::is_directory(dir, ec))
+                if (auto store = mcpp::provider::layout().system_home("/"); !store.empty()) dir = store;
             if (!dir.is_absolute() || !std::filesystem::is_directory(dir, ec)) {
                 // Fewer packages reused than asked for: said, not hidden.
                 mcpp::diag::degraded("xlings/upstream", std::format("[xlings] upstream {} is not a directory", entry),

src/fallback/probe_sysroot.cppm

@@ -99,10 +99,15 @@ bool sysroot_is_owned(std::string_view reportedPath,
 //
 // Both anchors are required. Registry alone would flag every legitimate
 // project-local tree; project alone would flag every payload sysroot.
+// In a Luban system (`systemRoot`, "/" but for a test) the sysroot `/` is the
+// system's own declared root (xlings design 2026-10-21 §3.7).
 bool sysroot_is_foreign(const std::filesystem::path& sysroot,
                         const std::filesystem::path& registryRoot,
-                        const std::filesystem::path& projectRoot) {
+                        const std::filesystem::path& projectRoot,
+                        const std::filesystem::path& systemRoot = "/") {
     if (sysroot.empty()) return false;
+    // A Luban system's own root: its compiler's `/` is the system it is.
+    if (sysroot.lexically_normal() == "/" && mcpp::provider::layout().declared_system_root(systemRoot)) return false;
     return !path_is_under(sysroot, registryRoot)
         && !path_is_under(sysroot, projectRoot);
 }

src/provider/provider.cppm

@@ -377,6 +377,13 @@ public:
     // Whether `name` is one of the provider's own indexes (its default and
     // the ones it adds itself), which stay where the provider keeps them.
     virtual bool is_provider_index(std::string_view name) const = 0;
+    // A system the provider made, as `root` (/, but for a test) says of
+    // itself (xlings design 2026-10-21 §3.7): its compiler's sysroot `/` is
+    // then the system's own declared root, not a tree nothing declared.
+    virtual bool declared_system_root(const std::filesystem::path& root) const = 0;
+    // The home such a system reads its packages from, by its root record;
+    // empty: not one, or its record says nothing usable.
+    virtual std::filesystem::path system_home(const std::filesystem::path& root) const = 0;
 };
 
 // What a provider says of a package's payload without installing anything

src/provider/xlings/provider.cppm

@@ -196,6 +196,36 @@ public:
             return {{dir, {}, {"xim"}}};
         return {};
     }
+    // A Luban system (xlings design 2026-10-21 §3.7): its os-release says
+    // ID=luban and /usr/lib/xlings-abi is the runtime xlings bound; its root
+    // record (/etc/xlings/usr) names its generation, <home>/subos/<name>/root/usr
+    // -- /xlings for one installed on a machine, the path of the home that
+    // made it for one made on a host.
+    bool declared_system_root(const std::filesystem::path& root) const override {
+        std::error_code ec;
+        if (!std::filesystem::exists(root / "usr/lib/xlings-abi", ec)) return false;
+        std::ifstream in(root / "etc/os-release");
+        for (std::string line; std::getline(in, line);) {
+            if (!line.starts_with("ID=")) continue;
+            auto id = line.substr(3);
+            if (id.size() >= 2 && (id.front() == '"' || id.front() == '\'')) id = id.substr(1, id.size() - 2);
+            return id == "luban";
+        }
+        return false;
+    }
+    std::filesystem::path system_home(const std::filesystem::path& root) const override {
+        if (!declared_system_root(root)) return {};
+        std::ifstream in(root / "etc/xlings/usr");
+        std::string line;
+        if (!std::getline(in, line)) return {};
+        while (!line.empty() && (line.back() == '\r' || line.back() == ' ')) line.pop_back();
+        const std::filesystem::path usr = std::filesystem::path(line).lexically_normal();
+        const auto rootDir = usr.parent_path(), instance = rootDir.parent_path(), subos = instance.parent_path();
+        if (!usr.is_absolute() || usr.filename() != "usr" || rootDir.filename() != "root" || subos.filename() != "subos") return {};
+        std::error_code ec;
+        const auto home = subos.parent_path();
+        return std::filesystem::is_directory(home, ec) ? home : std::filesystem::path{};
+    }
     bool is_provider_index(std::string_view name) const override {
         // xlings' default index and the ones it adds beside it.
         return name == "xim" || name == "awesome" || name == "scode" || name == "d2x";

tests/unit/test_luban_root.cpp

@@ -0,0 +1,66 @@
+// A Luban system says what it is (xlings design 2026-10-21 §3.7, J5): its
+// root is declared (os-release ID=luban and /usr/lib/xlings-abi), so its
+// compiler's sysroot `/` is not foreign there; and its store is the home its
+// root record names, which `[xlings] upstream = ["/xlings"]` means for a
+// system made on a host (whose home is not at /xlings).
+
+#include <gtest/gtest.h>
+
+import std;
+import mcpp.provider;
+import mcpp.provider.select;
+import mcpp.fallback.probe_sysroot;
+
+namespace fs = std::filesystem;
+
+namespace {
+
+struct Tree {
+    fs::path dir;
+    Tree() {
+        dir = fs::temp_directory_path() / std::format("mcpp-luban-root-{}-{}", ::testing::UnitTest::GetInstance()->random_seed(),
+                                                      std::chrono::steady_clock::now().time_since_epoch().count());
+        fs::create_directories(dir);
+    }
+    ~Tree() { std::error_code ec; fs::remove_all(dir, ec); }
+    void write(const fs::path& rel, std::string_view text) const {
+        fs::create_directories((dir / rel).parent_path());
+        std::ofstream(dir / rel) << text;
+    }
+};
+
+}  // namespace
+
+TEST(LubanRoot, ALubanSystemsRootIsDeclared) {
+    Tree t;
+    EXPECT_FALSE(mcpp::provider::layout().declared_system_root(t.dir));
+    t.write("etc/os-release", "NAME=\"Luban\"\nID=luban\nVERSION_ID=2026.10.22.1\n");
+    EXPECT_FALSE(mcpp::provider::layout().declared_system_root(t.dir)) << "its os-release alone is not enough";
+    t.write("usr/lib/xlings-abi/README", "");
+    EXPECT_TRUE(mcpp::provider::layout().declared_system_root(t.dir));
+    t.write("etc/os-release", "ID=\"debian\"\n");
+    EXPECT_FALSE(mcpp::provider::layout().declared_system_root(t.dir));
+}
+
+TEST(LubanRoot, ItsSysrootSlashIsNotForeign) {
+    Tree t;
+    const fs::path registry = "/home/u/.mcpp/registry", project = "/home/u/work/demo";
+    EXPECT_TRUE(mcpp::fallback::sysroot_is_foreign("/", registry, project, t.dir)) << "anywhere else, / is nothing declared";
+    t.write("etc/os-release", "ID=luban\n");
+    t.write("usr/lib/xlings-abi/README", "");
+    EXPECT_FALSE(mcpp::fallback::sysroot_is_foreign("/", registry, project, t.dir));
+    EXPECT_TRUE(mcpp::fallback::sysroot_is_foreign("/opt/other/sysroot", registry, project, t.dir)) << "only its own root";
+}
+
+TEST(LubanRoot, ItsStoreIsTheHomeItsRootRecordNames) {
+    Tree t;
+    t.write("etc/os-release", "ID=luban\n");
+    t.write("usr/lib/xlings-abi/README", "");
+    EXPECT_TRUE(mcpp::provider::layout().system_home(t.dir).empty()) << "no record";
+    const auto home = t.dir / "host-home";
+    fs::create_directories(home / "subos" / "box" / "root" / "usr");
+    t.write("etc/xlings/usr", (home / "subos" / "box" / "root" / "usr").string() + "\n");
+    EXPECT_EQ(mcpp::provider::layout().system_home(t.dir), home.lexically_normal());
+    t.write("etc/xlings/usr", "/somewhere/else/usr\n");
+    EXPECT_TRUE(mcpp::provider::layout().system_home(t.dir).empty()) << "a record of another shape";
+}