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fix(build): stage BMIs through mcpp instead of an in-place shell copy (#311)
Windows-only symptom, three overlapping defects. 1. `rule cp_bmi` overwrote the destination IN PLACE (`powershell Copy-Item -Force` / `cp -f`). mcpp writes the staged std BMI path into compile_commands.json so clangd can resolve `import std;`, clangd memory-maps that very file, and Windows refuses to replace a file with an open user-mapped section — error 1224, reported as a bare "build failed". Staging now runs through `mcpp stage` (new internal subcommand, same shape as `mcpp dyndep`): skip when the destination is already equivalent, else temp-file + rename, else in-place, with retries, and a failure message that names the file and the likely holder. Never downgraded to a warning — a stale BMI turns into a confusing "module 'std' not found" or a silently mismatched link. The rule is shared with Windows runtime-DLL deployment, which had the same hazard against a program still running from a previous `mcpp run`. 2. `default_cache_root()` was a private copy of the home resolution, unchanged since v0.0.1: no %USERPROFILE% branch, no self-contained detection. On Windows PowerShell (no $HOME) the std BMI cache landed in the *current working directory* as `.mcpp-bmi/` while dep BMIs went to %USERPROFILE%\.mcpp\bmi — two roots for one cache, the second cwd-dependent, which is what made a re-stage a routine event. New leaf module `mcpp.home` is now the single resolver (config, stdmod, prepare's git cache and two more copies in doctor all route through it). 3. The staging rule carried no `restat`, so any re-stage recompiled everything that imports std even when the bytes were identical. Adding `restat = 1` on top of the no-write path fixes that — measured: touching the cache-side BMI now runs the stage edge alone, and the next build is "no work to do". Note for the record: aligning the destination's mtime with the source defeats restat and re-triggers the cascade, so a skipped stage touches no timestamps at all. Also: `FAILED: <target>` survives the ninja output filter (normalized to `failed: <target>`) — dropping it is why the report couldn't tell a staging failure from a compile error; `mcpp new` ignores `.mcpp/`; `mcpp doctor` points at a leftover `.mcpp-bmi/`. Design + plan: .agents/docs/2026-07-30-issue311-*.md Tests: unit test_home (4), test_build_stage (10), test_ninja_backend (+4), e2e 170 (no-cascade + cache root), e2e 171 (held destination, PowerShell MemoryMappedFile on Windows — reproduces #311 without needing clangd).
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‎.agents/docs/2026-07-30-issue311-bmi-staging-and-cache-root-design.md‎

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# BMI staging 原语 + BMI 缓存根收敛 — 实施计划
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配套设计：`2026-07-30-issue311-bmi-staging-and-cache-root-design.md`
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关联 issue：#311
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建议目标版本：**2026.7.30.1**（常规迭代）
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单 PR 交付。阶段有依赖顺序：**P1 → P2 → P3**（P3 的 ninja 文本依赖 P2 的子命令存在，
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P2 的判据依赖 P1 收敛后的缓存根不再随 cwd 漂移）。P4/P5/P6 可与 P3 并行收尾。
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---
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## P1 — 单一 home 解析器（`mcpp.home`）
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**新建** `src/home.cppm`：
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```cpp
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export module mcpp.home;
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import std;
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import mcpp.platform;
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export namespace mcpp::home {
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std::filesystem::path root(); // MCPP_HOME
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std::filesystem::path bmi_root(); // root()/bmi
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}
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```
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- `root()` = 把 `src/config.cppm:313-353` 的 `default_mcpp_home()` + `home_dir()`
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**逐字搬迁**（含 `USERPROFILE` 分支、self-contained 探测、`target/` 与 `data/xpkgs/`
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两条 disqualify）。两个函数都是纯函数、无副作用，搬迁是安全的。
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- `bmi_root()` = `root() / "bmi"`。
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**改动点（4 处调用方 + 3 处旧实现）**：
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| 文件 | 位置 | 改动 |
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|---|---|---|
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| `src/config.cppm` | :313-353 | 删除两个本地函数 |
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| `src/config.cppm` | :500 / :510 | `cfg.mcppHome = mcpp::home::root()`；`cfg.bmiCacheDir = mcpp::home::bmi_root()` |
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| `src/toolchain/stdmod.cppm` | :179-187 | 整体替换为 `return mcpp::home::bmi_root();`（新增 `import mcpp.home;`） |
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| `src/build/prepare.cppm` | :2826-2832 | 内联 lambda → `mcpp::home::root()` |
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**编译系统**：`mcpp.toml` 若显式列源文件需同步；当前是 `src/**` 推导（`mcpp build -v`
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里 `Inferred sources`），无需改。
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**无环性核对**（改前必须自查一次）：`mcpp.home` 只 import `std` + `mcpp.platform`。
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`mcpp.config` 的闭包内无 `mcpp.toolchain.*`，故 `mcpp.toolchain.stdmod → mcpp.home` 不成环。
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**测试**（`tests/unit/test_config.cpp` 扩写，或新建 `tests/unit/test_home.cpp`）：
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| 用例 | 断言 |
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|---|---|
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| `MCPP_HOME` 优先 | 设环境变量 → `root()` == 该值；`bmi_root()` == `<it>/bmi` |
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| Windows 无 HOME | `is_windows` 下 `USERPROFILE` 生效（用 `if constexpr` 分支或跳过非 Windows） |
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| 兜底形态 | 两个变量都不设时，路径以 `.mcpp` 结尾、**不再**以 `.mcpp-bmi` 结尾 |
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| 与 config 一致 | `load()` 后 `cfg.bmiCacheDir == mcpp::home::bmi_root()`（同一进程同一环境下必须相等） |
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最后一条是这次的核心不变量，**必须机器校验**——它就是 D2 的回归闸。
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---
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## P2 — `mcpp stage` 子命令
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**新建** `src/build/stage.cppm`（`export module mcpp.build.stage;`），承载纯逻辑：
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```cpp
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struct StageResult { bool copied; }; // copied=false ⇒ 判等跳过
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struct StageError { std::string message; }; // 已含 hint 文案
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std::expected<StageResult, StageError> stage_file(
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const std::filesystem::path& src,
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const std::filesystem::path& dst,
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bool verify_hash);
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```
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实现按设计 §S1 语义表：
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1. `src` 不存在 → error。
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2. `create_directories(dst.parent_path())`。
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3. `dst` 存在 && `file_size` 相等 && (verify == Size || 逐字节相等)
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→ **不写字节、不碰时间戳** → `StageOutcome{.copied = false}`。
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（对齐 mtime 会让 `restat` 失效并重新引发级联——实测过，见设计 §S1 的表）
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4. 否则：`copy_file(src, dst.tmp.<pid>)` → `rename(tmp, dst)`。
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5. rename 失败 → `copy_file(src, dst, overwrite_existing)`。
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6. 4/5 均失败 → 睡 100 / 300 / 900 ms 重试整个 4-5 序列，共 3 轮。
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7. 仍失败 → `StageError`，文案照设计 §S4 的模板（file / from / os error / hint 四段，
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hint 必须点名 clangd 与 `compile_commands.json` 的因果）。
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8. 每次退出前清理残留 `dst.tmp.<pid>`。
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`Verify::Content` 由 `MCPP_STAGE_VERIFY=content` 或 `--verify content` 打开；实现是分块
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逐字节比较（同 I/O 成本、无碰撞面、可提前退出），不引入 hash 依赖。
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**CLI 接线**（照 `dyndep` 的形状）：
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| 文件 | 改动 |
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|---|---|
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| `src/cli/cmd_build.cppm` | 新增 `export int cmd_stage(const ParsedArgs&)`，就近放在 `cmd_dyndep`（:187）旁 |
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| `src/cli.cppm` | :496 附近新增 `.subcommand(cl::App("stage") ...)`，描述以 `(internal: invoked by ninja)` 开头，选项 `--output/-o`、`--verify`；`.action(wrap_rc(cmd_stage))` |
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| `src/cli.cppm` | :547-551 的 `known` 白名单加 `"stage"`，**数组长度 22 → 23**（写死的模板实参，漏改即编译失败/静默拒命令） |
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**测试**（新建 `tests/unit/test_build_stage.cpp`）：
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| 用例 | 断言 |
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|---|---|
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| 目标不存在 | 复制发生，`copied == true`，内容一致 |
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| 目标已存在且等长等内容 | `copied == false`，且**目标 mtime 一点没变**（不是"未变成 now"，是完全不变） |
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| 等长但内容不同 + `Verify::Content` | `copied == true` |
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| 等长但内容不同 + `Verify::Size` | `copied == false`（这是**有意的** fp 判据取舍，注释写清） |
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| `src` 缺失 | error，message 含 src 路径 |
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| 目标目录不存在 | 自动创建 |
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| 只读目标（POSIX `chmod 444`） | 走 rename 分支成功；断言最终内容正确 |
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| 错误文案 | 人为构造失败（目标是个目录）→ message 含 `clangd` 与 `hint:` |
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---
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## P3 — ninja 后端切到新 rule
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**文件**：`src/build/ninja_backend.cppm`
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1. **`mcpp` 变量提取**：把 :404 的
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`append(std::format("mcpp = {}\n", escape_ninja_path(mcpp_exe_path())))`
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移出 `if (dyndep)`（:403），改为无条件绑定；`scan_deps` 仍留在 `if (dyndep)` 内。
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2. **rule 重写**（:411-419），跨平台单一形态、不再分叉 PowerShell/`cp`：
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```
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rule stage_file
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command = $mcpp stage --output $out $in
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description = STAGE $out
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restat = 1
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```
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3. **rule 更名**：`cp_bmi` → `stage_file`，四处 staging edge（:793/:795/:807/:810）与
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DLL 部署（:1080）同步改名。
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4. `command_prefixes()`（:278-291）追加 `mcpp_exe_path()`。
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5. `filter_ninja_output()`（:323-345）：`FAILED:` 不再整行丢弃，改为归一成
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`failed: <target>` 保留。
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**文件**：`src/build/execute.cppm`
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6. `read_ninja_command_prefixes()`（:181-205）白名单 key 加 `"mcpp"`。
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**测试**（`tests/unit/test_ninja_backend.cpp`）：
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| 用例 | 断言 |
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|---|---|
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| rule 文本 | 含 `rule stage_file`、`$mcpp stage --output $out $in`、`restat = 1`；**不含** `Copy-Item`、`cp -f` |
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| `mcpp` 绑定 | dyndep 开/关两种 plan 下都出现 `mcpp = ` 行 |
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| staging edge | 四条 edge 的 rule 名是 `stage_file`；`std.compat` 仍有 `| pcm.cache/std.pcm` order-only 前置 |
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| DLL 部署 | `runtimeDeployFiles` 非空时用同一 rule 名 |
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| 过滤器 | `filter_ninja_output` 对含 `<mcpp路径> stage ...` 的回显行过滤掉、对 `failed:` 与 `hint:` 正文保留 |
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---
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## P4 — 兜底路径的可见性
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| 文件 | 改动 |
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|---|---|
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| `src/scaffold/create.cppm` | :284-287 的 `.gitignore` 模板：`target/` + `.mcpp/` |
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| `src/doctor.cppm` | :155/:192 附近：若 cwd 或工程根存在 `.mcpp-bmi/`，输出一行 `legacy BMI cache at <path> — safe to delete`（**不自动删**） |
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`.gitignore` 模板变更需同步 e2e 中断言过 scaffold 产物的用例（`grep -rn "gitignore" tests/e2e`
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先扫一遍）。
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---
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## P5 — e2e
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**新建 `tests/e2e/170_bmi_staging_no_cascade.sh`**（全平台跑，锁 D3 + S1 步 2）：
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1. `mcpp new` 一个 bin 工程 → `mcpp build`（产出 staged BMI）；
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2. 从 `mcpp build -v` 的 STAGE 行或 `build.ninja` 解析出 staging edge 的 `$in`
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（**不要**用 `awk '{print $NF}'` 直接切 `rule` 行——本次调查里就踩过，取到的是 `cp_bmi`
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这个字面量；正确做法是匹配 `^build .*: (stage_file|cp_bmi) ` 的行再取最后一个字段）；
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3. `touch "$in"` 让 edge 变脏；
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4. `mcpp build -v` 断言：
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- 退出 0；
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- 输出**不含** `src/main.cpp` 的编译行（反级联，今天会失败）；
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- staged BMI 的内容与 `$in` 一致。
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**新建 `tests/e2e/171_bmi_staging_locked_dest.sh`**：
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- **Windows 分支**（`case "$(uname -s)" in *NT*|MINGW*|MSYS*)`）：用 PowerShell 在子进程里
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映射住 staged BMI，**不依赖 clangd**：
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```powershell
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$f = [System.IO.MemoryMappedFiles.MemoryMappedFile]::CreateFromFile(
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$path, [System.IO.FileMode]::Open)
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Start-Sleep -Seconds 30 # 持有期覆盖被测构建
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```
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然后 `touch` 缓存侧 BMI（保持内容不变）→ `mcpp build` 必须**成功**（走判等跳过）。
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这就是 #311 的最小复现，且不需要装 clangd。
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- **POSIX 分支**：`chmod 444` staged BMI + 让缓存侧内容真的不同（改用另一个 fingerprint 的
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BMI 或人为构造一份等长-不同内容的假文件）→ 断言走 rename 分支成功。
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- 负例（两个平台）：把 staged BMI 换成一个**目录**同名占位 → 断言构建失败且 stderr 含
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`hint:` 与 `clangd`。
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`tests/e2e/run_all.sh` 若是显式清单则登记两个新脚本；编号接 169(上游 169 已被 semver 用例占用)。
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---
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## P6 — 收尾
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1. `CHANGELOG.md`：
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- fix(#311)：Windows 上被 clangd 映射的 std BMI 不再让构建失败；
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- **behavior change**：BMI 缓存根统一为 `$MCPP_HOME/bmi`（Windows 从 `<cwd>\.mcpp-bmi`、
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self-contained 安装从 `~/.mcpp/bmi` 迁走）；首次构建会重编一次 std（10–60 s），
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遗留目录可手动删除。
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2. 版本号：`mcpp.toml` → `2026.7.30.1`（注意 `git status` 里 `mcpp.toml` 已有本地改动，
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提交前先核对那处改动是否该一起进）。
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3. 发布闭环按既有 runbook 走（release → 镜像 xlings-res 双端 → xim-pkgindex → 真装验证 →
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bootstrap pin）。**bootstrap pin 与本次发布版本是两组，不要一起 bump**。
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4. 不要在 issue #311 下评论（按本次任务要求）；发布后再回复。
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---
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## 自查清单（提交前逐条打勾）
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- [ ] `grep -rn "\.mcpp-bmi" src/` 只剩 doctor 的遗留提示与注释，无路径构造
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- [ ] `grep -rn "getenv(\"HOME\")" src/` 不再出现在 BMI/home 解析路径上
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- [ ] `grep -rn "Copy-Item" src/` 归零
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- [ ] `grep -rn "cp_bmi" src/` 归零
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- [ ] `cli.cppm` 的 `known` 数组长度与元素数一致（22 → 23）
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- [ ] `if (dyndep)` 之外能拿到 `$mcpp`（用 GCC 非 dyndep plan 生成一次 build.ninja 目视核对）
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- [ ] 单测全绿 + `tests/e2e/170`、`171` 全绿（Linux 本机 + Windows CI）
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- [ ] Windows CI 上确认 STAGE 行不再 spawn PowerShell（顺带的启动开销收益）

‎src/build/execute.cppm‎

Lines changed: 5 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -190,7 +190,11 @@ std::vector<std::string> read_ninja_command_prefixes(const std::filesystem::path
190190
auto key = line.substr(0, eq);
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while (!key.empty() && std::isspace(static_cast<unsigned char>(key.back())))
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key.pop_back();
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if (key != "cxx" && key != "cc" && key != "ar" && key != "scan_deps")
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// `mcpp` drives the dyndep + stage_file rules; treating it as a command
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// prefix filters the echoed command line while keeping the diagnostic
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// mcpp itself printed (#311).
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if (key != "cxx" && key != "cc" && key != "ar" && key != "scan_deps"
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&& key != "mcpp")
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continue;
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std::string value = line.substr(eq + 1);

‎src/build/ninja_backend.cppm‎

Lines changed: 45 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -67,7 +67,7 @@ std::string escape_ninja_path(const std::filesystem::path& p) {
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// a response file that gcc/clang/GNU ar tokenize GNU-style, where
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// backslash is an ESCAPE character — `obj\cli.o` would arrive as
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// `objcli.o`. Every Windows consumer of these strings (CreateProcess
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// path resolution, cl.exe/link.exe, PowerShell Copy-Item, ninja itself)
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// path resolution, cl.exe/link.exe, `mcpp stage`, ninja itself)
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// accepts forward slashes; POSIX output is byte-identical.
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std::string s = p.generic_string();
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std::string out;
@@ -288,6 +288,10 @@ std::vector<std::string> command_prefixes(const CompileFlags& flags,
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add(flags.ccBinary);
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add(flags.arBinary);
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add(plan.scanDepsPath);
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// mcpp itself drives the dyndep and stage_file rules; its echoed command
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// line is noise, while the message it prints on failure is the diagnostic
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// we want to keep.
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add(mcpp_exe_path());
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return prefixes;
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}
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@@ -331,11 +335,26 @@ std::string filter_ninja_output(std::string_view output,
331335
auto trimmed = ltrim_copy(line);
332336
if (trimmed.starts_with("ninja: Entering directory")
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|| trimmed.starts_with("ninja: build stopped")
334-
|| trimmed.starts_with("FAILED:")
335338
|| is_ninja_progress_line(trimmed)
336339
|| is_command_line(trimmed, commandPrefixes)) {
337340
continue;
338341
}
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// Keep WHICH output failed. Dropping this line entirely (as we used to)
343+
// is why #311's report couldn't tell a BMI staging failure from a
344+
// compile error. Normalized to lowercase `failed:` so it reads as part
345+
// of mcpp's own diagnostics, and `[code=N]` is dropped as noise.
346+
if (trimmed.starts_with("FAILED:")) {
347+
auto target = ltrim_copy(trimmed.substr(std::string_view("FAILED:").size()));
348+
if (target.starts_with("[code=")) {
349+
if (auto close = target.find(']'); close != std::string::npos)
350+
target = ltrim_copy(target.substr(close + 1));
351+
}
352+
if (target.empty()) continue;
353+
filtered += "failed: ";
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filtered += target;
355+
filtered.push_back('\n');
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continue;
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}
339358
filtered += line;
340359
filtered.push_back('\n');
341360
}
@@ -400,23 +419,28 @@ std::string emit_ninja_string(const BuildPlan& plan) {
400419
flags.ldBinary.empty() ? std::string("link.exe")
401420
: escape_ninja_path(flags.ldBinary)));
402421
}
422+
// `$mcpp` is needed by stage_file in EVERY configuration (dyndep or not),
423+
// so the binding cannot live inside the `if (dyndep)` below.
424+
append(std::format("mcpp = {}\n", escape_ninja_path(mcpp_exe_path())));
403425
if (dyndep) {
404-
append(std::format("mcpp = {}\n", escape_ninja_path(mcpp_exe_path())));
405426
if (!plan.scanDepsPath.empty()) {
406427
append(std::format("scan_deps = {}\n", escape_ninja_path(plan.scanDepsPath)));
407428
}
408429
}
409430
append("\n");
410431

411-
append("rule cp_bmi\n");
412-
if constexpr (mcpp::platform::is_windows) {
413-
// Use PowerShell Copy-Item which handles both forward and back slashes.
414-
// cmd.exe `copy` breaks on forward-slash paths from ninja.
415-
append(" command = powershell -NoProfile -Command \"Copy-Item -Force '$in' -Destination '$out'\"\n");
416-
} else {
417-
append(" command = mkdir -p $$(dirname $out) && cp -f $in $out\n");
418-
}
419-
append(" description = STAGE $out\n\n");
432+
// Staging (cache → build dir) runs through mcpp itself instead of a
433+
// per-platform shell copy: it skips the write when the destination is
434+
// already equivalent, writes out-of-place + renames when it isn't, retries
435+
// transient sharing violations, and fails with a diagnostic that names the
436+
// likely holder. #311: `Copy-Item -Force` overwrote in place, so a std BMI
437+
// that clangd had memory-mapped failed the whole build with error 1224.
438+
// `restat = 1` is what makes the no-write path actually pay off — a skipped
439+
// stage must not dirty every importer of the staged BMI.
440+
append("rule stage_file\n");
441+
append(" command = $mcpp stage --output $out $in\n");
442+
append(" description = STAGE $out\n");
443+
append(" restat = 1\n\n");
420444

421445
// P1: per-file dyndep rule. Converts one .ddi → .dd independently.
422446
append(std::format(
@@ -790,9 +814,9 @@ std::string emit_ninja_string(const BuildPlan& plan) {
790814

791815
bool has_std_artifacts = !plan.stdBmiPath.empty() && !plan.stdObjectPath.empty();
792816
if (has_std_artifacts) {
793-
append(std::format("build {} : cp_bmi {}\n", escape_ninja_path(std_bmi_dst),
817+
append(std::format("build {} : stage_file {}\n", escape_ninja_path(std_bmi_dst),
794818
escape_ninja_path(plan.stdBmiPath)));
795-
append(std::format("build {} : cp_bmi {}\n\n", escape_ninja_path(std_o_dst),
819+
append(std::format("build {} : stage_file {}\n\n", escape_ninja_path(std_o_dst),
796820
escape_ninja_path(plan.stdObjectPath)));
797821
}
798822

@@ -804,10 +828,10 @@ std::string emit_ninja_string(const BuildPlan& plan) {
804828
if (has_std_compat) {
805829
// std.compat.pcm depends on std.pcm — ensure std.pcm is staged first
806830
// so clang can resolve the transitive dependency when loading std.compat.pcm.
807-
append(std::format("build {} : cp_bmi {} | {}\n", escape_ninja_path(compat_bmi_dst),
831+
append(std::format("build {} : stage_file {} | {}\n", escape_ninja_path(compat_bmi_dst),
808832
escape_ninja_path(plan.stdCompatBmiPath),
809833
escape_ninja_path(std_bmi_dst)));
810-
append(std::format("build {} : cp_bmi {}\n\n", escape_ninja_path(compat_o_dst),
834+
append(std::format("build {} : stage_file {}\n\n", escape_ninja_path(compat_o_dst),
811835
escape_ninja_path(plan.stdCompatObjectPath)));
812836
}
813837

@@ -1074,10 +1098,13 @@ std::string emit_ninja_string(const BuildPlan& plan) {
10741098
append("\n");
10751099

10761100
// Windows runtime-DLL deployment: one copy edge per staged dep DLL. Emitted
1077-
// once (deduped by dest in BuildPlan), reusing the generic cp_bmi copy rule.
1101+
// once (deduped by dest in BuildPlan), reusing the generic stage_file rule
1102+
// — which also means a DLL still loaded by a running program from a
1103+
// previous `mcpp run` gets the skip-if-equivalent treatment instead of a
1104+
// hard "cannot copy" failure.
10781105
// Inert on RPATH platforms where runtimeDeployFiles is empty.
10791106
for (auto const& d : plan.runtimeDeployFiles) {
1080-
append(std::format("build {} : cp_bmi {}\n",
1107+
append(std::format("build {} : stage_file {}\n",
10811108
escape_ninja_path(d.dest),
10821109
escape_ninja_path(d.source)));
10831110
}

‎src/build/prepare.cppm‎

Lines changed: 2 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -11,6 +11,7 @@ export module mcpp.build.prepare;
1111

1212
import std;
1313
import mcpp.diag;
14+
import mcpp.home;
1415
import mcpp.platform.axis;
1516
import mcpp.libs.json;
1617
import mcpp.log;
@@ -2823,13 +2824,7 @@ prepare_build(bool print_fingerprint,
28232824
// them to a commit before forming the cache key; this lets
28242825
// `mcpp update <dep>` pick up a moved branch without deleting
28252826
// unrelated git caches.
2826-
auto mcppHome = [] {
2827-
if (auto* e = std::getenv("MCPP_HOME"); e && *e)
2828-
return std::filesystem::path(e);
2829-
if (auto* e = std::getenv("HOME"); e && *e)
2830-
return std::filesystem::path(e) / ".mcpp";
2831-
return std::filesystem::current_path() / ".mcpp";
2832-
}();
2827+
auto mcppHome = mcpp::home::root(); // single resolver (#311)
28332828
std::string resolvedGitRev = spec.gitRev;
28342829
if (spec.gitRefKind == "branch") {
28352830
auto ref = std::format("refs/heads/{}", spec.gitRev);

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