fix: load cached action metadata before buffered ninja output

Author: Sunrisepeak

.agents/docs/2026-11-05-heterogeneous-build-implementation-plan.md

@@ -467,3 +467,24 @@ latest，实际执行输出 `mcpp 2026.11.5.1`；安装后的 ELF 与公开安
 索引分别经 PR #713、#714 合入、发布并通过普通客户端验证。musl 0.27.7 复用
 版本组推进的 PR #85；runtime 0.18.8 与 platform 0.23.8 继续按公开前置依赖
 依次验证。源码对照诊断不能替代引擎正式生产者与联合 UX 的资格验证。
+
+## 19. 缓存构建的内部进度首次读取（2026-11-05）
+
+收尾 CI 的 `1013_optional_action_progress_preserves_outputs` 暴露了一个核心显示
+缺口。缓存构建不重新规划，其步骤归属记录原先只在 Ninja 首次输出状态时读取；
+异步轮询在记录尚未加载时直接跳过 action-start 文件。Ninja 缓冲长动作输出时，
+合法进度文件虽已存在，界面仍不能显示正在执行的动作和内部阶段。该问题不说明
+动作本身停止，也不能解释引擎冷启动 CPU 的性能差额。
+
+修正使轮询先检查 action-start 是否新增内容，再按需读取已有归属记录。无工作
+的缓存构建仍不预读记录；快照计数仍不能完成动作，输出后置条件、命令退出码和
+取消处理保持原契约。没有通过延长工作、隐藏断言或调低门槛修复正式测试。
+
+固定原构建的 448 个对象，仅重编译进度模块并链接诊断二进制。新增测试在没有
+任何 Ninja stdout 或完成记录时要求观察到动作及内部阶段：原模块在两秒期限内
+失败，修正模块通过；完整进度测试文件的 25 项全部通过。原 1013 E2E 与额外的
+短动作观测控制均通过，包含各 role 的输出、增量不重跑、失败和取消。初次八路
+全量本地编译曾被资源回收终止，不记为通过；单模块控制也不替代正式 CI。
+
+源代码和回归测试继续复用 PR #889。其最新完整 CI 和随后独立生态审查仍是合入
+前置条件，已发布 tag 的源码与资产不作改写。

src/build/progress.cppm

@@ -733,7 +733,8 @@ struct Build::Impl {
 namespace {
 
 // The fast path has no plan: it reads the record the plan wrote, when ninja
-// reports its first step, so a build with nothing to do reads nothing.
+// reports its first step or an action writes its start, so a build with
+// nothing to do reads nothing. Ninja may buffer stdout until an action ends.
 void ensure_record(Build::Impl& b) {
     if (b.record || b.recordTried) return;
     b.recordTried = true;
@@ -1004,11 +1005,13 @@ void read_log(Report& r, Build::Impl& b, std::vector<std::string>& out) {
 // Reads the start file of an open pass; model lock held. A check or prepare
 // action that starts names its package: that is work.
 void read_starts(Report& r, Build::Impl& b, std::vector<std::string>& out) {
-    if (!b.inPass || !b.record) return;
+    if (!b.inPass) return;
     std::error_code ec;
     const auto path = b.dir / kStartsFile;
     const auto size = std::filesystem::file_size(path, ec);
     if (ec || size <= b.startsOffset) return;
+    ensure_record(b);
+    if (!b.record) return;
     std::ifstream f(path, std::ios::binary);
     f.seekg(static_cast<std::streamoff>(b.startsOffset));
     std::string text(static_cast<std::size_t>(size - b.startsOffset), '\0');

tests/unit/test_build_progress.cpp

@@ -3,6 +3,7 @@
 import std;
 import mcpp.ui;
 import mcpp.build.progress;
+import mcpp.build.action_progress;
 
 // The build's report (.agents/docs/2026-09-29-build-progress-display-design.md,
 // revised by .agents/docs/2026-09-30-build-output-refinement-design.md): its
@@ -331,6 +332,43 @@ TEST(ProgressText, ThePhaseIsAlignedWithTheVerbs) {
 
 // ─── When a package is named (revision 3, §5.3) ──────────────────────────
 
+TEST(ProgressModel, AnActionStartLoadsTheFastPathRecordBeforeNinjaWritesStdout) {
+    namespace telemetry = mcpp::build::action_progress;
+    Tmp tmp;
+    Record rec;
+    rec.packages = {{"progress-no-stdout", true, "progress-no-stdout", 0, 1, "v0.0.0 (.)", "project"}};
+    rec.owner = {{"gen/check.stamp", 0}};
+    rec.step = {{"gen/check.stamp", 0}};
+    rec.actions = {{"gen/check.stamp", "check-action"}};
+    rec.steps = 1;
+    write_record(tmp.path, rec);
+    Build build(tmp.path); // The cached path has a record on disk, not set_record().
+    build.pass_begin();
+    const auto env = build.environment();
+    const auto root = std::ranges::find(env, telemetry::kRootEnv, &decltype(env)::value_type::first);
+    ASSERT_NE(root, env.end());
+    std::ofstream(telemetry::channel(root->second, "gen/check.stamp"))
+        << R"({"v":1,"seq":1,"phase":"internal-check","done":1,"total":2})";
+    const auto unixMs = std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count();
+    append(tmp.path / kStartsFile, std::format("gen/check.stamp\t{}\n", unixMs));
+    testing::internal::CaptureStderr();
+    open(false);
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(2);
+    std::string row;
+    do {
+        row = status_row();
+        if (row.contains("internal-check 1/2")) break;
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    } while (std::chrono::steady_clock::now() < deadline);
+    close();
+    (void)testing::internal::GetCapturedStderr();
+    EXPECT_TRUE(build.has_record());
+    EXPECT_NE(row.find("check-action"), std::string::npos) << row;
+    EXPECT_NE(row.find("internal-check 1/2"), std::string::npos) << row;
+    // No Build::status() or completed ninja-log entry was supplied.
+}
+
 TEST(ProgressModel, APackageIsNamedOnceWhenItsFirstStepFinishes) {
     mcpp::ui::disable_color();
     Tmp tmp;