Merge pull request 'Track what a primary module interface imports' (#29) from claude/issue-26 into master

README.md

@@ -125,7 +125,7 @@ Per-import precise tracking for both within-project and cross-project module dep
 
 Diamond deps (`A → {B, C}; B → X; C → X`) build `X` exactly once via a `std::shared_future<BuildResult>` cache.
 
-Tracking is derived from each translation unit's `import` statements, which are scanned when the sources are declared. `cfg.dependencies` is often assigned *afterwards* — `AddTest` works that way — so `Build()` re-resolves any import that matched nothing at scan time before it compares mtimes. Without that, a consumer of a dependency's module carried no edge to it at all: adding a data member to that dependency's interface rebuilt the library, relinked the consumer, and left the consumer's object compiled against the old class layout. Nothing fails to link when a member is added, so the result was a working build and a crash later.
+Tracking is derived from each translation unit's `import` statements, which are scanned when the sources are declared. Every kind of unit is scanned — primary module interfaces, partitions and implementation units alike — and an interface that imports a sibling module in the same `Configuration` also gets its compile ordered behind it. `cfg.dependencies` is often assigned *afterwards* — `AddTest` works that way — so `Build()` re-resolves any import that matched nothing at scan time before it compares mtimes. Without all of that, a consumer of a dependency's module carried no edge to it at all: adding a data member to that dependency's interface rebuilt the library, relinked the consumer, and left the consumer's object compiled against the old class layout. Nothing fails to link when a member is added, so the result was a working build and a crash later.
 
 Everything that changes what gets built belongs in the variant id, since it names the `bin/` and `build/` directory. That includes project args crafter-build itself doesn't interpret: `crafter-build` and `crafter-build -- --no-webgpu` get separate directories rather than interleaving their outputs in one. The cached host PCMs under `<cache>/crafter.build/<target>-<march>/` are shared by every crafter-build on the machine, so they are invalidated by a hash of the module sources rather than by mtime — an mtime can't tell a newer PCM from one built by a different install.
 

implementations/Crafter.Build-Clang.cpp

@@ -73,6 +73,7 @@ void Configuration::ResolvePendingImports() {
         });
     };
     for(const std::unique_ptr<Module>& interface : interfaces) {
+        sweep(interface->pendingImports, interface->moduleDependencies, interface->externalModuleDependencies);
         for(const std::unique_ptr<ModulePartition>& partition : interface->partitions) {
             sweep(partition->pendingImports, partition->moduleDependencies, partition->externalModuleDependencies);
         }
@@ -104,19 +105,26 @@ void Configuration::GetInterfacesAndImplementations(std::span<fs::path> interfac
         tempModulePaths[i] = {file, fileContent, nullptr, nullptr};
     }
 
-    std::erase_if(tempModulePaths, [this](std::tuple<fs::path, std::string, ModulePartition*, Module*>& file) {
+    // Primary interface units first, so the partition pass below can find their
+    // parent Module. They stay in tempModulePaths — a primary unit's own
+    // `import X;` lines are layout dependencies exactly like a partition's, and
+    // dropping the entry here is what left them unrecorded (issue #26). Marked
+    // by a null partition slot with the Module slot filled in.
+    for(std::tuple<fs::path, std::string, ModulePartition*, Module*>& file : tempModulePaths) {
         std::smatch match;
         if (std::regex_search(std::get<1>(file), match, std::regex(R"(export module ([a-zA-Z0-9_\.\-]+);)"))) {
-            std::get<0>(file).replace_extension("");
+            fs::path pthCpy = std::get<0>(file);
-            this->interfaces.push_back(std::make_unique<Module>(std::move(match[1].str()), std::move(std::get<0>(file))));
+            pthCpy.replace_extension("");
-            return true;
+            this->interfaces.push_back(std::make_unique<Module>(std::move(match[1].str()), std::move(pthCpy)));
-        } else {
+            std::get<3>(file) = this->interfaces.back().get();
-            return false;
+        }
     }
-    });
 
         for(std::uint16_t i = 0; i < tempModulePaths.size(); i++) {
         std::smatch match;
+        if (std::get<3>(tempModulePaths[i]) != nullptr) {
+            continue;
+        }
         if (std::regex_search(std::get<1>(tempModulePaths[i]), match, std::regex(R"(export module ([a-zA-Z_0-9\.\-]+):([a-zA-Z_0-9\.\-]+);)"))) {
             for(const std::unique_ptr<Module>& modulee : this->interfaces) {
                 if(modulee->name == match[1]) {
@@ -142,6 +150,22 @@ void Configuration::GetInterfacesAndImplementations(std::span<fs::path> interfac
         Module* parentModule = std::get<3>(file);
         const std::string& fileContent = std::get<1>(file);
 
+        // Primary interface unit: record only its module imports. Its
+        // `[export] import :Part;` lines need no edge — Module::Check walks
+        // every partition it owns and Module::Compile builds them all before
+        // precompiling itself, so partitions are covered wholesale.
+        if (partition == nullptr) {
+            std::regex primaryPattern(R"(import ([a-zA-Z_0-9\.\-]+);)");
+            std::sregex_iterator primaryCurrent(fileContent.begin(), fileContent.end(), primaryPattern);
+            std::sregex_iterator primaryLast;
+            while (primaryCurrent != primaryLast) {
+                std::smatch match = *primaryCurrent;
+                resolveImport(match[1].str(), parentModule->moduleDependencies, parentModule->externalModuleDependencies, parentModule->pendingImports);
+                ++primaryCurrent;
+            }
+            continue;
+        }
+
         std::regex partitionPattern(R"(import :([a-zA-Z_\-0-9\.]+);)");
         std::sregex_iterator currentMatch(fileContent.begin(), fileContent.end(), partitionPattern);
         std::sregex_iterator lastMatch;
@@ -882,9 +906,24 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
         if (ext.latestArtifact > externalFloor) externalFloor = ext.latestArtifact;
     }
 
+    // Check every interface before compiling any of them. Module::Compile waits
+    // on the `compiled` flag of each sibling module it imports, and that flag is
+    // set either by a Compile that runs or by the Check that decides none is
+    // needed — so a Check still pending while another module's thread is already
+    // waiting would block on a flag nothing goes on to raise.
+    std::vector<Module*> staleInterfaces;
     for(std::unique_ptr<Module>& interface : config.interfaces) {
         if(interface->Check(pcmDir, externalFloor)) {
-            Module* mod = interface.get();
+            staleInterfaces.push_back(interface.get());
+            buildResult.repack = true;
+        }
+        files += std::format(" {}/{}.o", buildDir.string(),  interface->path.filename().string());
+        for(std::unique_ptr<ModulePartition>& part : interface->partitions) {
+            files += std::format(" {}/{}.o", buildDir.string(),  part->path.filename().string());
+        }
+    }
+
+    for(Module* mod : staleInterfaces) {
         threads.emplace_back([mod, &command, &pcmDir, &buildDir, &buildCancelled, &buildError]() {
             Progress::Task task(std::format("Compiling interface {}", mod->path.filename().string()));
             try {
@@ -896,12 +935,6 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
                 }
             }
         });
-            buildResult.repack = true;
-        }
-        files += std::format(" {}/{}.o", buildDir.string(),  interface->path.filename().string());
-        for(std::unique_ptr<ModulePartition>& part : interface->partitions) {
-            files += std::format(" {}/{}.o", buildDir.string(),  part->path.filename().string());
-        }
     }
 
     for(Implementation& implementation : config.implementations) {

implementations/Crafter.Build-Interface.cpp

@@ -39,6 +39,10 @@ namespace Crafter {
                 }
                 needsRecompiling = false;
                 compiled.store(true);
+                // Nothing waits on `compiled` until the compile threads start,
+                // which is after every Check has run — but notify anyway so the
+                // flag is never left set without a wake-up behind it.
+                compiled.notify_all();
                 return false;
             } else {
                 needsRecompiling = true;
@@ -100,24 +104,55 @@ namespace Crafter {
             std::string pcmPath = std::format("{}.pcm", (pcmDir/path.filename()).generic_string());
             std::string cppmPath = std::format("{}.cppm", path.generic_string());
             if(fs::exists(pcmPath) && std::max(fs::last_write_time(cppmPath), sourceFloor) < fs::last_write_time(pcmPath)) {
+                fs::file_time_type pcmTime = fs::last_write_time(pcmPath);
+                // Every partition gets Check()ed even once one is known stale:
+                // Compile() drives partition rebuilds off their own
+                // needsRecompiling flags, so short-circuiting here would leave
+                // the later ones unevaluated and silently unbuilt.
                 bool depCheck = false;
                 for(std::unique_ptr<ModulePartition>& partition : partitions) {
                     if(partition->Check(pcmDir, sourceFloor)) {
                         depCheck = true;
                     }
                 }
+                // Modules this interface unit imports directly. Local ones share
+                // our pcmDir and are resolved recursively; external ones are
+                // compared by PCM mtime, their owning Configuration having
+                // already finished building by the time we run.
+                for(Module* dependency : moduleDependencies) {
+                    if(dependency->Check(pcmDir, sourceFloor)) {
+                        depCheck = true;
+                    }
+                }
+                if(!depCheck) {
+                    for(const auto& [externalMod, externalPcmPath] : externalModuleDependencies) {
+                        std::error_code ec;
+                        fs::file_time_type t = fs::last_write_time(externalPcmPath, ec);
+                        if (!ec && t >= pcmTime) {
+                            depCheck = true;
+                            break;
+                        }
+                    }
+                }
                 if(depCheck) {
                     needsRecompiling = true;
                     return true;
                 } else {
                     needsRecompiling = false;
                     compiled.store(true);
+                    compiled.notify_all();
                     return false;
                 }
             } else {
+                // Already known stale, but the dependencies still need
+                // evaluating: an unchecked local module would never get built,
+                // and Compile() below waits on its `compiled` flag.
                 for(std::unique_ptr<ModulePartition>& partition : partitions) {
                     partition->Check(pcmDir, sourceFloor);
                 }
+                for(Module* dependency : moduleDependencies) {
+                    dependency->Check(pcmDir, sourceFloor);
+                }
                 needsRecompiling = true;
                 return true;
             }
@@ -127,6 +162,17 @@ namespace Crafter {
     }
 
     void Module::Compile(const std::string_view clang, const fs::path& pcmDir, const fs::path& buildDir, std::atomic<bool>& buildCancelled, std::string& buildError) {
+        // A sibling module in the same Configuration that this interface unit
+        // imports has to have its PCM on disk before we precompile against it.
+        // Safe to block here: Build() Checks every interface before it spawns
+        // any compile thread, so a dependency that needs no rebuild already has
+        // `compiled` set and this returns immediately.
+        for(Module* dependency : moduleDependencies) {
+            if(!dependency->compiled.load()) {
+                dependency->compiled.wait(false);
+            }
+        }
+
         std::vector<std::thread> threads;
         threads.reserve(partitions.size());
         for(std::unique_ptr<ModulePartition>& part : partitions) {

interfaces/Crafter.Build-Interface.cppm

@@ -22,7 +22,7 @@ namespace Crafter {
         // Names that never resolve (`std`, module-mapped externals) stay here.
         std::vector<std::string> pendingImports;
         std::atomic<bool> compiled;
-        bool needsRecompiling;
+        bool needsRecompiling = false;
         bool checked = false;
         std::string name;
         fs::path path;
@@ -33,8 +33,17 @@ namespace Crafter {
 
     export class Module {
         public:
+        // A primary module interface unit imports things too — `import Base;`
+        // in `export module Widget;` is as much a layout dependency as the
+        // same line in a partition. Recorded on the same three vectors, with
+        // the same meanings, so Check() can see through them and Compile()
+        // can order itself behind a sibling module in the same Configuration
+        // (issue #26).
+        std::vector<Module*> moduleDependencies;
+        std::vector<std::pair<Module*, fs::path>> externalModuleDependencies;
+        std::vector<std::string> pendingImports;
         std::atomic<bool> compiled;
-        bool needsRecompiling;
+        bool needsRecompiling = false;
         bool checked = false;
         std::vector<std::unique_ptr<ModulePartition>> partitions;
         std::string name;

project.cpp

@@ -108,6 +108,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("ModuleInterface").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("DependencyLink").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("IncrementalInterfaceChange").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("TransitiveInterfaceChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("CleanProject").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ShaderCompile").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("StandardArgs").Dependencies({ CrafterBuildLib.get() });

tests/TransitiveInterfaceChange/fixture/base/Base-grown.cppm.in

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+export module Base;
+import std;
+
+export namespace Demo {
+    class Payload {
+        public:
+        // The whole point of the fixture: `first` moves, and nothing about any
+        // signature or mangled name changes with it.
+        std::array<char, 4096> padding{};
+        std::int32_t first = 1;
+        void Stamp();
+    };
+}

tests/TransitiveInterfaceChange/fixture/base/Base.cpp

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module Base;
+import std;
+
+namespace Demo {
+    void Payload::Stamp() {
+        first = 11;
+    }
+}

tests/TransitiveInterfaceChange/fixture/base/Base.cppm

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+export module Base;
+import std;
+
+export namespace Demo {
+    class Payload {
+        public:
+        std::int32_t first = 1;
+        void Stamp();
+    };
+}

tests/TransitiveInterfaceChange/fixture/main.cpp

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Consumer of the layout under test. `Thing` lives on the stack here, so a
+// stale object file that still believes the old `sizeof(Payload)` lets the
+// library's Fill() write past what this frame reserved. Reports the disagreement
+// as a non-zero exit rather than relying on the corruption to fault.
+import Widget;
+import std;
+
+int main() {
+    Demo::Thing thing;
+    thing.Fill();
+    std::println("first={} tail={} sizeof={}", thing.payload.first, thing.tail, sizeof(Demo::Thing));
+    if (thing.payload.first != 11 || thing.tail != 7) {
+        return 1;
+    }
+    return 0;
+}

tests/TransitiveInterfaceChange/fixture/widget/Widget.cpp

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module Widget;
+import std;
+
+namespace Demo {
+    void Thing::Fill() {
+        payload.Stamp();
+        tail = 7;
+    }
+}

tests/TransitiveInterfaceChange/fixture/widget/Widget.cppm

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// A *primary* module interface unit — no partitions — that imports another
+// module and embeds one of its types by value. The import lives here rather
+// than in a partition on purpose: that is the edge issue #26 was missing.
+export module Widget;
+import std;
+export import Base;
+
+export namespace Demo {
+    class Thing {
+        public:
+        Payload payload;
+        std::int32_t tail = 0;
+        void Fill();
+    };
+}

tests/TransitiveInterfaceChange/main.cpp

@@ -0,0 +1,237 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Issue #26. A *primary* module interface unit — `export module Widget;`, no
+// partitions — records nothing about what it imports. The scanner registered
+// the Module and then dropped the file before the import pass ran, and Module
+// itself had nowhere to put the edge, so:
+//
+//   * `Module::Check` looked only at its own .cppm and its partitions. A layout
+//     change in an imported module left Widget.pcm and Widget.o untouched, the
+//     consumer's object untouched, and the executable relinked — mixing two
+//     layouts with no diagnostic. That is the reported failure: rebuild the
+//     dependency, relink the test, crash somewhere unrelated.
+//
+//   * `Module::Compile` waited on nothing. Two modules in one Configuration
+//     compile on concurrent threads, so importing a sibling was a coin flip
+//     between working and "module 'Base' not found".
+//
+// Partitions never had either problem — they carry the same three vectors and
+// Check/Compile honour them — which is why the gap only shows up on a module
+// whose interface is one flat unit. tests/IncrementalInterfaceChange covers the
+// neighbouring consumer-side edge (issue #27).
+
+namespace {
+    std::int32_t Failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++Failures;
+        }
+    }
+
+    // The fixture is mutated during the run, so work on a copy outside the repo.
+    fs::path StageFixture() {
+        fs::path source = fs::current_path() / "tests" / "TransitiveInterfaceChange" / "fixture";
+        fs::path staged = fs::temp_directory_path() / "crafter-build-transitive-interface-change";
+        fs::remove_all(staged);
+        fs::copy(source, staged, fs::copy_options::recursive);
+        return staged;
+    }
+
+    // Swap in the Base variant carrying the extra member. From a file rather
+    // than a string literal: the module scanner reads raw source, so a module
+    // declaration spelled inside this test would make the test itself look like
+    // an implementation unit of Base.
+    void GrowBase(const fs::path& staged) {
+        fs::copy_file(staged / "base" / "Base-grown.cppm.in", staged / "base" / "Base.cppm", fs::copy_options::overwrite_existing);
+        // copy_file carries the source's mtime across, which would leave the
+        // rewritten interface looking older than the BMI built from it.
+        fs::last_write_time(staged / "base" / "Base.cppm", fs::file_time_type::clock::now());
+    }
+
+    std::unique_ptr<Configuration> MakeBaseLib(const fs::path& staged) {
+        auto base = std::make_unique<Configuration>();
+        base->path = staged / "base";
+        base->name = "base";
+        base->outputName = "base";
+        base->target = HostTarget();
+        base->type = ConfigurationType::LibraryStatic;
+        std::array<fs::path, 1> ifaces = { "Base" };
+        std::array<fs::path, 1> impls = { "Base" };
+        base->GetInterfacesAndImplementations(ifaces, impls);
+        return base;
+    }
+
+    std::unique_ptr<Configuration> MakeWidgetLib(const fs::path& staged) {
+        auto widget = std::make_unique<Configuration>();
+        widget->path = staged / "widget";
+        widget->name = "widget";
+        widget->outputName = "widget";
+        widget->target = HostTarget();
+        widget->type = ConfigurationType::LibraryStatic;
+        std::array<fs::path, 1> ifaces = { "Widget" };
+        std::array<fs::path, 1> impls = { "Widget" };
+        widget->GetInterfacesAndImplementations(ifaces, impls);
+        return widget;
+    }
+
+    Module* FindModule(const Configuration& cfg, std::string_view name) {
+        for (const std::unique_ptr<Module>& mod : cfg.interfaces) {
+            if (mod->name == name) return mod.get();
+        }
+        return nullptr;
+    }
+
+    bool BuildOk(Configuration& cfg, std::string_view label) {
+        // A fresh depResults per pass: the map memoizes each Configuration's
+        // build for the duration of one pass, so reusing it would skip the
+        // dependencies' second build entirely.
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        BuildResult r = Build(cfg, depResults, depMutex);
+        if (!r.result.empty()) {
+            std::println(std::cerr, "FAIL: {} build failed: {}", label, r.result);
+            ++Failures;
+            return false;
+        }
+        return true;
+    }
+
+    // Non-zero exit here means the two ends of the link disagree about the
+    // layout — see the fixture's main.cpp.
+    void CheckAgrees(const fs::path& binary, std::string_view label) {
+        auto run = RunCommandWithTimeout(binary.string(), std::chrono::seconds(30));
+        Check(run.exitCode == 0 && !run.crashed && !run.timedOut, std::format("{}: consumer and library agree on the layout (exit={} output='{}')", label, run.exitCode, run.output));
+    }
+}
+
+int main() {
+    {
+        // The scan records a primary interface unit's imports, and the
+        // pending/resolve machinery reaches them the same way it reaches a
+        // partition's.
+        fs::path staged = StageFixture();
+        std::unique_ptr<Configuration> base = MakeBaseLib(staged);
+        std::unique_ptr<Configuration> widget = MakeWidgetLib(staged);
+
+        Module* mod = FindModule(*widget, "Widget");
+        Check(mod != nullptr, "the primary interface unit registered a module named Widget");
+        if (mod != nullptr) {
+            Check(mod->partitions.empty(), "Widget has no partitions, so nothing else could carry the edge");
+            Check(mod->externalModuleDependencies.empty(), "nothing resolvable before dependencies are assigned");
+            Check(std::ranges::find(mod->pendingImports, "Base") != mod->pendingImports.end(), "the unresolved import of Base is remembered as pending");
+            Check(std::ranges::find(mod->pendingImports, "std") != mod->pendingImports.end(), "std stays pending — it is supplied from outside the graph");
+
+            widget->dependencies = { base.get() };
+            widget->ResolvePendingImports();
+
+            Check(mod->externalModuleDependencies.size() == 1, "ResolvePendingImports places the external module edge");
+            if (mod->externalModuleDependencies.size() == 1) {
+                Check(mod->externalModuleDependencies[0].first->name == "Base", "the edge names the Base module");
+                Check(mod->externalModuleDependencies[0].second == base->PcmDir() / "Base.pcm", "the edge points at the dependency's BMI");
+            }
+            Check(std::ranges::find(mod->pendingImports, "Base") == mod->pendingImports.end(), "the resolved name is no longer pending");
+            Check(std::ranges::find(mod->pendingImports, "std") != mod->pendingImports.end(), "std is still pending after the sweep");
+
+            // Build() runs a sweep unconditionally on top of whatever callers
+            // already did, so a repeat must not duplicate the edge.
+            widget->ResolvePendingImports();
+            Check(mod->externalModuleDependencies.size() == 1, "ResolvePendingImports is idempotent");
+        }
+    }
+
+    {
+        // Two modules in one Configuration, declared in the order that used to
+        // hide the problem: Widget — which imports Base — is registered first.
+        // Every primary unit is registered before any import is resolved, so
+        // declaration order must not matter.
+        fs::path staged = StageFixture();
+        Configuration cfg;
+        cfg.path = staged;
+        cfg.name = "sibling";
+        cfg.outputName = "sibling";
+        cfg.target = HostTarget();
+        cfg.type = ConfigurationType::Executable;
+        std::array<fs::path, 2> ifaces = { "widget/Widget", "base/Base" };
+        std::array<fs::path, 3> impls = { "widget/Widget", "base/Base", "main" };
+        cfg.GetInterfacesAndImplementations(ifaces, impls);
+
+        Module* widgetMod = FindModule(cfg, "Widget");
+        Module* baseMod = FindModule(cfg, "Base");
+        Check(widgetMod != nullptr && baseMod != nullptr, "both sibling modules registered");
+        if (widgetMod != nullptr && baseMod != nullptr) {
+            Check(std::ranges::find(widgetMod->moduleDependencies, baseMod) != widgetMod->moduleDependencies.end(), "Widget's import of its sibling resolved locally despite being declared first");
+            Check(widgetMod->externalModuleDependencies.empty(), "a sibling in the same Configuration is not an external edge");
+            Check(baseMod->moduleDependencies.empty(), "Base imports no sibling");
+        }
+
+        // That edge is also the compile-ordering guarantee: Widget's precompile
+        // needs Base.pcm on disk. Repeated from scratch because the failure was
+        // a thread race, and one lucky pass proves nothing.
+        for (std::int32_t attempt = 0; attempt < 4; ++attempt) {
+            fs::remove_all(cfg.BuildDir());
+            fs::remove_all(cfg.BinDir());
+            if (!BuildOk(cfg, std::format("sibling pass {}", attempt))) break;
+            CheckAgrees(cfg.BinDir() / "sibling", std::format("sibling pass {}", attempt));
+        }
+    }
+
+    {
+        // End to end, the reported shape: a consumer executable, a library whose
+        // primary interface unit imports a second library's module, and a member
+        // added to a class in that second module.
+        fs::path staged = StageFixture();
+        std::unique_ptr<Configuration> base = MakeBaseLib(staged);
+        std::unique_ptr<Configuration> widget = MakeWidgetLib(staged);
+        widget->dependencies = { base.get() };
+
+        Configuration app;
+        app.path = staged;
+        app.name = "consumer";
+        app.outputName = "consumer";
+        app.target = HostTarget();
+        app.type = ConfigurationType::Executable;
+        std::array<fs::path, 0> appIfaces = {};
+        std::array<fs::path, 1> appImpls = { "main" };
+        app.GetInterfacesAndImplementations(appIfaces, appImpls);
+        app.dependencies = { widget.get() };
+
+        fs::path binary = app.BinDir() / "consumer";
+        fs::path widgetInterfaceObject = widget->BuildDir() / "Widget.o";
+        fs::path widgetImplObject = widget->BuildDir() / "Widget_impl.o";
+        fs::path consumerObject = app.BuildDir() / "main_impl.o";
+
+        if (BuildOk(app, "first pass")) {
+            CheckAgrees(binary, "first pass");
+
+            fs::file_time_type interfaceBefore = fs::last_write_time(widgetInterfaceObject);
+            fs::file_time_type implBefore = fs::last_write_time(widgetImplObject);
+            fs::file_time_type consumerBefore = fs::last_write_time(consumerObject);
+
+            GrowBase(staged);
+
+            if (BuildOk(app, "second pass")) {
+                // All three sat on the old layout before the fix, and all three
+                // were left alone while base rebuilt and both binaries relinked.
+                Check(fs::last_write_time(widgetInterfaceObject) > interfaceBefore, "the importing interface unit is recompiled");
+                Check(fs::last_write_time(widgetImplObject) > implBefore, "that module's implementation unit is recompiled");
+                Check(fs::last_write_time(consumerObject) > consumerBefore, "the consumer's translation unit is recompiled");
+
+                CheckAgrees(binary, "second pass");
+            }
+        }
+    }
+
+    if (Failures > 0) {
+        std::println(std::cerr, "{} assertions failed", Failures);
+        return 1;
+    }
+    return 0;
+}