fix(external): fetch and build each clone once per build pass

`crafter-build test` builds a project's tests side by side, and a
project whose tests each copy its externalDependencies hands every one
of them the same unpinned dependency. Each test's Build() pulled the
shared clone. Before the clone had a lock, those pulls ran at the same
time and broke its .git/FETCH_HEAD, which failed every test of
Crafter.Graphics (issue #34). Since 653dd0b they wait for each other,
but every one is still a network round trip: 26 tests cost 26 pulls of
each dependency, one after another, and for a CMake dependency as many
configure checks and builds.

depResults already records one build pass, keyed by the PcmDir of every
dependency configuration built in it. An external dependency now goes
in the same map, keyed by the directory its build leaves
(ExternalBuildDir: the clone, or for CMake its build directory). The
first configuration that needs it fetches and builds it; the others
wait for that and take its outcome. Keying on the build directory
rather than the clone keeps a CMake declaration from taking the outcome
of a header-only declaration of the same clone, which only fetched it.
A fresh depResults is a fresh pass, so the next build still pulls, and
every caller of Build() gets this: RunTests, a plain build, and the
pragma dependencies LoadProject builds.

To make that possible BuildExternal is split. BuildExternal is now the
shared work, lock, fetch, configure and build, and returns the error.
ExternalOutputs computes what one declaration compiles and links with,
because two declarations of one clone can differ in includeDirs,
libDirs and libs; ExternalBuildResult loses its error field. The work
no longer stops between steps when the configuration that started it
is cancelled: others may be waiting on it, and a run cut short would
leave them a clone nobody fetched. A dependency configuration's Build()
already runs to completion for the same reason.

The promise and future the dependency threads used to build or wait for
a configuration are now one helper, OncePerPass, which both kinds use.
An exception thrown in BuildExternal used to escape its thread into
std::terminate; it is now a build error, as for a dependency.

With eight tests that share a local dependency, a test run fetched it
eight times before this and fetches it once now (SharedExternalFetch,
next commit).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -86,7 +86,7 @@ Conventions the existing tests follow:
 - Collect failures with a `Check(cond, msg)` helper that prints `FAIL: ...` to stderr and counts, then return nonzero at the end. Do not stop at the first failure unless later steps are meaningless without it.
 - Drive the engine in-process: `Build(cfg, depResults, depMutex)`, `RunTests`, the lint driver. Do not shell out to `bin/crafter-build` and grep its stdout.
 - Fixtures under `tests/<Name>/fixture/` are inputs. A test that edits or touches files first stages a copy under `fs::temp_directory_path() / "crafter-build-<name>"` and removes it on the next run. Variant sources are checked in as `*.in` files and copied over the original inside the staged copy.
-- Use a fresh `depResults` map per build pass: the map memoises each configuration's result for one pass, so reusing it skips rebuilds you meant to observe. `Build()` itself is safe to call repeatedly on the same `Configuration`.
+- Use a fresh `depResults` map per build pass: the map memoises each configuration's result, and each external dependency's fetch and build, for one pass, so reusing it skips rebuilds and pulls you meant to observe. `Build()` itself is safe to call repeatedly on the same `Configuration`.
 - Set `cfg.path` before calling `GetInterfacesAndImplementations`, and spell `cFiles` as absolute paths: both resolve against the working directory, not the configuration.
 
 A bug fix ships with a test that fails before the fix and passes after it, in its own `test:` commit when it documents the behaviour on its own.

implementations/Crafter.Build-Clang.cpp

@@ -423,6 +423,28 @@ CompileCommand Crafter::GetCompileCommand(const Configuration& config) {
     return out;
 }
 
+namespace {
+    // The outcome of `work` for `key` in this build pass. The first caller
+    // runs it; every other caller, whether at the same time or later, waits
+    // for that run and gets its result or its exception. The keys are the
+    // directories the work writes, a configuration's PcmDir and an external
+    // dependency's ExternalBuildDir, so the two kinds never collide.
+    std::shared_future<BuildResult> OncePerPass(std::unordered_map<fs::path, std::shared_future<BuildResult>>& depResults, std::mutex& depMutex, const fs::path& key, const std::function<BuildResult()>& work) {
+        std::promise<BuildResult> promise;
+        std::shared_future<BuildResult> outcome = promise.get_future().share();
+        {
+            std::lock_guard lock(depMutex);
+            if (auto [it, inserted] = depResults.try_emplace(key, outcome); !inserted) return it->second;
+        }
+        try {
+            promise.set_value(work());
+        } catch (...) {
+            promise.set_exception(std::current_exception());
+        }
+        return outcome;
+    }
+}
+
 BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, std::shared_future<BuildResult>>& depResults, std::mutex& depMutex) {
     // Before anything is spawned: one message naming the missing compiler
     // beats a clang++-not-found per translation unit.
@@ -678,14 +700,30 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
     externalThreads.reserve(config.externalDependencies.size());
     for (std::size_t i = 0; i < config.externalDependencies.size(); ++i) {
         externalThreads.emplace_back([&, i]() {
-            Progress::Task task(std::format("Building external dep {}", config.externalDependencies[i].name));
+            const ExternalDependency& dep = config.externalDependencies[i];
+            Progress::Task task(std::format("Building external dep {}", dep.name));
             if (buildCancelled.load(std::memory_order_relaxed)) return;
-            externalResults[i] = BuildExternal(config.externalDependencies[i], config.target, buildCancelled);
-            if (!externalResults[i].error.empty()) {
+            std::string error;
+            try {
+                // Every configuration of the pass that declares this
+                // dependency shares one fetch and build of it. A project's
+                // tests often each copy its externalDependencies, and RunTests
+                // builds them side by side: each of them pulling the clone
+                // cost a network round trip per test, and before the clone had
+                // a lock, the simultaneous pulls broke its FETCH_HEAD and
+                // failed every test (issue #34).
+                const std::shared_future<BuildResult> built = OncePerPass(depResults, depMutex, ExternalBuildDir(dep, config.target), [&] {
+                    return BuildResult{BuildExternal(dep, config.target), false, {}};
+                });
+                error = built.get().result;
+                if (error.empty()) externalResults[i] = ExternalOutputs(dep, config.target);
+            } catch (const std::exception& e) {
+                error = std::format("external dep {} threw: {}", dep.name, e.what());
+            }
+            if (error.empty()) return;
             bool expected = false;
             if (buildCancelled.compare_exchange_strong(expected, true)) {
-                    buildError = externalResults[i].error;
-                }
+                buildError = std::move(error);
             }
         });
     }
@@ -746,35 +784,8 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
             try {
                 if (buildCancelled.load(std::memory_order_relaxed)) return;
 
-                std::shared_ptr<std::promise<BuildResult>> promise;
-                std::shared_future<BuildResult> resultFuture;
-                bool isBuilder = false;
-
-                depMutex.lock();
-                fs::path cacheKey = dep->PcmDir();
-                auto it = depResults.find(cacheKey);
-                if (it == depResults.end()) {
-                    isBuilder = true;
-                    promise = std::make_shared<std::promise<BuildResult>>();
-                    resultFuture = promise->get_future().share();
-                    depResults.emplace(cacheKey, resultFuture);
-                } else {
-                    resultFuture = it->second;
-                }
-                depMutex.unlock();
-
-                if (isBuilder) {
-                    BuildResult built;
-                    try {
-                        built = Build(*dep, depResults, depMutex);
-                    } catch (...) {
-                        promise->set_exception(std::current_exception());
-                        throw;
-                    }
-                    promise->set_value(std::move(built));
-                }
-
-                const BuildResult& result = resultFuture.get();
+                const std::shared_future<BuildResult> built = OncePerPass(depResults, depMutex, dep->PcmDir(), [&] { return Build(*dep, depResults, depMutex); });
+                const BuildResult& result = built.get();
                 fileMutex.lock();
                 for (const std::string& lib : result.libs) libSet.insert(lib);
                 for (const std::string& f : result.publicCompileFlags) publicFlagSet.insert(f);

implementations/Crafter.Build-External.cpp

@@ -231,15 +231,16 @@ std::vector<std::string> Crafter::ExternalIncludeFlags(const ExternalDependency&
     return flags;
 }
 
-ExternalBuildResult Crafter::BuildExternal(const ExternalDependency& dep, std::string_view target, std::atomic<bool>& cancelled) {
-    ExternalBuildResult result;
-
-    if (cancelled.load(std::memory_order_relaxed)) return result;
+fs::path Crafter::ExternalBuildDir(const ExternalDependency& dep, std::string_view target) {
+    fs::path cloneDir = ExternalCloneDir(dep, target);
+    if (cloneDir.empty() || dep.builder != ExternalBuilder::CMake) return cloneDir;
+    return cloneDir / "build";
+}
 
+std::string Crafter::BuildExternal(const ExternalDependency& dep, std::string_view target) {
     std::string name = dep.name.empty() ? DeriveName(dep.source) : dep.name;
     if (name.empty()) {
-        result.error = std::format("Could not derive name for external dependency from URL '{}'", dep.source.url);
-        return result;
+        return std::format("Could not derive name for external dependency from URL '{}'", dep.source.url);
     }
 
     fs::path externalRoot = GetCacheDir() / "external";
@@ -247,41 +248,38 @@ ExternalBuildResult Crafter::BuildExternal(const ExternalDependency& dep, std::s
         std::error_code ec;
         fs::create_directories(externalRoot, ec);
         if (ec) {
-            result.error = std::format("Failed to create {}: {}", externalRoot.string(), ec.message());
-            return result;
+            return std::format("Failed to create {}: {}", externalRoot.string(), ec.message());
         }
     }
 
     fs::path cloneDir = ExternalCloneDir(dep, target);
-    // Builds running side by side (tests built in parallel, two
-    // crafter-builds) share the clone: one fetches and builds it at a time.
+    // Build() makes one call for all the configurations of a pass, but two
+    // crafter-builds, or two passes, can still reach the same clone at once,
+    // and two pulls in one clone race on its .git/FETCH_HEAD.
     FileLock lock(fs::path(std::format("{}.lock", cloneDir.string())));
 
-    std::string fetchErr = FetchGit(dep.source, cloneDir);
-    if (!fetchErr.empty()) {
-        result.error = std::format("git fetch for '{}': {}", name, fetchErr);
-        return result;
+    if (std::string err = FetchGit(dep.source, cloneDir); !err.empty()) {
+        return std::format("git fetch for '{}': {}", name, err);
     }
+    if (dep.builder != ExternalBuilder::CMake) return "";
 
-    if (cancelled.load(std::memory_order_relaxed)) return result;
-
-    fs::path cmakeBuildDir;
-    if (dep.builder == ExternalBuilder::CMake) {
-        cmakeBuildDir = cloneDir / "build";
+    const fs::path cmakeBuildDir = ExternalBuildDir(dep, target);
     if (std::string err = ConfigureCMake(cloneDir, cmakeBuildDir, dep.options, target); !err.empty()) {
-            result.error = std::format("cmake configure for '{}': {}", name, err);
-            return result;
+        return std::format("cmake configure for '{}': {}", name, err);
     }
-        if (cancelled.load(std::memory_order_relaxed)) return result;
     if (std::string err = BuildCMake(cmakeBuildDir); !err.empty()) {
-            result.error = std::format("cmake build for '{}': {}", name, err);
-            return result;
-        }
+        return std::format("cmake build for '{}': {}", name, err);
     }
+    return "";
+}
 
+ExternalBuildResult Crafter::ExternalOutputs(const ExternalDependency& dep, std::string_view target) {
+    ExternalBuildResult result;
     result.compileFlags = ExternalIncludeFlags(dep, target);
 
-    if (dep.builder == ExternalBuilder::CMake) {
+    const fs::path cmakeBuildDir = ExternalBuildDir(dep, target);
+    if (dep.builder != ExternalBuilder::CMake || cmakeBuildDir.empty()) return result;
+
     // Each search path gets both a -L (link-time) and a -Wl,-rpath
     // (runtime-loader). The rpath stays embedded in the produced
     // binary so it picks up shared deps from the cache without any
@@ -319,7 +317,6 @@ ExternalBuildResult Crafter::BuildExternal(const ExternalDependency& dep, std::s
             if (!ec && t > result.latestArtifact) result.latestArtifact = t;
         }
     }
-    }
 
     return result;
 }

interfaces/Crafter.Build-Clang.cppm

@@ -656,6 +656,13 @@ export namespace Crafter {
     };
     CompileCommand GetCompileCommand(const Configuration& config);
 
+    // Builds `config` and the configurations it depends on. depResults is the
+    // record of one build pass, guarded by depMutex and keyed by the directory
+    // each piece of work writes: a dependency's PcmDir, an external
+    // dependency's ExternalBuildDir. Configurations built with the same map,
+    // as RunTests builds its tests, build a shared dependency and fetch a
+    // shared external once between them. A fresh map starts a new pass, which
+    // checks everything again and pulls unpinned externals again.
     BuildResult Build(Configuration& config, std::unordered_map<fs::path, std::shared_future<BuildResult>>& depResults, std::mutex& depMutex);
 
     // Takes main's argv verbatim so the CLI entry point is a one-liner;

interfaces/Crafter.Build-External.cppm

@@ -36,7 +36,6 @@ export namespace Crafter {
     };
 
     struct ExternalBuildResult {
-        std::string error;
         std::vector<std::string> compileFlags;
         std::vector<std::string> linkFlags;
         fs::file_time_type latestArtifact = fs::file_time_type::min();
@@ -47,12 +46,30 @@ export namespace Crafter {
     // through this, so a caller that only needs to know where the headers ended
     // up cannot drift from where they actually are.
     fs::path ExternalCloneDir(const ExternalDependency& dep, std::string_view target);
+    // Where BuildExternal leaves this dependency: the CMake build directory,
+    // or the clone itself when it is only fetched. Pure, like ExternalCloneDir.
+    // Build() keys a pass's record of the work on it rather than on the clone,
+    // so a declaration that shares the clone but builds it with CMake is not
+    // handed the outcome of a fetch alone.
+    fs::path ExternalBuildDir(const ExternalDependency& dep, std::string_view target);
     // The -I flags this dependency contributes, from its declared includeDirs.
     // Also pure, which is what lets the linter's AST layer parse a source that
     // includes an external's headers without running a build to find out where
     // they are. The paths only resolve once something has fetched the clone.
     std::vector<std::string> ExternalIncludeFlags(const ExternalDependency& dep, std::string_view target);
-    ExternalBuildResult BuildExternal(const ExternalDependency& dep, std::string_view target, std::atomic<bool>& cancelled);
+    // Fetches the clone and, for ExternalBuilder::CMake, configures and builds
+    // it. Empty on success, otherwise what failed. A lock on the clone is held
+    // throughout, so another crafter-build, or another build pass in this one,
+    // works on it before or after this call and never during it. Not
+    // cancellable once started: Build() hands the outcome to every
+    // configuration of the pass that declares the dependency, and a run cut
+    // short would leave them a clone nobody fetched.
+    std::string BuildExternal(const ExternalDependency& dep, std::string_view target);
+    // What a configuration that declares `dep` compiles and links with once
+    // BuildExternal has run: the -I flags and, for CMake, the -L, -rpath and -l
+    // flags and the newest library. Per declaration, because two that share a
+    // clone can still differ in includeDirs, libDirs and libs.
+    ExternalBuildResult ExternalOutputs(const ExternalDependency& dep, std::string_view target);
 
     // Specification for a sibling crafter-build project to fetch and depend on.
     // GitSource picks the revision: leave branch + commit empty for the

tests/ExternalArtifactTime/main.cpp

@@ -7,8 +7,8 @@ namespace fs = std::filesystem;
 using namespace Crafter;
 
 // A CMake dependency's newest library decides when its consumers are stale
-// (BuildExternal's latestArtifact). Only files directly in the CMake build
-// directory named *.a or *.so were looked at, so msquic's
+// (the latestArtifact of ExternalOutputs). Only files directly in the CMake
+// build directory named *.a or *.so were looked at, so msquic's
 // bin/Release/libmsquic.so.2, in one of the dependency's libDirs and with a
 // version after the .so, never counted: a rebuilt msquic left everything
 // built against the old one in place.
@@ -69,9 +69,9 @@ int main() {
     dep.builder = ExternalBuilder::CMake;
     dep.libDirs = { "bin/Release" };
     dep.libs = { "versioned" };
-    std::atomic<bool> cancelled = false;
-    const ExternalBuildResult result = BuildExternal(dep, HostTarget(), cancelled);
-    Check(result.error.empty(), std::format("the dependency builds: {}", result.error));
+    const std::string error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("the dependency builds: {}", error));
+    const ExternalBuildResult result = ExternalOutputs(dep, HostTarget());
 
     const fs::path library = ExternalCloneDir(dep, HostTarget()) / "build" / "bin" / "Release" / "libversioned.so.2.0.0";
     Check(fs::exists(library), std::format("the library is where its CMake project puts it: {}", library.string()));