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

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`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>
This commit is contained in:
Jorijn van der Graaf 2026-10-31 02:09:24 +01:00
commit f77d913171
Signed by: jorijnvdgraaf
GPG key ID: 2937E59CDCC1BCFB
6 changed files with 141 additions and 109 deletions
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83
implementations/Crafter.Build-Clang.cpp
View file

@ -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()) {
bool expected = false;
if (buildCancelled.compare_exchange_strong(expected, true)) {
buildError = externalResults[i].error;
}
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 = 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);