src/libutil/async-collect.hh

@@ -0,0 +1,101 @@
+#pragma once
+/// @file
+
+#include <kj/async.h>
+#include <kj/common.h>
+#include <kj/vector.h>
+#include <list>
+#include <optional>
+#include <type_traits>
+
+namespace nix {
+
+template<typename K, typename V>
+class AsyncCollect
+{
+public:
+    using Item = std::conditional_t<std::is_void_v<V>, K, std::pair<K, V>>;
+
+private:
+    kj::ForkedPromise<void> allPromises;
+    std::list<Item> results;
+    size_t remaining;
+
+    kj::ForkedPromise<void> signal;
+    kj::Maybe<kj::Own<kj::PromiseFulfiller<void>>> notify;
+
+    void oneDone(Item item)
+    {
+        results.emplace_back(std::move(item));
+        remaining -= 1;
+        KJ_IF_MAYBE (n, notify) {
+            (*n)->fulfill();
+            notify = nullptr;
+        }
+    }
+
+    kj::Promise<void> collectorFor(K key, kj::Promise<V> promise)
+    {
+        if constexpr (std::is_void_v<V>) {
+            return promise.then([this, key{std::move(key)}] { oneDone(std::move(key)); });
+        } else {
+            return promise.then([this, key{std::move(key)}](V v) {
+                oneDone(Item{std::move(key), std::move(v)});
+            });
+        }
+    }
+
+    kj::ForkedPromise<void> waitForAll(kj::Array<std::pair<K, kj::Promise<V>>> & promises)
+    {
+        kj::Vector<kj::Promise<void>> wrappers;
+        for (auto & [key, promise] : promises) {
+            wrappers.add(collectorFor(std::move(key), std::move(promise)));
+        }
+
+        return kj::joinPromisesFailFast(wrappers.releaseAsArray()).fork();
+    }
+
+public:
+    AsyncCollect(kj::Array<std::pair<K, kj::Promise<V>>> && promises)
+        : allPromises(waitForAll(promises))
+        , remaining(promises.size())
+        , signal{nullptr}
+    {
+    }
+
+    kj::Promise<std::optional<Item>> next()
+    {
+        if (remaining == 0 && results.empty()) {
+            return {std::nullopt};
+        }
+
+        if (!results.empty()) {
+            auto result = std::move(results.front());
+            results.pop_front();
+            return {{std::move(result)}};
+        }
+
+        if (notify == nullptr) {
+            auto pair = kj::newPromiseAndFulfiller<void>();
+            notify = std::move(pair.fulfiller);
+            signal = pair.promise.fork();
+        }
+
+        return signal.addBranch().exclusiveJoin(allPromises.addBranch()).then([this] {
+            return next();
+        });
+    }
+};
+
+/**
+  * Collect the results of a list of promises, in order of completion.
+  * Once any input promise is rejected all promises that have not been
+  * resolved or rejected will be cancelled and the exception rethrown.
+  */
+template<typename K, typename V>
+AsyncCollect<K, V> asyncCollect(kj::Array<std::pair<K, kj::Promise<V>>> promises)
+{
+    return AsyncCollect<K, V>(std::move(promises));
+}
+
+}

src/libutil/meson.build

@@ -53,6 +53,7 @@ libutil_headers = files(
   'archive.hh',
   'args/root.hh',
   'args.hh',
+  'async-collect.hh',
   'async-semaphore.hh',
   'backed-string-view.hh',
   'box_ptr.hh',

tests/unit/libutil/async-collect.cc

@@ -0,0 +1,104 @@
+#include "async-collect.hh"
+
+#include <gtest/gtest.h>
+#include <kj/array.h>
+#include <kj/async.h>
+#include <kj/exception.h>
+#include <stdexcept>
+
+namespace nix {
+
+TEST(AsyncCollect, void)
+{
+    kj::EventLoop loop;
+    kj::WaitScope waitScope(loop);
+
+    auto a = kj::newPromiseAndFulfiller<void>();
+    auto b = kj::newPromiseAndFulfiller<void>();
+    auto c = kj::newPromiseAndFulfiller<void>();
+    auto d = kj::newPromiseAndFulfiller<void>();
+
+    auto collect = asyncCollect(kj::arr(
+        std::pair(1, std::move(a.promise)),
+        std::pair(2, std::move(b.promise)),
+        std::pair(3, std::move(c.promise)),
+        std::pair(4, std::move(d.promise))
+    ));
+
+    auto p = collect.next();
+    ASSERT_FALSE(p.poll(waitScope));
+
+    // collection is ordered
+    c.fulfiller->fulfill();
+    b.fulfiller->fulfill();
+
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_EQ(p.wait(waitScope), 3);
+
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_EQ(p.wait(waitScope), 2);
+
+    p = collect.next();
+    ASSERT_FALSE(p.poll(waitScope));
+
+    // exceptions propagate
+    a.fulfiller->rejectIfThrows([] { throw std::runtime_error("test"); });
+
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_THROW(p.wait(waitScope), kj::Exception);
+
+    // first exception aborts collection
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_THROW(p.wait(waitScope), kj::Exception);
+}
+
+TEST(AsyncCollect, nonVoid)
+{
+    kj::EventLoop loop;
+    kj::WaitScope waitScope(loop);
+
+    auto a = kj::newPromiseAndFulfiller<int>();
+    auto b = kj::newPromiseAndFulfiller<int>();
+    auto c = kj::newPromiseAndFulfiller<int>();
+    auto d = kj::newPromiseAndFulfiller<int>();
+
+    auto collect = asyncCollect(kj::arr(
+        std::pair(1, std::move(a.promise)),
+        std::pair(2, std::move(b.promise)),
+        std::pair(3, std::move(c.promise)),
+        std::pair(4, std::move(d.promise))
+    ));
+
+    auto p = collect.next();
+    ASSERT_FALSE(p.poll(waitScope));
+
+    // collection is ordered
+    c.fulfiller->fulfill(1);
+    b.fulfiller->fulfill(2);
+
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_EQ(p.wait(waitScope), std::pair(3, 1));
+
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_EQ(p.wait(waitScope), std::pair(2, 2));
+
+    p = collect.next();
+    ASSERT_FALSE(p.poll(waitScope));
+
+    // exceptions propagate
+    a.fulfiller->rejectIfThrows([] { throw std::runtime_error("test"); });
+
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_THROW(p.wait(waitScope), kj::Exception);
+
+    // first exception aborts collection
+    p = collect.next();
+    ASSERT_TRUE(p.poll(waitScope));
+    ASSERT_THROW(p.wait(waitScope), kj::Exception);
+}
+}

tests/unit/meson.build

@@ -39,6 +39,7 @@ liblixutil_test_support = declare_dependency(
 )
 
 libutil_tests_sources = files(
+  'libutil/async-collect.cc',
   'libutil/async-semaphore.cc',
   'libutil/canon-path.cc',
   'libutil/checked-arithmetic.cc',