11 files changed, 272 insertions, 254 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 827c9f541..f40611b31 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -11,7 +11,10 @@
 #include "drv-output-substitution-goal.hh"
 #include "strings.hh"
 
+#include <boost/outcome/try.hpp>
 #include <fstream>
+#include <kj/async-unix.h>
+#include <kj/debug.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/un.h>
@@ -780,7 +783,7 @@ try {
                     buildResult.startTime = time(0); // inexact
                     state = &DerivationGoal::buildDone;
                     started();
-                    return WaitForWorld{std::move(a.fds), false};
+                    return WaitForWorld{std::move(a.promise), false};
                 },
                 [&](HookReply::Postpone) -> std::optional<WorkResult> {
                     /* Not now; wait until at least one child finishes or
@@ -992,9 +995,6 @@ try {
     buildResult.timesBuilt++;
     buildResult.stopTime = time(0);
 
-    /* So the child is gone now. */
-    worker.childTerminated(this);
-
     /* Close the read side of the logger pipe. */
     closeReadPipes();
 
@@ -1266,12 +1266,8 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
     /* Create the log file and pipe. */
     Path logFile = openLogFile();
 
-    std::set<int> fds;
-    fds.insert(hook->fromHook.get());
-    fds.insert(hook->builderOut.get());
     builderOutFD = &hook->builderOut;
-
-    return HookReply::Accept{std::move(fds)};
+    return HookReply::Accept{handleChildOutput()};
 }
 
 
@@ -1331,23 +1327,69 @@ void DerivationGoal::closeLogFile()
 }
 
 
-Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data)
+Goal::Finished DerivationGoal::tooMuchLogs()
 {
-    assert(builderOutFD);
+    killChild();
+    return done(
+        BuildResult::LogLimitExceeded, {},
+        Error("%s killed after writing more than %d bytes of log output",
+            getName(), settings.maxLogSize));
+}
 
-    auto tooMuchLogs = [&] {
-        killChild();
-        return done(
-            BuildResult::LogLimitExceeded, {},
-            Error("%s killed after writing more than %d bytes of log output",
-                getName(), settings.maxLogSize));
-    };
+struct DerivationGoal::InputStream final : private kj::AsyncObject
+{
+    int fd;
+    kj::UnixEventPort::FdObserver observer;
+
+    InputStream(kj::UnixEventPort & ep, int fd)
+        : fd(fd)
+        , observer(ep, fd, kj::UnixEventPort::FdObserver::OBSERVE_READ)
+    {
+        int flags = fcntl(fd, F_GETFL);
+        if (flags < 0) {
+            throw SysError("fcntl(F_GETFL) failed on fd %i", fd);
+        }
+        if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+            throw SysError("fcntl(F_SETFL) failed on fd %i", fd);
+        }
+    }
+
+    kj::Promise<std::string_view> read(kj::ArrayPtr<char> buffer)
+    {
+        const auto res = ::read(fd, buffer.begin(), buffer.size());
+        // closing a pty endpoint causes EIO on the other endpoint. stock kj streams
+        // do not handle this and throw exceptions we can't ask for errno instead :(
+        // (we can't use `errno` either because kj may well have mangled it by now.)
+        if (res == 0 || (res == -1 && errno == EIO)) {
+            return std::string_view{};
+        }
+
+        KJ_NONBLOCKING_SYSCALL(res) {}
+
+        if (res > 0) {
+            return std::string_view{buffer.begin(), static_cast<size_t>(res)};
+        }
+
+        return observer.whenBecomesReadable().then([this, buffer] {
+            return read(buffer);
+        });
+    }
+};
+
+kj::Promise<Outcome<void, Goal::Finished>> DerivationGoal::handleBuilderOutput(InputStream & in) noexcept
+try {
+    auto buf = kj::heapArray<char>(4096);
+    while (true) {
+        auto data = co_await in.read(buf);
+        lastChildActivity = worker.aio.provider->getTimer().now();
+
+        if (data.empty()) {
+            co_return result::success();
+        }
 
-    // local & `ssh://`-builds are dealt with here.
-    if (fd == builderOutFD->get()) {
         logSize += data.size();
         if (settings.maxLogSize && logSize > settings.maxLogSize) {
-            return tooMuchLogs();
+            co_return tooMuchLogs();
         }
 
         for (auto c : data)
@@ -1362,10 +1404,22 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
             }
 
         if (logSink) (*logSink)(data);
-        return StillAlive{};
     }
+} catch (...) {
+    co_return std::current_exception();
+}
+
+kj::Promise<Outcome<void, Goal::Finished>> DerivationGoal::handleHookOutput(InputStream & in) noexcept
+try {
+    auto buf = kj::heapArray<char>(4096);
+    while (true) {
+        auto data = co_await in.read(buf);
+        lastChildActivity = worker.aio.provider->getTimer().now();
+
+        if (data.empty()) {
+            co_return result::success();
+        }
 
-    if (hook && fd == hook->fromHook.get()) {
         for (auto c : data)
             if (c == '\n') {
                 auto json = parseJSONMessage(currentHookLine);
@@ -1381,7 +1435,7 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
                                 (fields.size() > 0 ? fields[0].get<std::string>() : "") + "\n";
                             logSize += logLine.size();
                             if (settings.maxLogSize && logSize > settings.maxLogSize) {
-                                return tooMuchLogs();
+                                co_return tooMuchLogs();
                             }
                             (*logSink)(logLine);
                         } else if (type == resSetPhase && ! fields.is_null()) {
@@ -1405,16 +1459,83 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
             } else
                 currentHookLine += c;
     }
-
-    return StillAlive{};
+} catch (...) {
+    co_return std::current_exception();
 }
 
+kj::Promise<Outcome<void, Goal::Finished>> DerivationGoal::handleChildOutput() noexcept
+try {
+    assert(builderOutFD);
+
+    auto builderIn = kj::heap<InputStream>(worker.aio.unixEventPort, builderOutFD->get());
+    kj::Own<InputStream> hookIn;
+    if (hook) {
+        hookIn = kj::heap<InputStream>(worker.aio.unixEventPort, hook->fromHook.get());
+    }
+
+    auto handlers = handleChildStreams(*builderIn, hookIn.get()).attach(std::move(builderIn), std::move(hookIn));
 
-void DerivationGoal::handleEOF(int fd)
+    if (respectsTimeouts() && settings.buildTimeout != 0) {
+        handlers = handlers.exclusiveJoin(
+            worker.aio.provider->getTimer()
+                .afterDelay(settings.buildTimeout.get() * kj::SECONDS)
+                .then([this]() -> Outcome<void, Finished> {
+                    return timedOut(
+                        Error("%1% timed out after %2% seconds", name, settings.buildTimeout)
+                    );
+                })
+        );
+    }
+
+    return handlers.then([this](auto r) -> Outcome<void, Finished> {
+        if (!currentLogLine.empty()) flushLine();
+        return r;
+    });
+} catch (...) {
+    return {std::current_exception()};
+}
+
+kj::Promise<Outcome<void, Goal::Finished>> DerivationGoal::monitorForSilence() noexcept
 {
-    if (!currentLogLine.empty()) flushLine();
+    while (true) {
+        const auto stash = lastChildActivity;
+        auto waitUntil = lastChildActivity + settings.maxSilentTime.get() * kj::SECONDS;
+        co_await worker.aio.provider->getTimer().atTime(waitUntil);
+        if (lastChildActivity == stash) {
+            co_return timedOut(
+                Error("%1% timed out after %2% seconds of silence", name, settings.maxSilentTime)
+            );
+        }
+    }
 }
 
+kj::Promise<Outcome<void, Goal::Finished>>
+DerivationGoal::handleChildStreams(InputStream & builderIn, InputStream * hookIn) noexcept
+{
+    lastChildActivity = worker.aio.provider->getTimer().now();
+
+    auto handlers = kj::joinPromisesFailFast([&] {
+        kj::Vector<kj::Promise<Outcome<void, Finished>>> parts{2};
+
+        parts.add(handleBuilderOutput(builderIn));
+        if (hookIn) {
+            parts.add(handleHookOutput(*hookIn));
+        }
+
+        return parts.releaseAsArray();
+    }());
+
+    if (respectsTimeouts() && settings.maxSilentTime != 0) {
+        handlers = handlers.exclusiveJoin(monitorForSilence().then([](auto r) {
+            return kj::arr(std::move(r));
+        }));
+    }
+
+    for (auto r : co_await handlers) {
+        BOOST_OUTCOME_CO_TRYV(r);
+    }
+    co_return result::success();
+}
 
 void DerivationGoal::flushLine()
 {
diff --git a/src/libstore/build/derivation-goal.hh b/src/libstore/build/derivation-goal.hh
index 020388d5a..46b07fc0b 100644
--- a/src/libstore/build/derivation-goal.hh
+++ b/src/libstore/build/derivation-goal.hh
@@ -8,6 +8,7 @@
 #include "store-api.hh"
 #include "pathlocks.hh"
 #include "goal.hh"
+#include <kj/time.h>
 
 namespace nix {
 
@@ -17,7 +18,7 @@ struct HookInstance;
 
 struct HookReplyBase {
     struct [[nodiscard]] Accept {
-        std::set<int> fds;
+        kj::Promise<Outcome<void, Goal::Finished>> promise;
     };
     struct [[nodiscard]] Decline {};
     struct [[nodiscard]] Postpone {};
@@ -70,6 +71,8 @@ struct InitialOutput {
  */
 struct DerivationGoal : public Goal
 {
+    struct InputStream;
+
     /**
      * Whether to use an on-disk .drv file.
      */
@@ -242,7 +245,7 @@ struct DerivationGoal : public Goal
         BuildMode buildMode = bmNormal);
     virtual ~DerivationGoal() noexcept(false);
 
-    Finished timedOut(Error && ex) override;
+    Finished timedOut(Error && ex);
 
     std::string key() override;
 
@@ -312,13 +315,19 @@ struct DerivationGoal : public Goal
     virtual void cleanupPostOutputsRegisteredModeCheck();
     virtual void cleanupPostOutputsRegisteredModeNonCheck();
 
-    /**
-     * Callback used by the worker to write to the log.
-     */
-    WorkResult handleChildOutput(int fd, std::string_view data) override;
-    void handleEOF(int fd) override;
+protected:
+    kj::TimePoint lastChildActivity = kj::minValue;
+
+    kj::Promise<Outcome<void, Finished>> handleChildOutput() noexcept;
+    kj::Promise<Outcome<void, Finished>>
+    handleChildStreams(InputStream & builderIn, InputStream * hookIn) noexcept;
+    kj::Promise<Outcome<void, Finished>> handleBuilderOutput(InputStream & in) noexcept;
+    kj::Promise<Outcome<void, Finished>> handleHookOutput(InputStream & in) noexcept;
+    kj::Promise<Outcome<void, Finished>> monitorForSilence() noexcept;
+    Finished tooMuchLogs();
     void flushLine();
 
+public:
     /**
      * Wrappers around the corresponding Store methods that first consult the
      * derivation.  This is currently needed because when there is no drv file
@@ -357,6 +366,11 @@ struct DerivationGoal : public Goal
 
     void waiteeDone(GoalPtr waitee) override;
 
+    virtual bool respectsTimeouts()
+    {
+        return false;
+    }
+
     StorePathSet exportReferences(const StorePathSet & storePaths);
 
     JobCategory jobCategory() const override {
diff --git a/src/libstore/build/drv-output-substitution-goal.cc b/src/libstore/build/drv-output-substitution-goal.cc
index 7986123cc..fdee53699 100644
--- a/src/libstore/build/drv-output-substitution-goal.cc
+++ b/src/libstore/build/drv-output-substitution-goal.cc
@@ -69,24 +69,26 @@ try {
        some other error occurs), so it must not touch `this`. So put
        the shared state in a separate refcounted object. */
     downloadState = std::make_shared<DownloadState>();
-    downloadState->outPipe.create();
+    auto pipe = kj::newPromiseAndCrossThreadFulfiller<void>();
+    downloadState->outPipe = kj::mv(pipe.fulfiller);
 
     downloadState->result =
         std::async(std::launch::async, [downloadState{downloadState}, id{id}, sub{sub}] {
+            Finally updateStats([&]() { downloadState->outPipe->fulfill(); });
             ReceiveInterrupts receiveInterrupts;
-            Finally updateStats([&]() { downloadState->outPipe.writeSide.close(); });
             return sub->queryRealisation(id);
         });
 
     state = &DrvOutputSubstitutionGoal::realisationFetched;
-    return {WaitForWorld{{downloadState->outPipe.readSide.get()}, true}};
+    return {WaitForWorld{
+        pipe.promise.then([]() -> Outcome<void, Finished> { return result::success(); }), true
+    }};
 } catch (...) {
     return {std::current_exception()};
 }
 
 kj::Promise<Result<Goal::WorkResult>> DrvOutputSubstitutionGoal::realisationFetched(bool inBuildSlot) noexcept
 try {
-    worker.childTerminated(this);
     maintainRunningSubstitutions.reset();
 
     try {
diff --git a/src/libstore/build/drv-output-substitution-goal.hh b/src/libstore/build/drv-output-substitution-goal.hh
index f33196665..a35bf67ee 100644
--- a/src/libstore/build/drv-output-substitution-goal.hh
+++ b/src/libstore/build/drv-output-substitution-goal.hh
@@ -45,7 +45,7 @@ class DrvOutputSubstitutionGoal : public Goal {
 
     struct DownloadState
     {
-        Pipe outPipe;
+        kj::Own<kj::CrossThreadPromiseFulfiller<void>> outPipe;
         std::future<std::shared_ptr<const Realisation>> result;
     };
 
@@ -74,8 +74,6 @@ public:
     kj::Promise<Result<WorkResult>> outPathValid(bool inBuildSlot) noexcept;
     kj::Promise<Result<WorkResult>> finished() noexcept;
 
-    Finished timedOut(Error && ex) override { abort(); };
-
     std::string key() override;
 
     kj::Promise<Result<WorkResult>> work(bool inBuildSlot) noexcept override;
diff --git a/src/libstore/build/goal.hh b/src/libstore/build/goal.hh
index 189505308..3f6e8396e 100644
--- a/src/libstore/build/goal.hh
+++ b/src/libstore/build/goal.hh
@@ -114,6 +114,8 @@ struct Goal
 
 public:
 
+    struct Finished;
+
     struct [[nodiscard]] StillAlive {};
     struct [[nodiscard]] WaitForSlot {};
     struct [[nodiscard]] WaitForAWhile {};
@@ -122,7 +124,7 @@ public:
         Goals goals;
     };
     struct [[nodiscard]] WaitForWorld {
-        std::set<int> fds;
+        kj::Promise<Outcome<void, Finished>> promise;
         bool inBuildSlot;
     };
     struct [[nodiscard]] Finished {
@@ -167,20 +169,6 @@ public:
 
     virtual void waiteeDone(GoalPtr waitee) { }
 
-    virtual WorkResult handleChildOutput(int fd, std::string_view data)
-    {
-        abort();
-    }
-
-    virtual void handleEOF(int fd)
-    {
-    }
-
-    virtual bool respectsTimeouts()
-    {
-        return false;
-    }
-
     void trace(std::string_view s);
 
     std::string getName() const
@@ -188,13 +176,6 @@ public:
         return name;
     }
 
-    /**
-     * Callback in case of a timeout.  It should wake up its waiters,
-     * get rid of any running child processes that are being monitored
-     * by the worker (important!), etc.
-     */
-    virtual Finished timedOut(Error && ex) = 0;
-
     virtual std::string key() = 0;
 
     virtual void cleanup() { }
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index f14d09652..040fa7461 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -121,8 +121,6 @@ LocalStore & LocalDerivationGoal::getLocalStore()
 void LocalDerivationGoal::killChild()
 {
     if (pid) {
-        worker.childTerminated(this);
-
         /* If we're using a build user, then there is a tricky race
            condition: if we kill the build user before the child has
            done its setuid() to the build user uid, then it won't be
@@ -243,14 +241,14 @@ try {
     try {
 
         /* Okay, we have to build. */
-        auto fds = startBuilder();
+        auto promise = startBuilder();
 
         /* This state will be reached when we get EOF on the child's
            log pipe. */
         state = &DerivationGoal::buildDone;
 
         started();
-        return {WaitForWorld{std::move(fds), true}};
+        return {WaitForWorld{std::move(promise), true}};
 
     } catch (BuildError & e) {
         outputLocks.unlock();
@@ -390,7 +388,9 @@ void LocalDerivationGoal::cleanupPostOutputsRegisteredModeNonCheck()
     cleanupPostOutputsRegisteredModeCheck();
 }
 
-std::set<int> LocalDerivationGoal::startBuilder()
+// NOTE this one isn't noexcept because it's called from places that expect
+// exceptions to signal failure to launch. we should change this some time.
+kj::Promise<Outcome<void, Goal::Finished>> LocalDerivationGoal::startBuilder()
 {
     if ((buildUser && buildUser->getUIDCount() != 1)
         #if __linux__
@@ -779,7 +779,7 @@ std::set<int> LocalDerivationGoal::startBuilder()
         msgs.push_back(std::move(msg));
     }
 
-    return {builderOutPTY.get()};
+    return handleChildOutput();
 }
 
 
diff --git a/src/libstore/build/local-derivation-goal.hh b/src/libstore/build/local-derivation-goal.hh
index cd040bc15..6239129ab 100644
--- a/src/libstore/build/local-derivation-goal.hh
+++ b/src/libstore/build/local-derivation-goal.hh
@@ -218,7 +218,7 @@ struct LocalDerivationGoal : public DerivationGoal
     /**
      * Start building a derivation.
      */
-    std::set<int> startBuilder();
+    kj::Promise<Outcome<void, Finished>> startBuilder();
 
     /**
      * Fill in the environment for the builder.
diff --git a/src/libstore/build/substitution-goal.cc b/src/libstore/build/substitution-goal.cc
index bd0ffcb9b..058f858d4 100644
--- a/src/libstore/build/substitution-goal.cc
+++ b/src/libstore/build/substitution-goal.cc
@@ -208,16 +208,17 @@ try {
 
     maintainRunningSubstitutions = worker.runningSubstitutions.addTemporarily(1);
 
-    outPipe.create();
+    auto pipe = kj::newPromiseAndCrossThreadFulfiller<void>();
+    outPipe = kj::mv(pipe.fulfiller);
 
     thr = std::async(std::launch::async, [this]() {
+        /* Wake up the worker loop when we're done. */
+        Finally updateStats([this]() { outPipe->fulfill(); });
+
         auto & fetchPath = subPath ? *subPath : storePath;
         try {
             ReceiveInterrupts receiveInterrupts;
 
-            /* Wake up the worker loop when we're done. */
-            Finally updateStats([this]() { outPipe.writeSide.close(); });
-
             Activity act(*logger, actSubstitute, Logger::Fields{worker.store.printStorePath(storePath), sub->getUri()});
             PushActivity pact(act.id);
 
@@ -234,7 +235,9 @@ try {
     });
 
     state = &PathSubstitutionGoal::finished;
-    return {WaitForWorld{{outPipe.readSide.get()}, true}};
+    return {WaitForWorld{
+        pipe.promise.then([]() -> Outcome<void, Finished> { return result::success(); }), true
+    }};
 } catch (...) {
     return {std::current_exception()};
 }
@@ -244,8 +247,6 @@ kj::Promise<Result<Goal::WorkResult>> PathSubstitutionGoal::finished(bool inBuil
 try {
     trace("substitute finished");
 
-    worker.childTerminated(this);
-
     try {
         thr.get();
     } catch (std::exception & e) {
@@ -288,22 +289,13 @@ try {
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::handleChildOutput(int fd, std::string_view data)
-{
-    return StillAlive{};
-}
-
-
 void PathSubstitutionGoal::cleanup()
 {
     try {
         if (thr.valid()) {
             // FIXME: signal worker thread to quit.
             thr.get();
-            worker.childTerminated(this);
         }
-
-        outPipe.close();
     } catch (...) {
         ignoreException();
     }
diff --git a/src/libstore/build/substitution-goal.hh b/src/libstore/build/substitution-goal.hh
index 3c97b19fd..91e256fd7 100644
--- a/src/libstore/build/substitution-goal.hh
+++ b/src/libstore/build/substitution-goal.hh
@@ -46,7 +46,7 @@ struct PathSubstitutionGoal : public Goal
     /**
      * Pipe for the substituter's standard output.
      */
-    Pipe outPipe;
+    kj::Own<kj::CrossThreadPromiseFulfiller<void>> outPipe;
 
     /**
      * The substituter thread.
@@ -90,8 +90,6 @@ public:
     );
     ~PathSubstitutionGoal();
 
-    Finished timedOut(Error && ex) override { abort(); };
-
     /**
      * We prepend "a$" to the key name to ensure substitution goals
      * happen before derivation goals.
@@ -112,11 +110,6 @@ public:
     kj::Promise<Result<WorkResult>> tryToRun(bool inBuildSlot) noexcept;
     kj::Promise<Result<WorkResult>> finished(bool inBuildSlot) noexcept;
 
-    /**
-     * Callback used by the worker to write to the log.
-     */
-    WorkResult handleChildOutput(int fd, std::string_view data) override;
-
     /* Called by destructor, can't be overridden */
     void cleanup() override final;
 
diff --git a/src/libstore/build/worker.cc b/src/libstore/build/worker.cc
index ee45c7e3f..284adbc50 100644
--- a/src/libstore/build/worker.cc
+++ b/src/libstore/build/worker.cc
@@ -7,10 +7,19 @@
 #include "signals.hh"
 #include "hook-instance.hh" // IWYU pragma: keep
 
-#include <poll.h>
-
 namespace nix {
 
+namespace {
+struct ErrorHandler : kj::TaskSet::ErrorHandler
+{
+    void taskFailed(kj::Exception && e) override
+    {
+        printError("unexpected async failure in Worker: %s", kj::str(e).cStr());
+        abort();
+    }
+} errorHandler;
+}
+
 Worker::Worker(Store & store, Store & evalStore, kj::AsyncIoContext & aio)
     : act(*logger, actRealise)
     , actDerivations(*logger, actBuilds)
@@ -18,6 +27,7 @@ Worker::Worker(Store & store, Store & evalStore, kj::AsyncIoContext & aio)
     , store(store)
     , evalStore(evalStore)
     , aio(aio)
+    , children(errorHandler)
 {
     /* Debugging: prevent recursive workers. */
     nrLocalBuilds = 0;
@@ -33,6 +43,7 @@ Worker::~Worker()
        are in trouble, since goals may call childTerminated() etc. in
        their destructors). */
     topGoals.clear();
+    children.clear();
 
     assert(expectedSubstitutions == 0);
     assert(expectedDownloadSize == 0);
@@ -209,7 +220,9 @@ void Worker::handleWorkResult(GoalPtr goal, Goal::WorkResult how)
                     dep->waiters.insert(goal);
                 }
             },
-            [&](Goal::WaitForWorld & w) { childStarted(goal, w.fds, w.inBuildSlot); },
+            [&](Goal::WaitForWorld & w) {
+                childStarted(goal, std::move(w.promise), w.inBuildSlot);
+            },
             [&](Goal::Finished & f) { goalFinished(goal, f); },
         },
         how
@@ -244,16 +257,22 @@ void Worker::wakeUp(GoalPtr goal)
 }
 
 
-void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
+void Worker::childStarted(GoalPtr goal, kj::Promise<Outcome<void, Goal::Finished>> promise,
     bool inBuildSlot)
 {
-    Child child;
-    child.goal = goal;
-    child.goal2 = goal.get();
-    child.fds = fds;
-    child.timeStarted = child.lastOutput = steady_time_point::clock::now();
-    child.inBuildSlot = inBuildSlot;
-    children.emplace_back(child);
+    children.add(promise
+        .then([this, goal](auto result) {
+            if (result.has_value()) {
+                handleWorkResult(goal, Goal::ContinueImmediately{});
+            } else if (result.has_error()) {
+                handleWorkResult(goal, std::move(result.assume_error()));
+            } else {
+                childException = result.assume_exception();
+            }
+        })
+        .attach(Finally{[this, goal, inBuildSlot] {
+            childTerminated(goal, inBuildSlot);
+        }}));
     if (inBuildSlot) {
         switch (goal->jobCategory()) {
         case JobCategory::Substitution:
@@ -269,13 +288,13 @@ void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
 }
 
 
-void Worker::childTerminated(Goal * goal)
+void Worker::childTerminated(GoalPtr goal, bool inBuildSlot)
 {
-    auto i = std::find_if(children.begin(), children.end(),
-        [&](const Child & child) { return child.goal2 == goal; });
-    if (i == children.end()) return;
+    if (childFinished) {
+        childFinished->fulfill();
+    }
 
-    if (i->inBuildSlot) {
+    if (inBuildSlot) {
         switch (goal->jobCategory()) {
         case JobCategory::Substitution:
             assert(nrSubstitutions > 0);
@@ -290,8 +309,6 @@ void Worker::childTerminated(Goal * goal)
         }
     }
 
-    children.erase(i);
-
     /* Wake up goals waiting for a build slot. */
     for (auto & j : wantingToBuild) {
         GoalPtr goal = j.lock();
@@ -390,11 +407,15 @@ Goals Worker::run(std::function<Goals (GoalFactory &)> req)
         if (topGoals.empty()) break;
 
         /* Wait for input. */
-        if (!children.empty() || !waitingForAWhile.empty())
+        if (!children.isEmpty() || !waitingForAWhile.empty())
             waitForInput();
         else {
             assert(!awake.empty());
         }
+
+        if (childException) {
+            std::rethrow_exception(childException);
+        }
     }
 
     /* If --keep-going is not set, it's possible that the main goal
@@ -402,7 +423,7 @@ Goals Worker::run(std::function<Goals (GoalFactory &)> req)
        --keep-going *is* set, then they must all be finished now. */
     assert(!settings.keepGoing || awake.empty());
     assert(!settings.keepGoing || wantingToBuild.empty());
-    assert(!settings.keepGoing || children.empty());
+    assert(!settings.keepGoing || children.isEmpty());
 
     return _topGoals;
 }
@@ -411,139 +432,51 @@ void Worker::waitForInput()
 {
     printMsg(lvlVomit, "waiting for children");
 
+    auto childFinished = [&]{
+        auto pair = kj::newPromiseAndFulfiller<void>();
+        this->childFinished = kj::mv(pair.fulfiller);
+        return kj::mv(pair.promise);
+    }();
+
     /* Process output from the file descriptors attached to the
        children, namely log output and output path creation commands.
        We also use this to detect child termination: if we get EOF on
        the logger pipe of a build, we assume that the builder has
        terminated. */
 
-    bool useTimeout = false;
-    long timeout = 0;
+    std::optional<long> timeout = 0;
     auto before = steady_time_point::clock::now();
 
-    /* If we're monitoring for silence on stdout/stderr, or if there
-       is a build timeout, then wait for input until the first
-       deadline for any child. */
-    auto nearest = steady_time_point::max(); // nearest deadline
-    if (settings.minFree.get() != 0)
-        // Periodicallty wake up to see if we need to run the garbage collector.
-        nearest = before + std::chrono::seconds(10);
-    for (auto & i : children) {
-        if (auto goal = i.goal.lock()) {
-            if (!goal->respectsTimeouts()) continue;
-            if (0 != settings.maxSilentTime)
-                nearest = std::min(nearest, i.lastOutput + std::chrono::seconds(settings.maxSilentTime));
-            if (0 != settings.buildTimeout)
-                nearest = std::min(nearest, i.timeStarted + std::chrono::seconds(settings.buildTimeout));
-        }
-    }
-    if (nearest != steady_time_point::max()) {
-        timeout = std::max(1L, (long) std::chrono::duration_cast<std::chrono::seconds>(nearest - before).count());
-        useTimeout = true;
+    // Periodicallty wake up to see if we need to run the garbage collector.
+    if (settings.minFree.get() != 0) {
+        timeout = 10;
     }
 
     /* If we are polling goals that are waiting for a lock, then wake
        up after a few seconds at most. */
     if (!waitingForAWhile.empty()) {
-        useTimeout = true;
         if (lastWokenUp == steady_time_point::min() || lastWokenUp > before) lastWokenUp = before;
         timeout = std::max(1L,
             (long) std::chrono::duration_cast<std::chrono::seconds>(
                 lastWokenUp + std::chrono::seconds(settings.pollInterval) - before).count());
     } else lastWokenUp = steady_time_point::min();
 
-    if (useTimeout)
-        vomit("sleeping %d seconds", timeout);
-
-    /* Use select() to wait for the input side of any logger pipe to
-       become `available'.  Note that `available' (i.e., non-blocking)
-       includes EOF. */
-    std::vector<struct pollfd> pollStatus;
-    std::map<int, size_t> fdToPollStatus;
-    for (auto & i : children) {
-        for (auto & j : i.fds) {
-            pollStatus.push_back((struct pollfd) { .fd = j, .events = POLLIN });
-            fdToPollStatus[j] = pollStatus.size() - 1;
-        }
-    }
-
-    if (poll(pollStatus.data(), pollStatus.size(),
-            useTimeout ? timeout * 1000 : -1) == -1) {
-        if (errno == EINTR) return;
-        throw SysError("waiting for input");
-    }
-
-    auto after = steady_time_point::clock::now();
-
-    /* Process all available file descriptors. FIXME: this is
-       O(children * fds). */
-    decltype(children)::iterator i;
-    for (auto j = children.begin(); j != children.end(); j = i) {
-        i = std::next(j);
-
-        checkInterrupt();
+    if (timeout)
+        vomit("sleeping %d seconds", *timeout);
 
-        GoalPtr goal = j->goal.lock();
-        assert(goal);
-
-        if (!goal->exitCode.has_value() &&
-            0 != settings.maxSilentTime &&
-            goal->respectsTimeouts() &&
-            after - j->lastOutput >= std::chrono::seconds(settings.maxSilentTime))
-        {
-            handleWorkResult(
-                goal,
-                goal->timedOut(Error(
-                    "%1% timed out after %2% seconds of silence",
-                    goal->getName(),
-                    settings.maxSilentTime
-                ))
-            );
-            continue;
+    auto waitFor = [&] {
+        if (timeout) {
+            return aio.provider->getTimer()
+                .afterDelay(*timeout * kj::SECONDS)
+                .exclusiveJoin(kj::mv(childFinished));
+        } else {
+            return std::move(childFinished);
         }
+    }();
 
-        else if (!goal->exitCode.has_value() &&
-            0 != settings.buildTimeout &&
-            goal->respectsTimeouts() &&
-            after - j->timeStarted >= std::chrono::seconds(settings.buildTimeout))
-        {
-            handleWorkResult(
-                goal,
-                goal->timedOut(
-                    Error("%1% timed out after %2% seconds", goal->getName(), settings.buildTimeout)
-                )
-            );
-            continue;
-        }
+    waitFor.wait(aio.waitScope);
 
-        std::set<int> fds2(j->fds);
-        std::vector<unsigned char> buffer(4096);
-        for (auto & k : fds2) {
-            const auto fdPollStatusId = get(fdToPollStatus, k);
-            assert(fdPollStatusId);
-            assert(*fdPollStatusId < pollStatus.size());
-            if (pollStatus.at(*fdPollStatusId).revents) {
-                ssize_t rd = ::read(k, buffer.data(), buffer.size());
-                // FIXME: is there a cleaner way to handle pt close
-                // than EIO? Is this even standard?
-                if (rd == 0 || (rd == -1 && errno == EIO)) {
-                    debug("%1%: got EOF", goal->getName());
-                    goal->handleEOF(k);
-                    handleWorkResult(goal, Goal::ContinueImmediately{});
-                    j->fds.erase(k);
-                } else if (rd == -1) {
-                    if (errno != EINTR)
-                        throw SysError("%s: read failed", goal->getName());
-                } else {
-                    printMsg(lvlVomit, "%1%: read %2% bytes",
-                        goal->getName(), rd);
-                    std::string_view data(charptr_cast<char *>(buffer.data()), rd);
-                    j->lastOutput = after;
-                    handleWorkResult(goal, goal->handleChildOutput(k, data));
-                }
-            }
-        }
-    }
+    auto after = steady_time_point::clock::now();
 
     if (!waitingForAWhile.empty() && lastWokenUp + std::chrono::seconds(settings.pollInterval) <= after) {
         lastWokenUp = after;
diff --git a/src/libstore/build/worker.hh b/src/libstore/build/worker.hh
index 6735ea0b9..37d80ba7b 100644
--- a/src/libstore/build/worker.hh
+++ b/src/libstore/build/worker.hh
@@ -21,24 +21,6 @@ class DrvOutputSubstitutionGoal;
 
 typedef std::chrono::time_point<std::chrono::steady_clock> steady_time_point;
 
-/**
- * A mapping used to remember for each child process to what goal it
- * belongs, and file descriptors for receiving log data and output
- * path creation commands.
- */
-struct Child
-{
-    WeakGoalPtr goal;
-    Goal * goal2; // ugly hackery
-    std::set<int> fds;
-    bool inBuildSlot;
-    /**
-     * Time we last got output on stdout/stderr
-     */
-    steady_time_point lastOutput;
-    steady_time_point timeStarted;
-};
-
 /* Forward definition. */
 struct HookInstance;
 
@@ -117,11 +99,6 @@ private:
     WeakGoals wantingToBuild;
 
     /**
-     * Child processes currently running.
-     */
-    std::list<Child> children;
-
-    /**
      * Number of build slots occupied.  This includes local builds but does not
      * include substitutions or remote builds via the build hook.
      */
@@ -179,6 +156,8 @@ private:
     void goalFinished(GoalPtr goal, Goal::Finished & f);
     void handleWorkResult(GoalPtr goal, Goal::WorkResult how);
 
+    kj::Own<kj::PromiseFulfiller<void>> childFinished;
+
     /**
      * Put `goal` to sleep until a build slot becomes available (which
      * might be right away).
@@ -212,10 +191,15 @@ private:
      * Registers a running child process.  `inBuildSlot` means that
      * the process counts towards the jobs limit.
      */
-    void childStarted(GoalPtr goal, const std::set<int> & fds,
+    void childStarted(GoalPtr goal, kj::Promise<Outcome<void, Goal::Finished>> promise,
         bool inBuildSlot);
 
     /**
+     * Unregisters a running child process.
+     */
+    void childTerminated(GoalPtr goal, bool inBuildSlot);
+
+    /**
       * Pass current stats counters to the logger for progress bar updates.
       */
     void updateStatistics();
@@ -240,6 +224,11 @@ public:
     Store & evalStore;
     kj::AsyncIoContext & aio;
 
+private:
+    kj::TaskSet children;
+    std::exception_ptr childException;
+
+public:
     struct HookState {
         std::unique_ptr<HookInstance> instance;
 
@@ -303,11 +292,6 @@ private:
 
 public:
     /**
-     * Unregisters a running child process.
-     */
-    void childTerminated(Goal * goal);
-
-    /**
      * Loop until the specified top-level goals have finished.
      */
     Goals run(std::function<Goals (GoalFactory &)> req);