Use coroutines for worker child I/O

This will enable way more RAII going forward.

src/libstore/build/derivation-building-goal.cc

@@ -71,14 +71,6 @@ void DerivationBuildingGoal::killChild()
 #endif
 }
 
-void DerivationBuildingGoal::timedOut(TimedOut && ex)
-{
-    killChild();
-    // We're not inside a coroutine, hence we can't use co_return here.
-    // Thus we ignore the return value.
-    [[maybe_unused]] Done _ = doneFailure(std::move(ex));
-}
-
 std::string showKnownOutputs(const StoreDirConfig & store, const Derivation & drv)
 {
     std::string msg;
@@ -443,7 +435,20 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
     if (useHook) {
         buildResult.startTime = time(0); // inexact
         started();
-        co_await Suspend{};
+
+        while (true) {
+            auto event = co_await WaitForChildEvent{};
+            if (auto * output = std::get_if<ChildOutput>(&event)) {
+                co_await processChildOutput(output->fd, output->data);
+            } else if (std::get_if<ChildEOF>(&event)) {
+                if (!currentLogLine.empty())
+                    flushLine();
+                break;
+            } else if (auto * timeout = std::get_if<TimedOut>(&event)) {
+                killChild();
+                co_return doneFailure(std::move(*timeout));
+            }
+        }
 
 #ifndef _WIN32
         assert(hook);
@@ -664,7 +669,20 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
     worker.childStarted(shared_from_this(), {builderOut}, true, true);
 
     started();
-    co_await Suspend{};
+
+    while (true) {
+        auto event = co_await WaitForChildEvent{};
+        if (auto * output = std::get_if<ChildOutput>(&event)) {
+            co_await processChildOutput(output->fd, output->data);
+        } else if (std::get_if<ChildEOF>(&event)) {
+            if (!currentLogLine.empty())
+                flushLine();
+            break;
+        } else if (auto * timeout = std::get_if<TimedOut>(&event)) {
+            killChild();
+            co_return doneFailure(std::move(*timeout));
+        }
+    }
 
     trace("build done");
 
@@ -997,7 +1015,7 @@ bool DerivationBuildingGoal::isReadDesc(Descriptor fd)
 #endif
 }
 
-void DerivationBuildingGoal::handleChildOutput(Descriptor fd, std::string_view data)
+Goal::Co DerivationBuildingGoal::processChildOutput(Descriptor fd, std::string_view data)
 {
     // local & `ssh://`-builds are dealt with here.
     auto isWrittenToLog = isReadDesc(fd);
@@ -1005,14 +1023,11 @@ void DerivationBuildingGoal::handleChildOutput(Descriptor fd, std::string_view d
         logSize += data.size();
         if (settings.maxLogSize && logSize > settings.maxLogSize) {
             killChild();
-            // We're not inside a coroutine, hence we can't use co_return here.
-            // Thus we ignore the return value.
-            [[maybe_unused]] Done _ = doneFailure(BuildError(
+            co_return doneFailure(BuildError(
                 BuildResult::Failure::LogLimitExceeded,
                 "%s killed after writing more than %d bytes of log output",
                 getName(),
                 settings.maxLogSize));
-            return;
         }
 
         for (auto c : data)
@@ -1065,13 +1080,7 @@ void DerivationBuildingGoal::handleChildOutput(Descriptor fd, std::string_view d
                 currentHookLine += c;
     }
 #endif
-}
-
-void DerivationBuildingGoal::handleEOF(Descriptor fd)
-{
-    if (!currentLogLine.empty())
-        flushLine();
-    worker.wakeUp(shared_from_this());
+    co_return Return{};
 }
 
 void DerivationBuildingGoal::flushLine()

src/libstore/build/drv-output-substitution-goal.cc

@@ -66,7 +66,16 @@ Goal::Co DrvOutputSubstitutionGoal::init()
             true,
             false);
 
-        co_await Suspend{};
+        while (true) {
+            auto event = co_await WaitForChildEvent{};
+            if (std::get_if<ChildOutput>(&event)) {
+                // Doesn't process child output
+            } else if (std::get_if<ChildEOF>(&event)) {
+                break;
+            } else if (std::get_if<TimedOut>(&event)) {
+                unreachable();
+            }
+        }
 
         worker.childTerminated(this);
 
@@ -149,9 +158,4 @@ std::string DrvOutputSubstitutionGoal::key()
     return "a$" + std::string(id.to_string());
 }
 
-void DrvOutputSubstitutionGoal::handleEOF(Descriptor fd)
-{
-    worker.wakeUp(shared_from_this());
-}
-
 } // namespace nix

src/libstore/build/goal.cc

@@ -12,6 +12,50 @@ TimedOut::TimedOut(time_t maxDuration)
 
 using Co = nix::Goal::Co;
 using promise_type = nix::Goal::promise_type;
+using ChildEvents = decltype(promise_type::childEvents);
+
+void ChildEvents::pushChildEvent(ChildOutput event)
+{
+    if (childTimeout)
+        return; // Already timed out, ignore
+    childOutputs.push(std::move(event));
+}
+
+void ChildEvents::pushChildEvent(ChildEOF event)
+{
+    if (childTimeout)
+        return; // Already timed out, ignore
+    assert(!childEOF);
+    childEOF = std::move(event);
+}
+
+void ChildEvents::pushChildEvent(TimedOut event)
+{
+    // Timeout is immediate - flush pending events
+    childOutputs = {};
+    childEOF.reset();
+    childTimeout = std::move(event);
+}
+
+bool ChildEvents::hasChildEvent() const
+{
+    return !childOutputs.empty() || childEOF || childTimeout;
+}
+
+Goal::ChildEvent ChildEvents::popChildEvent()
+{
+    if (!childOutputs.empty()) {
+        auto event = std::move(childOutputs.front());
+        childOutputs.pop();
+        return event;
+    }
+    if (childEOF)
+        return *std::exchange(childEOF, std::nullopt);
+    if (childTimeout)
+        return *std::exchange(childTimeout, std::nullopt);
+    unreachable();
+}
+
 using handle_type = nix::Goal::handle_type;
 using Suspend = nix::Goal::Suspend;
 
@@ -212,6 +256,27 @@ void Goal::work()
     assert(top_co || exitCode != ecBusy);
 }
 
+void Goal::handleChildOutput(Descriptor fd, std::string_view data)
+{
+    assert(top_co);
+    top_co->handle.promise().childEvents.pushChildEvent(ChildOutput{fd, std::string{data}});
+    worker.wakeUp(shared_from_this());
+}
+
+void Goal::handleEOF(Descriptor fd)
+{
+    assert(top_co);
+    top_co->handle.promise().childEvents.pushChildEvent(ChildEOF{fd});
+    worker.wakeUp(shared_from_this());
+}
+
+void Goal::timedOut(TimedOut && ex)
+{
+    assert(top_co);
+    top_co->handle.promise().childEvents.pushChildEvent(std::move(ex));
+    worker.wakeUp(shared_from_this());
+}
+
 Goal::Co Goal::yield()
 {
     worker.wakeUp(shared_from_this());

src/libstore/build/substitution-goal.cc

@@ -258,7 +258,16 @@ Goal::Co PathSubstitutionGoal::tryToRun(
         true,
         false);
 
-    co_await Suspend{};
+    while (true) {
+        auto event = co_await WaitForChildEvent{};
+        if (std::get_if<ChildOutput>(&event)) {
+            // Substitution doesn't process child output
+        } else if (std::get_if<ChildEOF>(&event)) {
+            break;
+        } else if (std::get_if<TimedOut>(&event)) {
+            unreachable(); // Substitution doesn't use timeouts
+        }
+    }
 
     trace("substitute finished");
 
@@ -310,11 +319,6 @@ Goal::Co PathSubstitutionGoal::tryToRun(
     co_return doneSuccess(BuildResult::Success::Substituted);
 }
 
-void PathSubstitutionGoal::handleEOF(Descriptor fd)
-{
-    worker.wakeUp(shared_from_this());
-}
-
 void PathSubstitutionGoal::cleanup()
 {
     try {

src/libstore/include/nix/store/build/derivation-building-goal.hh

@@ -100,8 +100,6 @@ private:
 
     std::map<ActivityId, Activity> builderActivities;
 
-    void timedOut(TimedOut && ex) override;
-
     std::string key() override;
 
     /**
@@ -129,10 +127,10 @@ private:
     bool isReadDesc(Descriptor fd);
 
     /**
-     * Callback used by the worker to write to the log.
+     * Process output from a child process.
      */
-    void handleChildOutput(Descriptor fd, std::string_view data) override;
-    void handleEOF(Descriptor fd) override;
+    Co processChildOutput(Descriptor fd, std::string_view data);
+
     void flushLine();
 
     /**

src/libstore/include/nix/store/build/derivation-goal.hh

@@ -52,11 +52,6 @@ struct DerivationGoal : public Goal
         bool storeDerivation);
     ~DerivationGoal() = default;
 
-    void timedOut(TimedOut && ex) override
-    {
-        unreachable();
-    };
-
     std::string key() override;
 
     JobCategory jobCategory() const override

src/libstore/include/nix/store/build/derivation-resolution-goal.hh

@@ -43,8 +43,6 @@ struct DerivationResolutionGoal : public Goal
      */
     std::unique_ptr<std::pair<StorePath, BasicDerivation>> resolvedDrv;
 
-    void timedOut(TimedOut && ex) override {}
-
 private:
 
     /**

src/libstore/include/nix/store/build/derivation-trampoline-goal.hh

@@ -109,8 +109,6 @@ struct DerivationTrampolineGoal : public Goal
 
     virtual ~DerivationTrampolineGoal();
 
-    void timedOut(TimedOut && ex) override {}
-
     std::string key() override;
 
     JobCategory jobCategory() const override

src/libstore/include/nix/store/build/drv-output-substitution-goal.hh

@@ -33,15 +33,8 @@ public:
 
     Co init();
 
-    void timedOut(TimedOut && ex) override
-    {
-        unreachable();
-    };
-
     std::string key() override;
 
-    void handleEOF(Descriptor fd) override;
-
     JobCategory jobCategory() const override
     {
         return JobCategory::Substitution;

src/libstore/include/nix/store/build/goal.hh

@@ -5,6 +5,8 @@
 #include "nix/store/build-result.hh"
 
 #include <coroutine>
+#include <queue>
+#include <variant>
 
 namespace nix {
 
@@ -145,6 +147,29 @@ public:
         friend Goal;
     };
 
+    /**
+     * Event types for child process communication, delivered via coroutines.
+     */
+    struct ChildOutput
+    {
+        Descriptor fd;
+        std::string data;
+    };
+
+    struct ChildEOF
+    {
+        Descriptor fd;
+    };
+
+    using ChildEvent = std::variant<ChildOutput, ChildEOF, TimedOut>;
+
+    /**
+     * Tag type for `co_await`-ing child events.
+     * Returns a `ChildEvent` when resumed.
+     */
+    struct WaitForChildEvent
+    {};
+
     // forward declaration of promise_type, see below
     struct promise_type;
 
@@ -283,6 +308,28 @@ public:
          */
         bool alive = true;
 
+        class
+        {
+            /**
+             * Structured queue of child events:
+             * - outputs: stream of data from child
+             * - eof: optional end-of-stream marker
+             * - timeout: optional timeout that flushes/overrides other events
+             */
+            std::queue<ChildOutput> childOutputs;
+            std::optional<ChildEOF> childEOF;
+            std::optional<TimedOut> childTimeout;
+
+        public:
+
+            void pushChildEvent(ChildOutput event);
+            void pushChildEvent(ChildEOF event);
+            void pushChildEvent(TimedOut event);
+            bool hasChildEvent() const;
+            ChildEvent popChildEvent();
+
+        } childEvents;
+
         /**
          * The awaiter used by @ref final_suspend.
          */
@@ -376,13 +423,66 @@ public:
             return static_cast<Co &&>(co);
         }
 
+        /**
+         * Awaiter for @ref Suspend. Always suspends, but asserts
+         * there are no pending child events (those should be
+         * consumed first via @ref WaitForChildEvent).
+         */
+        struct SuspendAwaiter
+        {
+            promise_type & promise;
+
+            bool await_ready()
+            {
+                assert(!promise.childEvents.hasChildEvent());
+                return false;
+            }
+
+            void await_suspend(handle_type) {}
+
+            void await_resume() {}
+        };
+
         /**
          * Allows awaiting a @ref Suspend.
          * Always suspends.
          */
-        std::suspend_always await_transform(Suspend)
+        SuspendAwaiter await_transform(Suspend)
         {
-            return {};
+            return SuspendAwaiter{*this};
+        };
+
+        /**
+         * Awaiter for child events. Suspends and returns the
+         * pending child event when resumed.
+         */
+        struct ChildEventAwaiter
+        {
+            handle_type handle;
+
+            bool await_ready()
+            {
+                return handle && handle.promise().childEvents.hasChildEvent();
+            }
+
+            void await_suspend(handle_type h)
+            {
+                handle = h;
+            }
+
+            ChildEvent await_resume()
+            {
+                assert(handle);
+                return handle.promise().childEvents.popChildEvent();
+            }
+        };
+
+        /**
+         * Allows awaiting child events (output, EOF, timeout).
+         */
+        ChildEventAwaiter await_transform(WaitForChildEvent)
+        {
+            return ChildEventAwaiter{handle_type::from_promise(*this)};
         };
     };
 
@@ -439,15 +539,23 @@ public:
 
     void work();
 
-    virtual void handleChildOutput(Descriptor fd, std::string_view data)
-    {
-        unreachable();
-    }
+    /**
+     * Called by the worker when data is received from a child process.
+     * Stores the event and resumes the coroutine.
+     */
+    void handleChildOutput(Descriptor fd, std::string_view data);
 
-    virtual void handleEOF(Descriptor fd)
-    {
-        unreachable();
-    }
+    /**
+     * Called by the worker when EOF is received from a child process.
+     * Stores the event and resumes the coroutine.
+     */
+    void handleEOF(Descriptor fd);
+
+    /**
+     * Called by the worker when a build times out.
+     * Stores the event and resumes the coroutine.
+     */
+    void timedOut(TimedOut && ex);
 
     void trace(std::string_view s);
 
@@ -456,13 +564,6 @@ 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 void timedOut(TimedOut && ex) = 0;
-
     /**
      * Used for comparisons. The order matters a bit for scheduling. We
      * want:

src/libstore/include/nix/store/build/substitution-goal.hh

@@ -53,11 +53,6 @@ public:
         std::optional<ContentAddress> ca = std::nullopt);
     ~PathSubstitutionGoal();
 
-    void timedOut(TimedOut && ex) override
-    {
-        unreachable();
-    };
-
     std::string key() override
     {
         return "a$" + std::string(storePath.name()) + "$" + worker.store.printStorePath(storePath);
@@ -72,12 +67,6 @@ public:
         StorePath subPath, nix::ref<Store> sub, std::shared_ptr<const ValidPathInfo> info, bool & substituterFailed);
     Co finished();
 
-    /**
-     * Callback used by the worker to write to the log.
-     */
-    void handleChildOutput(Descriptor fd, std::string_view data) override {};
-    void handleEOF(Descriptor fd) override;
-
     /* Called by destructor, can't be overridden */
     void cleanup() override final;