libstore: make waiting for a while a promise

this simplifies waitForInput quite a lot, and at the same time makes
polling less thundering-herd-y. it even fixes early polling wakeups!

Change-Id: I6dfa62ce91729b8880342117d71af5ae33366414

src/libstore/build/derivation-goal.cc

@@ -736,7 +736,7 @@ try {
         if (!actLock)
             actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
                 fmt("waiting for lock on %s", Magenta(showPaths(lockFiles))));
-        return {WaitForAWhile{}};
+        return waitForAWhile();
     }
 
     actLock.reset();
@@ -776,32 +776,32 @@ try {
         auto hookReply = tryBuildHook(inBuildSlot);
         auto result = std::visit(
             overloaded{
-                [&](HookReply::Accept & a) -> std::optional<WorkResult> {
+                [&](HookReply::Accept & a) -> std::optional<kj::Promise<Result<WorkResult>>> {
                     /* Yes, it has started doing so.  Wait until we get
                        EOF from the hook. */
                     actLock.reset();
                     buildResult.startTime = time(0); // inexact
                     state = &DerivationGoal::buildDone;
                     started();
-                    return WaitForWorld{std::move(a.promise), false};
+                    return {{WaitForWorld{std::move(a.promise), false}}};
                 },
-                [&](HookReply::Postpone) -> std::optional<WorkResult> {
+                [&](HookReply::Postpone) -> std::optional<kj::Promise<Result<WorkResult>>> {
                     /* Not now; wait until at least one child finishes or
                        the wake-up timeout expires. */
                     if (!actLock)
                         actLock = std::make_unique<Activity>(*logger, lvlTalkative, actBuildWaiting,
                             fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
                     outputLocks.unlock();
-                    return WaitForAWhile{};
+                    return waitForAWhile();
                 },
-                [&](HookReply::Decline) -> std::optional<WorkResult> {
+                [&](HookReply::Decline) -> std::optional<kj::Promise<Result<WorkResult>>> {
                     /* We should do it ourselves. */
                     return std::nullopt;
                 },
             },
             hookReply);
         if (result) {
-            return {std::move(*result)};
+            return std::move(*result);
         }
     }
 

src/libstore/build/goal.cc

@@ -1,4 +1,6 @@
 #include "goal.hh"
+#include "worker.hh"
+#include <kj/time.h>
 
 namespace nix {
 
@@ -15,4 +17,15 @@ void Goal::trace(std::string_view s)
     debug("%1%: %2%", name, s);
 }
 
+kj::Promise<Result<Goal::WorkResult>> Goal::waitForAWhile()
+try {
+    trace("wait for a while");
+    /* If we are polling goals that are waiting for a lock, then wake
+       up after a few seconds at most. */
+    co_await worker.aio.provider->getTimer().afterDelay(settings.pollInterval.get() * kj::SECONDS);
+    co_return ContinueImmediately{};
+} catch (...) {
+    co_return std::current_exception();
+}
+
 }

src/libstore/build/goal.hh

@@ -118,7 +118,6 @@ public:
 
     struct [[nodiscard]] StillAlive {};
     struct [[nodiscard]] WaitForSlot {};
-    struct [[nodiscard]] WaitForAWhile {};
     struct [[nodiscard]] ContinueImmediately {};
     struct [[nodiscard]] WaitForGoals {
         Goals goals;
@@ -140,7 +139,6 @@ public:
     struct [[nodiscard]] WorkResult : std::variant<
                                           StillAlive,
                                           WaitForSlot,
-                                          WaitForAWhile,
                                           ContinueImmediately,
                                           WaitForGoals,
                                           WaitForWorld,
@@ -150,6 +148,11 @@ public:
         using variant::variant;
     };
 
+protected:
+    kj::Promise<Result<WorkResult>> waitForAWhile();
+
+public:
+
     /**
      * Exception containing an error message, if any.
      */

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

@@ -212,7 +212,7 @@ try {
             if (!actLock)
                 actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
                     fmt("waiting for a free build user ID for '%s'", Magenta(worker.store.printStorePath(drvPath))));
-            return {WaitForAWhile{}};
+            return waitForAWhile();
         }
     }
 

src/libstore/build/worker.cc

@@ -32,7 +32,6 @@ Worker::Worker(Store & store, Store & evalStore, kj::AsyncIoContext & aio)
     /* Debugging: prevent recursive workers. */
     nrLocalBuilds = 0;
     nrSubstitutions = 0;
-    lastWokenUp = steady_time_point::min();
 }
 
 
@@ -212,7 +211,6 @@ void Worker::handleWorkResult(GoalPtr goal, Goal::WorkResult how)
         overloaded{
             [&](Goal::StillAlive) {},
             [&](Goal::WaitForSlot) { waitForBuildSlot(goal); },
-            [&](Goal::WaitForAWhile) { waitForAWhile(goal); },
             [&](Goal::ContinueImmediately) { wakeUp(goal); },
             [&](Goal::WaitForGoals & w) {
                 for (auto & dep : w.goals) {
@@ -221,12 +219,25 @@ void Worker::handleWorkResult(GoalPtr goal, Goal::WorkResult how)
                 }
             },
             [&](Goal::WaitForWorld & w) {
-                childStarted(goal, std::move(w.promise), w.inBuildSlot);
+                childStarted(
+                    goal,
+                    w.promise.then([](auto r) -> Result<Goal::WorkResult> {
+                        if (r.has_value()) {
+                            return {Goal::ContinueImmediately{}};
+                        } else if (r.has_error()) {
+                            return {std::move(r).error()};
+                        } else {
+                            return r.exception();
+                        }
+                    }),
+                    w.inBuildSlot
+                );
             },
             [&](Goal::Finished & f) { goalFinished(goal, f); },
         },
         how
     );
+    updateStatistics();
 }
 
 void Worker::removeGoal(GoalPtr goal)
@@ -257,17 +268,15 @@ void Worker::wakeUp(GoalPtr goal)
 }
 
 
-void Worker::childStarted(GoalPtr goal, kj::Promise<Outcome<void, Goal::Finished>> promise,
+void Worker::childStarted(GoalPtr goal, kj::Promise<Result<Goal::WorkResult>> promise,
     bool inBuildSlot)
 {
     children.add(promise
         .then([this, goal](auto result) {
             if (result.has_value()) {
-                handleWorkResult(goal, Goal::ContinueImmediately{});
+                handleWorkResult(goal, std::move(result.assume_value()));
-            } else if (result.has_error()) {
-                handleWorkResult(goal, std::move(result.assume_error()));
             } else {
-                childException = result.assume_exception();
+                childException = result.assume_error();
             }
         })
         .attach(Finally{[this, goal, inBuildSlot] {
@@ -331,13 +340,6 @@ void Worker::waitForBuildSlot(GoalPtr goal)
 }
 
 
-void Worker::waitForAWhile(GoalPtr goal)
-{
-    debug("wait for a while");
-    waitingForAWhile.insert(goal);
-}
-
-
 void Worker::updateStatistics()
 {
     // only update progress info while running. this notably excludes updating
@@ -397,8 +399,12 @@ Goals Worker::run(std::function<Goals (GoalFactory &)> req)
                 const bool inSlot = goal->jobCategory() == JobCategory::Substitution
                     ? nrSubstitutions < std::max(1U, (unsigned int) settings.maxSubstitutionJobs)
                     : nrLocalBuilds < settings.maxBuildJobs;
-                handleWorkResult(goal, goal->work(inSlot).wait(aio.waitScope).value());
+                auto result = goal->work(inSlot);
-                updateStatistics();
+                if (result.poll(aio.waitScope)) {
+                    handleWorkResult(goal, result.wait(aio.waitScope).value());
+                } else {
+                    childStarted(goal, std::move(result), false);
+                }
 
                 if (topGoals.empty()) break; // stuff may have been cancelled
             }
@@ -407,7 +413,7 @@ Goals Worker::run(std::function<Goals (GoalFactory &)> req)
         if (topGoals.empty()) break;
 
         /* Wait for input. */
-        if (!children.isEmpty() || !waitingForAWhile.empty())
+        if (!children.isEmpty())
             waitForInput();
         else {
             assert(!awake.empty());
@@ -445,22 +451,12 @@ void Worker::waitForInput()
        terminated. */
 
     std::optional<long> timeout = 0;
-    auto before = steady_time_point::clock::now();
 
     // 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()) {
-        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 (timeout)
         vomit("sleeping %d seconds", *timeout);
 
@@ -475,17 +471,6 @@ void Worker::waitForInput()
     }();
 
     waitFor.wait(aio.waitScope);
-
-    auto after = steady_time_point::clock::now();
-
-    if (!waitingForAWhile.empty() && lastWokenUp + std::chrono::seconds(settings.pollInterval) <= after) {
-        lastWokenUp = after;
-        for (auto & i : waitingForAWhile) {
-            GoalPtr goal = i.lock();
-            if (goal) wakeUp(goal);
-        }
-        waitingForAWhile.clear();
-    }
 }
 
 

src/libstore/build/worker.hh

@@ -117,16 +117,6 @@ private:
     std::map<StorePath, std::weak_ptr<PathSubstitutionGoal>> substitutionGoals;
     std::map<DrvOutput, std::weak_ptr<DrvOutputSubstitutionGoal>> drvOutputSubstitutionGoals;
 
-    /**
-     * Goals sleeping for a few seconds (polling a lock).
-     */
-    WeakGoals waitingForAWhile;
-
-    /**
-     * Last time the goals in `waitingForAWhile` where woken up.
-     */
-    steady_time_point lastWokenUp;
-
     /**
      * Cache for pathContentsGood().
      */
@@ -164,14 +154,6 @@ private:
      */
     void waitForBuildSlot(GoalPtr goal);
 
-    /**
-     * Wait for a few seconds and then retry this goal.  Used when
-     * waiting for a lock held by another process.  This kind of
-     * polling is inefficient, but POSIX doesn't really provide a way
-     * to wait for multiple locks in the main select() loop.
-     */
-    void waitForAWhile(GoalPtr goal);
-
     /**
      * Wake up a goal (i.e., there is something for it to do).
      */
@@ -191,7 +173,7 @@ private:
      * Registers a running child process.  `inBuildSlot` means that
      * the process counts towards the jobs limit.
      */
-    void childStarted(GoalPtr goal, kj::Promise<Outcome<void, Goal::Finished>> promise,
+    void childStarted(GoalPtr goal, kj::Promise<Result<Goal::WorkResult>> promise,
         bool inBuildSlot);
 
     /**