libstore: turn periodic gc attempt into a promise

notably we will check whether we want to do GC at all only once during
startup, and we'll only attempt GC every ten seconds rather than every
time a goal has finished a partial work call. this shouldn't cause any
problems in practice since relying on auto-gc is not deterministic and
stores in which builds can fill all remaining free space in merely ten
seconds are severely troubled even when gargage collection runs a lot.

Change-Id: I1175a56bf7f4e531f8be90157ad88750ff2ddec4

src/libstore/build/worker.cc

@@ -298,6 +298,13 @@ std::vector<GoalPtr> Worker::run(std::function<Targets (GoalFactory &)> req)
     }
 
     auto promise = runImpl();
+
+    // TODO GC interface?
+    if (auto localStore = dynamic_cast<LocalStore *>(&store); localStore && settings.minFree != 0) {
+        // Periodically wake up to see if we need to run the garbage collector.
+        promise = promise.exclusiveJoin(boopGC(*localStore));
+    }
+
     promise.wait(aio.waitScope).value();
 
     std::vector<GoalPtr> results;
@@ -315,10 +322,6 @@ try {
 
         checkInterrupt();
 
-        // TODO GC interface?
-        if (auto localStore = dynamic_cast<LocalStore *>(&store))
-            localStore->autoGC(false);
-
         /* Call every wake goal (in the ordering established by
            CompareGoalPtrs). */
         while (!awake.empty() && !topGoals.empty()) {
@@ -356,22 +359,23 @@ try {
     co_return result::failure(std::current_exception());
 }
 
+kj::Promise<Result<void>> Worker::boopGC(LocalStore & localStore)
+try {
+    while (true) {
+        co_await aio.provider->getTimer().afterDelay(10 * kj::SECONDS);
+        localStore.autoGC(false);
+    }
+} catch (...) {
+    co_return result::failure(std::current_exception());
+}
+
 kj::Promise<Result<void>> Worker::waitForInput()
 try {
     printMsg(lvlVomit, "waiting for children");
 
-    auto waitFor = [&]{
     auto pair = kj::newPromiseAndFulfiller<void>();
     this->childFinished = kj::mv(pair.fulfiller);
-        return kj::mv(pair.promise);
+    co_await pair.promise;
-    }();
-
-    if (settings.minFree.get() != 0) {
-        // Periodicallty wake up to see if we need to run the garbage collector.
-        waitFor = waitFor.exclusiveJoin(aio.provider->getTimer().afterDelay(10 * kj::SECONDS));
-    }
-
-    co_await waitFor;
     co_return result::success();
 } catch (...) {
     co_return result::failure(std::current_exception());

src/libstore/build/worker.hh

@@ -20,6 +20,7 @@ namespace nix {
 struct DerivationGoal;
 struct PathSubstitutionGoal;
 class DrvOutputSubstitutionGoal;
+class LocalStore;
 
 typedef std::chrono::time_point<std::chrono::steady_clock> steady_time_point;
 
@@ -189,6 +190,7 @@ private:
     }
 
     kj::Promise<Result<void>> runImpl();
+    kj::Promise<Result<void>> boopGC(LocalStore & localStore);
 
 public: