libstore: turn Worker in a kj event loop user

using a proper event loop basis we no longer have to worry about most of
the intricacies of poll(), or platform-dependent replacements for it. we
may even be able to use the event loop and its promise system for all of
our scheduling in the future. we don't do any real async processing yet,
this is just preparation to separate the first such change from the huge
api design difference with the async framework we chose (kj from capnp):

kj::Promise, unlike std::future, doesn't return exceptions unmangled. it
instead wraps any non-kj exception into a kj exception, erasing all type
information and preserving mostly the what() string in the process. this
makes sense in the capnp rpc use case where unrestricted exception types
can't be transferred, and since it moves error handling styles closer to
a world we'd actually like there's no harm in doing it only here for now

Change-Id: I20f888de74d525fb2db36ca30ebba4bcfe9cc838

1 files changed, 8 insertions, 6 deletions

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 7553f1e79..4baa525d9 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -149,8 +149,8 @@ void LocalDerivationGoal::killSandbox(bool getStats)
 }
 
 
-Goal::WorkResult LocalDerivationGoal::tryLocalBuild(bool inBuildSlot)
-{
+kj::Promise<Result<Goal::WorkResult>> LocalDerivationGoal::tryLocalBuild(bool inBuildSlot) noexcept
+try {
 #if __APPLE__
     additionalSandboxProfile = parsedDrv->getStringAttr("__sandboxProfile").value_or("");
 #endif
@@ -159,7 +159,7 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild(bool inBuildSlot)
         state = &DerivationGoal::tryToBuild;
         outputLocks.unlock();
         if (0U != settings.maxBuildJobs) {
-            return WaitForSlot{};
+            return {WaitForSlot{}};
         }
         if (getMachines().empty()) {
             throw Error(
@@ -214,7 +214,7 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild(bool inBuildSlot)
             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{}};
         }
     }
 
@@ -250,15 +250,17 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild(bool inBuildSlot)
         state = &DerivationGoal::buildDone;
 
         started();
-        return WaitForWorld{std::move(fds), true};
+        return {WaitForWorld{std::move(fds), true}};
 
     } catch (BuildError & e) {
         outputLocks.unlock();
         buildUser.reset();
         auto report = done(BuildResult::InputRejected, {}, std::move(e));
         report.permanentFailure = true;
-        return report;
+        return {std::move(report)};
     }
+} catch (...) {
+    return {std::current_exception()};
 }