libstore: replace Goal::WaitForSlot with semaphores

now that we have an event loop in the worker we can use it and its
magical execution suspending properties to replace the slot counts
we managed explicitly with semaphores and raii tokens. technically
this would not have needed an event loop base to be doable, but it
is a whole lot easier to wait for a token to be available if there
is a callback mechanism ready for use that doesn't require a whole
damn dedicated abstract method in Goal to work, and specific calls
to that dedicated method strewn all over the worker implementation

Change-Id: I1da7cf386d94e2bbf2dba9b53ff51dbce6a0cff7

1 files changed, 8 insertions, 4 deletions

diff --git a/src/libstore/build/drv-output-substitution-goal.cc b/src/libstore/build/drv-output-substitution-goal.cc
index fdee53699..6ef00d1ff 100644
--- a/src/libstore/build/drv-output-substitution-goal.cc
+++ b/src/libstore/build/drv-output-substitution-goal.cc
@@ -42,7 +42,10 @@ try {
     trace("trying next substituter");
 
     if (!inBuildSlot) {
-        return {WaitForSlot{}};
+        return worker.substitutions.acquire().then([this](auto token) {
+            slotToken = std::move(token);
+            return work();
+        });
     }
 
     maintainRunningSubstitutions = worker.runningSubstitutions.addTemporarily(1);
@@ -81,7 +84,7 @@ try {
 
     state = &DrvOutputSubstitutionGoal::realisationFetched;
     return {WaitForWorld{
-        pipe.promise.then([]() -> Outcome<void, Finished> { return result::success(); }), true
+        pipe.promise.then([]() -> Outcome<void, Finished> { return result::success(); })
     }};
 } catch (...) {
     return {std::current_exception()};
@@ -90,6 +93,7 @@ try {
 kj::Promise<Result<Goal::WorkResult>> DrvOutputSubstitutionGoal::realisationFetched(bool inBuildSlot) noexcept
 try {
     maintainRunningSubstitutions.reset();
+    slotToken = {};
 
     try {
         outputInfo = downloadState->result.get();
@@ -168,9 +172,9 @@ std::string DrvOutputSubstitutionGoal::key()
     return "a$" + std::string(id.to_string());
 }
 
-kj::Promise<Result<Goal::WorkResult>> DrvOutputSubstitutionGoal::work(bool inBuildSlot) noexcept
+kj::Promise<Result<Goal::WorkResult>> DrvOutputSubstitutionGoal::work() noexcept
 {
-    return (this->*state)(inBuildSlot);
+    return (this->*state)(slotToken.valid());
 }