libstore: always release build/substitution slot tokens

not doing this can freeze slots until the goal that occupied them is
freed (rather than simply complete), and then can freeze the system.

fixes #549

Change-Id: I042df04222f8ffbaa18ef4a4eae6cbd6f89b679e

src/libstore/build/goal.cc

@@ -22,6 +22,12 @@ kj::Promise<void> Goal::waitForAWhile()
 
 kj::Promise<Result<Goal::WorkResult>> Goal::work() noexcept
 try {
+    // always clear the slot token, no matter what happens. not doing this
+    // can cause builds to get stuck on exceptions (or other early exist).
+    // ideally we'd use scoped slot tokens instead of keeping them in some
+    // goal member variable, but we cannot do this yet for legacy reasons.
+    KJ_DEFER({ slotToken = {}; });
+
     BOOST_OUTCOME_CO_TRY(auto result, co_await workImpl());
 
     trace("done");