hydra-queue-runner: do not assume reservation is available when build step throws unexpectedly

`reservation` might be null when an uncaught exception has been thrown.
We copy the data we need to report the error.

Signed-off-by: Raito Bezarius <raito@lix.systems>

src/hydra-queue-runner/builder.cc

@@ -28,6 +28,9 @@ void State::builder(MachineReservation::ptr reservation)
 
     {
         auto activeStep = std::make_shared<ActiveStep>();
+        auto machineSshName = reservation->machine->sshName;
+        auto drvPath = reservation->step->drvPath;
+
         activeStep->step = reservation->step;
         activeSteps_.lock()->insert(activeStep);
 
@@ -39,9 +42,12 @@ void State::builder(MachineReservation::ptr reservation)
             auto destStore = getDestStore();
             res = doBuildStep(destStore, reservation, activeStep);
         } catch (std::exception & e) {
+            if (!reservation) {
+                printMsg(lvlTalkative, "machine '%s' has been released earlier (e.g. at copying outputs phase)", machineSshName);
+            }
             printMsg(lvlError, "uncaught exception building ‘%s’ on ‘%s’: %s",
-                localStore->printStorePath(reservation->step->drvPath),
-                reservation->machine->sshName,
+                localStore->printStorePath(drvPath),
+                machineSshName,
                 e.what());
         }
     }