Keep machine stats even when a machine is removed from the machines file

This is important for the Hydra provisioner, since it needs to be able
to see whether a disabled machine still has jobs running on it.

src/hydra-queue-runner/dispatcher.cc

@@ -89,6 +89,7 @@ system_time State::doDispatch()
             auto machines_(machines.lock());
             for (auto & m : *machines_) {
                 auto info(m.second->state->connectInfo.lock());
+                if (!m.second->enabled) continue;
                 if (info->consecutiveFailures && info->disabledUntil > now) {
                     if (info->disabledUntil < sleepUntil)
                         sleepUntil = info->disabledUntil;

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -74,8 +74,14 @@ void State::parseMachines(const std::string & contents)
     }
 
     for (auto & m : oldMachines)
-        if (newMachines.find(m.first) == newMachines.end())
+        if (newMachines.find(m.first) == newMachines.end()) {
             printMsg(lvlInfo, format("removing machine ‘%1%’") % m.first);
+            /* Add a disabled Machine object to make sure stats are
+               maintained. */
+            auto machine = std::make_shared<Machine>(*(m.second));
+            machine->enabled = false;
+            newMachines[m.first] = machine;
+        }
 
     auto machines_(machines.lock());
     *machines_ = newMachines;
@@ -489,6 +495,7 @@ void State::dumpStatus(Connection & conn, bool log)
                 auto & s(m->state);
                 nested.attr(m->sshName);
                 JSONObject nested2(out);
+                nested2.attr("enabled", m->enabled);
                 nested2.attr("currentJobs", s->currentJobs);
                 if (s->currentJobs == 0)
                     nested2.attr("idleSince", s->idleSince);

src/hydra-queue-runner/state.hh

@@ -201,6 +201,8 @@ struct Machine
 {
     typedef std::shared_ptr<Machine> ptr;
 
+    bool enabled{true};
+
     std::string sshName, sshKey;
     std::set<std::string> systemTypes, supportedFeatures, mandatoryFeatures;
     unsigned int maxJobs = 1;