diff --git a/src/hydra-queue-runner/hydra-queue-runner.cc b/src/hydra-queue-runner/hydra-queue-runner.cc
index d2edf32c..6b9d4aae 100644
--- a/src/hydra-queue-runner/hydra-queue-runner.cc
+++ b/src/hydra-queue-runner/hydra-queue-runner.cc
@@ -8,6 +8,8 @@
 #include <pqxx/pqxx>
 
 #include "build-result.hh"
+#include "sync.hh"
+
 #include "store-api.hh"
 #include "derivations.hh"
 #include "shared.hh"
@@ -16,6 +18,13 @@
 using namespace nix;
 
 
+template <class C, class V>
+bool has(const C & c, const V & v)
+{
+    return c.find(v) != c.end();
+}
+
+
 std::mutex exitRequestMutex;
 std::condition_variable exitRequest;
 bool exitRequested(false);
@@ -76,6 +85,9 @@ struct Connection : pqxx::connection
 typedef unsigned int BuildID;
 
 
+struct Step;
+
+
 struct Build
 {
     typedef std::shared_ptr<Build> ptr;
@@ -84,10 +96,18 @@ struct Build
     BuildID id;
     Path drvPath;
     std::map<string, Path> outputs;
+    std::string fullJobName;
+
+    std::shared_ptr<Step> toplevel;
 
     bool finishedInDB;
 
     Build() : finishedInDB(false) { }
+
+    ~Build()
+    {
+        printMsg(lvlError, format("destroying build %1%") % id);
+    }
 };
 
 
@@ -95,17 +115,28 @@ struct Step
 {
     typedef std::shared_ptr<Step> ptr;
     typedef std::weak_ptr<Step> wptr;
+
     Path drvPath;
     Derivation drv;
 
-    /* The build steps on which this step depends. */
-    std::set<Step::ptr> deps;
+    struct State
+    {
+        /* The build steps on which this step depends. */
+        std::set<Step::ptr> deps;
 
-    /* The build steps that depend on this step. */
-    std::vector<Step::wptr> rdeps;
+        /* The build steps that depend on this step. */
+        std::vector<Step::wptr> rdeps;
 
-    /* Builds that have this step as the top-level derivation. */
-    std::vector<Build::wptr> builds;
+        /* Builds that have this step as the top-level derivation. */
+        std::vector<Build::wptr> builds;
+    };
+
+    Sync<State> state;
+
+    ~Step()
+    {
+        printMsg(lvlError, format("destroying step %1%") % drvPath);
+    }
 };
 
 
@@ -116,17 +147,21 @@ private:
     std::thread queueMonitorThread;
 
     /* The queued builds. */
-    std::map<BuildID, Build::ptr> builds;
+    typedef std::map<BuildID, Build::ptr> Builds;
+    Sync<Builds> builds;
 
     /* All active or pending build steps (i.e. dependencies of the
-       queued builds). */
-    std::map<Path, Step::ptr> steps;
+       queued builds). Note that these are weak pointers. Steps are
+       kept alive by being reachable from Builds or by being in
+       progress. */
+    typedef std::map<Path, Step::wptr> Steps;
+    Sync<Steps> steps;
 
     /* Build steps that have no unbuilt dependencies. */
-    std::set<Step::ptr> runnable;
+    typedef std::list<Step::wptr> Runnable;
+    Sync<Runnable> runnable;
 
-    std::mutex runnableMutex;
-    std::condition_variable runnableCV;
+    std::condition_variable_any runnableCV;
 
 public:
     State();
@@ -147,7 +182,8 @@ public:
 
     void getQueuedBuilds(std::shared_ptr<StoreAPI> store, pqxx::connection & conn);
 
-    Step::ptr createStep(std::shared_ptr<StoreAPI> store, const Path & drvPath);
+    Step::ptr createStep(std::shared_ptr<StoreAPI> store, const Path & drvPath,
+        std::set<Step::ptr> & newRunnable);
 
     void destroyStep(Step::ptr step, bool proceed);
 
@@ -254,26 +290,55 @@ void State::getQueuedBuilds(std::shared_ptr<StoreAPI> store, pqxx::connection &
 {
     printMsg(lvlError, "checking the queue...");
 
-    pqxx::work txn(conn);
+#if 0
+    {
+        auto runnable_(runnable.lock());
+        auto builds_(builds.lock());
+        auto steps_(steps.lock());
+        printMsg(lvlError, format("%1% builds, %2% steps, %3% runnable steps")
+            % builds_->size()
+            % steps_->size()
+            % runnable_->size());
+    }
+#endif
 
-    // FIXME: query only builds with ID higher than the previous
-    // highest.
-    auto res = txn.exec("select * from Builds where finished = 0");
+    /* Grab the queued builds from the database, but don't process
+       them yet (since we don't want a long-running transaction). */
+    std::list<Build::ptr> newBuilds; // FIXME: use queue
 
-    // FIXME: don't process inside a txn.
-    for (auto const & row : res) {
-        BuildID id = row["id"].as<BuildID>();
-        if (builds.find(id) != builds.end()) continue;
+    {
+        pqxx::work txn(conn);
 
-        Build::ptr build(new Build);
-        build->id = id;
-        build->drvPath = row["drvPath"].as<string>();
+        // FIXME: query only builds with ID higher than the previous
+        // highest.
+        auto res = txn.exec("select * from Builds where finished = 0 order by id");
 
-        printMsg(lvlInfo, format("loading build %1% (%2%:%3%:%4%)") % id % row["project"] % row["jobset"] % row["job"]);
+        auto builds_(builds.lock());
+
+        for (auto const & row : res) {
+            BuildID id = row["id"].as<BuildID>();
+            if (has(*builds_, id)) continue;
+
+            auto build = std::make_shared<Build>();
+            build->id = id;
+            build->drvPath = row["drvPath"].as<string>();
+            build->fullJobName = row["project"].as<string>() + ":" + row["jobset"].as<string>() + ":" + row["job"].as<string>();
+
+            newBuilds.push_back(build);
+        }
+    }
+
+    /* Now instantiate build steps for each new build. The builder
+       threads can start building the runnable build steps right away,
+       even while we're still processing other new builds. */
+    for (auto & build : newBuilds) {
+        // FIXME: remove build from newBuilds to ensure quick destruction
+        // FIXME: exception handling
+
+        printMsg(lvlInfo, format("loading build %1% (%2%)") % build->id % build->fullJobName);
 
         if (!store->isValidPath(build->drvPath)) {
             /* Derivation has been GC'ed prematurely. */
-            Connection conn;
             pqxx::work txn(conn);
             txn.parameterized
                 ("update Builds set finished = 1, buildStatus = $2, startTime = $3, stopTime = $3, errorMsg = $4 where id = $1")
@@ -285,12 +350,15 @@ void State::getQueuedBuilds(std::shared_ptr<StoreAPI> store, pqxx::connection &
             continue;
         }
 
-        Step::ptr step = createStep(store, build->drvPath);
+        std::set<Step::ptr> newRunnable;
+        Step::ptr step = createStep(store, build->drvPath, newRunnable);
+
+        /* If we didn't get a step, it means the step's outputs are
+           all valid. So we mark this as a finished, cached build. */
         if (!step) {
             Derivation drv = readDerivation(build->drvPath);
             BuildResult res = getBuildResult(store, drv);
 
-            Connection conn;
             pqxx::work txn(conn);
             time_t now = time(0);
             markSucceededBuild(txn, build, res, true, now, now);
@@ -299,21 +367,49 @@ void State::getQueuedBuilds(std::shared_ptr<StoreAPI> store, pqxx::connection &
             continue;
         }
 
-        step->builds.push_back(build);
+        /* Note: if we exit this scope prior to this, the build and
+           all newly created steps are destroyed. */
 
-        builds[id] = build;
+        {
+            auto builds_(builds.lock());
+            auto step_(step->state.lock());
+            (*builds_)[build->id] = build;
+            step_->builds.push_back(build);
+            build->toplevel = step;
+        }
+
+        /* Prior to this, the build is not visible to
+           getDependentBuilds().  Now it is, so the build can be
+           failed if a dependency fails. (It can't succeed right away
+           because its top-level is not runnable yet). */
+
+        /* Add the new runnable build steps to ‘runnable’ and wake up
+           the builder threads. */
+        for (auto & r : newRunnable)
+            makeRunnable(r);
     }
 }
 
 
-Step::ptr State::createStep(std::shared_ptr<StoreAPI> store, const Path & drvPath)
+Step::ptr State::createStep(std::shared_ptr<StoreAPI> store, const Path & drvPath,
+    std::set<Step::ptr> & newRunnable)
 {
-    auto prev = steps.find(drvPath);
-    if (prev != steps.end()) return prev->second;
+    /* Check if the requested step already exists. */
+    {
+        auto steps_(steps.lock());
+        auto prev = steps_->find(drvPath);
+        if (prev != steps_->end()) {
+            auto step = prev->second.lock();
+            /* Since ‘step’ is a strong pointer, the referred Step
+               object won't be deleted after this. */
+            if (step) return step;
+            steps_->erase(drvPath); // remove stale entry
+        }
+    }
 
     printMsg(lvlInfo, format("considering derivation ‘%1%’") % drvPath);
 
-    Step::ptr step(new Step);
+    auto step = std::make_shared<Step>();
     step->drvPath = drvPath;
     step->drv = readDerivation(drvPath);
 
@@ -333,17 +429,25 @@ Step::ptr State::createStep(std::shared_ptr<StoreAPI> store, const Path & drvPat
     printMsg(lvlInfo, format("creating build step ‘%1%’") % drvPath);
 
     /* Create steps for the dependencies. */
+    bool hasDeps = false;
     for (auto & i : step->drv.inputDrvs) {
-        Step::ptr dep = createStep(store, i.first);
+        Step::ptr dep = createStep(store, i.first, newRunnable);
         if (dep) {
-            step->deps.insert(dep);
-            dep->rdeps.push_back(step);
+            hasDeps = true;
+            auto step_(step->state.lock());
+            auto dep_(dep->state.lock());
+            step_->deps.insert(dep);
+            dep_->rdeps.push_back(step);
         }
     }
 
-    steps[drvPath] = step;
+    {
+        auto steps_(steps.lock());
+        assert(steps_->find(drvPath) == steps_->end());
+        (*steps_)[drvPath] = step;
+    }
 
-    if (step->deps.empty()) makeRunnable(step);
+    if (!hasDeps) newRunnable.insert(step);
 
     return step;
 }
@@ -351,30 +455,48 @@ Step::ptr State::createStep(std::shared_ptr<StoreAPI> store, const Path & drvPat
 
 void State::destroyStep(Step::ptr step, bool proceed)
 {
-    steps.erase(step->drvPath);
+    printMsg(lvlInfo, format("destroying build step ‘%1%’") % step->drvPath);
 
-    for (auto & rdep_ : step->rdeps) {
+    {
+        auto steps_(steps.lock());
+        steps_->erase(step->drvPath);
+    }
+
+    std::vector<Step::wptr> rdeps;
+
+    {
+        auto step_(step->state.lock());
+        rdeps = step_->rdeps;
+
+        /* Sanity checks. */
+        for (auto & build_ : step_->builds) {
+            auto build = build_.lock();
+            if (!build) continue;
+            assert(build->drvPath == step->drvPath);
+            assert(build->finishedInDB);
+        }
+    }
+
+    for (auto & rdep_ : rdeps) {
         auto rdep = rdep_.lock();
         if (!rdep) continue;
-        assert(rdep->deps.find(step) != rdep->deps.end());
-        rdep->deps.erase(step);
+        bool runnable = false;
+        {
+            auto rdep_(rdep->state.lock());
+            assert(has(rdep_->deps, step));
+            rdep_->deps.erase(step);
+            if (rdep_->deps.empty()) runnable = true;
+        }
         if (proceed) {
             /* If this rdep has no other dependencies, then we can now
                build it. */
-            if (rdep->deps.empty())
+            if (runnable)
                 makeRunnable(rdep);
         } else
-            /* If ‘step’ failed, then delete all dependent steps as
-               well. */
+            /* If ‘step’ failed or was cancelled, then delete all
+               dependent steps as well. */
             destroyStep(rdep, false);
     }
-
-    for (auto & build_ : step->builds) {
-        auto build = build_.lock();
-        if (!build) continue;
-        assert(build->drvPath == step->drvPath);
-        assert(build->finishedInDB);
-    }
 }
 
 
@@ -386,17 +508,27 @@ std::set<Build::ptr> State::getDependentBuilds(Step::ptr step)
     std::function<void(Step::ptr)> visit;
 
     visit = [&](Step::ptr step) {
-        if (done.find(step) != done.end()) return;
+        if (has(done, step)) return;
         done.insert(step);
 
-        for (auto & build : step->builds) {
-            auto build2 = build.lock();
-            if (build2) res.insert(build2);
+        std::vector<Step::wptr> rdeps;
+
+        {
+            auto step_(step->state.lock());
+
+            for (auto & build : step_->builds) {
+                auto build_ = build.lock();
+                if (build_) res.insert(build_);
+            }
+
+            /* Make a copy of rdeps so that we don't hold the lock for
+               very long. */
+            rdeps = step_->rdeps;
         }
 
-        for (auto & rdep : step->rdeps) {
-            auto rdep2 = rdep.lock();
-            if (rdep2) visit(rdep2);
+        for (auto & rdep : rdeps) {
+            auto rdep_ = rdep.lock();
+            if (rdep_) visit(rdep_);
         }
     };
 
@@ -408,11 +540,14 @@ std::set<Build::ptr> State::getDependentBuilds(Step::ptr step)
 
 void State::makeRunnable(Step::ptr step)
 {
-    assert(step->deps.empty());
+    {
+        auto step_(step->state.lock());
+        assert(step_->deps.empty());
+    }
 
     {
-        std::lock_guard<std::mutex> lock(runnableMutex);
-        runnable.insert(step);
+        auto runnable_(runnable.lock());
+        runnable_->push_back(step);
     }
 
     runnableCV.notify_one();
@@ -424,17 +559,20 @@ void State::builderThreadEntry(int slot)
     auto store = openStore(); // FIXME: pool
 
     while (true)  {
+        /* Sleep until a runnable build step becomes available. */
         Step::ptr step;
         {
-            std::unique_lock<std::mutex> lock(runnableMutex);
-            while (runnable.empty())
-                runnableCV.wait(lock);
-            step = *runnable.begin();
-            runnable.erase(step);
+            auto runnable_(runnable.lock());
+            while (runnable_->empty())
+                runnable_.wait(runnableCV);
+            auto weak = *runnable_->begin();
+            runnable_->pop_front();
+            step = weak.lock();
+            if (!step) continue;
         }
 
+        /* Build it. */
         printMsg(lvlError, format("slot %1%: got build step ‘%2%’") % slot % step->drvPath);
-
         doBuildStep(store, step);
     }
 
@@ -444,34 +582,38 @@ void State::builderThreadEntry(int slot)
 
 void State::doBuildStep(std::shared_ptr<StoreAPI> store, Step::ptr step)
 {
-    assert(step->deps.empty());
-
     /* There can be any number of builds in the database that depend
-       on this derivation. Arbitrarily pick one (though preferring
-       those build of which this is the top-level derivation) for the
+       on this derivation. Arbitrarily pick one (though preferring a
+       build of which this is the top-level derivation) for the
        purpose of creating build steps. We could create a build step
        record for every build, but that could be very expensive
        (e.g. a stdenv derivation can be a dependency of tens of
        thousands of builds), so we don't. */
     Build::ptr build;
 
-    auto builds = getDependentBuilds(step);
+    {
+        auto dependents = getDependentBuilds(step);
 
-    if (builds.empty()) {
-        /* Apparently all builds that depend on this derivation are
-           gone (e.g. cancelled). So don't bother. */
-        printMsg(lvlInfo, format("cancelling build step ‘%1%’") % step->drvPath);
-        destroyStep(step, true);
-        return;
+        if (dependents.empty()) {
+            /* Apparently all builds that depend on this derivation
+               are gone (e.g. cancelled). So don't bother. (This is
+               very unlikely to happen, because normally Steps are
+               only kept alive by being reachable from a
+               Build). FIXME: what if a new Build gets a reference to
+               this step? */
+            printMsg(lvlInfo, format("cancelling build step ‘%1%’") % step->drvPath);
+            destroyStep(step, false);
+            return;
+        }
+
+        for (auto build2 : dependents)
+            if (build2->drvPath == step->drvPath) { build = build2; break; }
+
+        if (!build) build = *dependents.begin();
+
+        printMsg(lvlInfo, format("performing build step ‘%1%’ (needed by %2% builds)") % step->drvPath % dependents.size());
     }
 
-    for (auto build2 : builds)
-        if (build2->drvPath == step->drvPath) { build = build2; break; }
-
-    if (!build) build = *builds.begin();
-
-    printMsg(lvlInfo, format("performing build step ‘%1%’ (needed by %2% builds)") % step->drvPath % builds.size());
-
     /* Create a build step record indicating that we started
        building. */
     Connection conn;
@@ -499,8 +641,30 @@ void State::doBuildStep(std::shared_ptr<StoreAPI> store, Step::ptr step)
 
     // FIXME: handle failed-with-output
 
-    // FIXME: handle new builds having been added in the meantime.
+    /* Remove this step. After this, incoming builds that depend on
+       drvPath will either see that the output paths exist, or will
+       create a new build step for drvPath. The latter is fine - it
+       won't conflict with this one, because we're removing it. In any
+       case, the set of dependent builds for ‘step’ can't increase
+       anymore because ‘step’ is no longer visible to createStep(). */
+    {
+        auto steps_(steps.lock());
+        steps_->erase(step->drvPath);
+    }
 
+    /* Get the final set of dependent builds. */
+    auto dependents = getDependentBuilds(step);
+
+    std::set<Build::ptr> direct;
+    {
+        auto step_(step->state.lock());
+        for (auto & build : step_->builds) {
+            auto build_ = build.lock();
+            if (build_) direct.insert(build_);
+        }
+    }
+
+    /* Update the database. */
     {
         pqxx::work txn(conn);
 
@@ -510,24 +674,21 @@ void State::doBuildStep(std::shared_ptr<StoreAPI> store, Step::ptr step)
 
             /* Mark all builds of which this derivation is the top
                level as succeeded. */
-            for (auto build2_ : step->builds) {
-                auto build2 = build2_.lock();
-                if (!build2) continue;
+            for (auto build2 : direct)
                 markSucceededBuild(txn, build2, res, false, startTime, stopTime);
-            }
 
         } else {
             /* Create failed build steps for every build that depends
                on this. */
             finishBuildStep(txn, stopTime, build->id, stepNr, bssFailed, errorMsg);
 
-            for (auto build2 : builds) {
+            for (auto build2 : dependents) {
                 if (build == build2) continue;
                 createBuildStep(txn, stopTime, build2, step, bssFailed, errorMsg, build->id);
             }
 
             /* Mark all builds that depend on this derivation as failed. */
-            for (auto build2 : builds) {
+            for (auto build2 : dependents) {
                 txn.parameterized
                     ("update Builds set finished = 1, isCachedBuild = 0, buildStatus = $2, startTime = $3, stopTime = $4 where id = $1")
                     (build2->id)
@@ -539,10 +700,21 @@ void State::doBuildStep(std::shared_ptr<StoreAPI> store, Step::ptr step)
         }
 
         txn.commit();
-
     }
 
-    /* Remove the build step from the graph. */
+    /* In case of success, destroy all Build objects of which ‘step’
+       is the top-level derivation. In case of failure, destroy all
+       dependent Build objects. Any Steps not referenced by other
+       Builds will be destroyed as well. */
+    for (auto build2 : dependents)
+        if (build2->toplevel == step || !success) {
+            auto builds_(builds.lock());
+            builds_->erase(build2->id);
+        }
+
+    /* Remove the step from the graph. In case of success, make
+       dependent build steps runnable if they have no other
+       dependencies. */
     destroyStep(step, success);
 }
 
@@ -590,8 +762,6 @@ void State::run()
 
     queueMonitorThread = std::thread(&State::queueMonitorThreadEntry, this);
 
-    sleep(1);
-
     for (int n = 0; n < 4; n++)
         std::thread(&State::builderThreadEntry, this, n).detach();
 
diff --git a/src/hydra-queue-runner/sync.hh b/src/hydra-queue-runner/sync.hh
new file mode 100644
index 00000000..6f5f9e6a
--- /dev/null
+++ b/src/hydra-queue-runner/sync.hh
@@ -0,0 +1,53 @@
+#pragma once
+
+#include <mutex>
+#include <condition_variable>
+
+/* This template class ensures synchronized access to a value of type
+   T. It is used as follows:
+
+     struct Data { int x; ... };
+
+     Sync<Data> data;
+
+     {
+       auto data_(data.lock());
+       data_->x = 123;
+     }
+
+   Here, "data" is automatically unlocked when "data_" goes out of
+   scope.
+*/
+
+template <class T>
+class Sync
+{
+private:
+    std::mutex mutex;
+    T data;
+
+public:
+
+    class Lock
+    {
+    private:
+        Sync * s;
+        friend Sync;
+        Lock(Sync * s) : s(s) { s->mutex.lock(); }
+    public:
+        Lock(Lock && l) : s(l.s) { l.s = 0; }
+        Lock(const Lock & l) = delete;
+        ~Lock() { if (s) s->mutex.unlock(); }
+        T * operator -> () { return &s->data; }
+        T & operator * () { return s->data; }
+
+        /* FIXME: performance impact of condition_variable_any? */
+        void wait(std::condition_variable_any & cv)
+        {
+            assert(s);
+            cv.wait(s->mutex);
+        }
+    };
+
+    Lock lock() { return Lock(this); }
+};