Add ability to run a batch completion job in the proxy thread

For very small jobs (i.e. just setting a flag or something) this is
going to be faster than dispatching to a thread.

lokimq/batch.h

@@ -38,7 +38,8 @@ namespace detail {
 
 enum class BatchStatus {
     running, // there are still jobs to run (or running)
-    complete, // the batch is complete but still has a completion function to call
+    complete, // the batch is complete but still has a completion job to call
+    complete_proxy, // same as `complete`, but the completion job should be invoked immediately in the proxy thread (be very careful)
     done // the batch is complete and has no completion function
 };
 
@@ -154,6 +155,7 @@ private:
     std::vector<job_result<R>> results;
     CompletionFunc complete;
     std::size_t jobs_outstanding = 0;
+    bool complete_in_proxy = false;
     bool started = false;
 
     void check_not_started() {
@@ -184,9 +186,22 @@ public:
     /// then jobs simply run and results are discarded.
     void completion(CompletionFunc comp) {
         check_not_started();
+        if (complete)
+            throw std::logic_error("Completion function can only be set once");
         complete = std::move(comp);
     }
 
+    /// Sets a completion function to invoke *IN THE PROXY THREAD* after all jobs have finished.  Be
+    /// very, very careful: this should not be a job that takes any significant amount of CPU time
+    /// or can block for any reason (NO MUTEXES).
+    void completion_proxy(CompletionFunc comp) {
+        check_not_started();
+        if (complete)
+            throw std::logic_error("Completion function can only be set once");
+        complete = std::move(comp);
+        complete_in_proxy = true;
+    }
+
 private:
 
     std::size_t size() const override {
@@ -212,7 +227,9 @@ private:
         if (jobs_outstanding)
             return detail::BatchStatus::running;
         if (complete)
-            return detail::BatchStatus::complete;
+            return complete_in_proxy
+                ? detail::BatchStatus::complete_proxy
+                : detail::BatchStatus::complete;
         return detail::BatchStatus::done;
     }
 

lokimq/lokimq.cpp

@@ -1001,6 +1001,17 @@ void LokiMQ::proxy_worker_message(std::vector<zmq::message_t>& parts) {
                 auto status = run.batch->job_finished();
                 if (status == detail::BatchStatus::complete) {
                     batch_jobs.emplace(run.batch, -1);
+                } else if (status == detail::BatchStatus::complete_proxy) {
+                    try {
+                        run.batch->job_completion(); // RUN DIRECTLY IN PROXY THREAD
+                    } catch (const std::exception &e) {
+                        // Raise these to error levels: you really shouldn't be doing anything
+                        // complicated in an in-proxy completion function!
+                        LMQ_LOG(error, "proxy thread caught exception when processing in-proxy completion command: ", e.what());
+                    } catch (...) {
+                        LMQ_LOG(error, "proxy thread caught non-standard exception when processing in-proxy completion command");
+                    }
+                    clear_job = true;
                 } else if (status == detail::BatchStatus::done) {
                     clear_job = true;
                 }