Implement file::flush()

reactor.cc

@@ -189,6 +189,15 @@ reactor::open_file_dma(sstring name) {
     });
 }
 
+future<>
+reactor::flush(file& f) {
+    return _thread_pool.submit<syscall_result<int>>([&f] {
+        return wrap_syscall<int>(::fsync(f._fd));
+    }).then([] (syscall_result<int> sr) {
+        assert(sr.result != -1);
+        return make_ready_future<>();
+    });
+}
 
 void reactor::run() {
     std::vector<std::unique_ptr<task>> current_tasks;

reactor.hh

@@ -423,6 +423,7 @@ public:
     future<size_t> write_all(pollable_fd_state& fd, const void* buffer, size_t size);
 
     future<file> open_file_dma(sstring name);
+    future<> flush(file& f);
 
     void run();
 
@@ -603,7 +604,9 @@ public:
         return the_reactor.write_dma(*this, pos, std::move(iov));
     }
 
-    future<> flush();
+    future<> flush() {
+        return the_reactor.flush(*this);
+    }
 
     friend class reactor;
 };