touch_directory: EEXIST-ignoring make directory

There are many situations in which we would like to make sure a directory
exists.  We can do that by creating the directory we want, and just ignoring
the relevant error.

It is a lot of code though, and I believe it is an idiom common enough to exist
on its own.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>

core/reactor.cc

@@ -1917,6 +1917,18 @@ future<> make_directory(sstring name) {
     return engine().make_directory(std::move(name));
 }
 
+future<> touch_directory(sstring name) {
+    return make_directory(name).then_wrapped([] (future<> f) {
+        try {
+            f.get();
+        } catch (std::system_error& e) {
+            if (e.code() != std::error_code(EEXIST, std::system_category())) {
+                throw;
+            }
+        }
+    });
+}
+
 future<> remove_file(sstring pathname) {
     return engine().remove_file(std::move(pathname));
 }

core/seastar.hh

@@ -108,5 +108,6 @@ future<connected_socket> connect(socket_address sa);
 future<file> open_file_dma(sstring name, open_flags flags);
 future<file> open_directory(sstring name);
 future<> make_directory(sstring name);
+future<> touch_directory(sstring name);
 future<> remove_file(sstring pathname);