PrivHelper: introduce functions for [de]serializing optional values

Summary:
# This diff

some NFS mount options are optional, and therefore we may need to essentially pass "std::nullopt" across the wire. Since std::optional<T> are not writeable (it is not an arithmetic type), we'll have to invent our own method for serializing/deserializing those values.

in this diff, I introduce [de]serializeOptional() functions which will:
1) serialize a bool to indicate whether the optional value is present or not.
2) If the value is present, then we continue to [de]serialize the value
3) If the value is std::nullopt, then we can skip #2 because there's no value to [de]serialize

# Context

NFS mounts are extremely configurable, especially on macOS. The configurations vary a ton and impact mount perf/reliability in many different ways. Instead of passing around all these mount options as a list of parameters, I decided to clean up the code and pass them around in a struct. That will keep the nfsMount() signature from growing out of hand, and overall make the code cleaner. This clean up is motivated by a desire to add even more NFS Mount Option configurations in the future (which is done later in this stack).

Reviewed By: jdelliot

Differential Revision: D68602730

fbshipit-source-id: 0e157bac918af0df07e3f0a250c68278bcc412bf

Author: MichaelCuevas

eden/fs/privhelper/PrivHelperConn.cpp

@@ -68,6 +68,33 @@ UnixSocket::Message serializeRequestPacket(
   return msg;
 }
 
+// Template function for serializing an optional value of any type that can be
+// trivially serialized (i.e. not strings or socket addresses)
+template <
+    typename T,
+    std::enable_if_t<std::is_arithmetic<T>::value, bool> = true>
+[[maybe_unused]] void serializeOption(Appender& a, std::optional<T> val) {
+  bool is_some = val.has_value();
+  a.write<bool>(is_some);
+  if (is_some) {
+    a.write<T>(val.value());
+  }
+}
+
+// Template function for deserializing an optional value of any type that can be
+//  trivially deserialized (i.e. not strings or socket addresses)
+template <
+    typename T,
+    std::enable_if_t<std::is_arithmetic<T>::value, bool> = true>
+[[maybe_unused]] std::optional<T> deserializeOption(Cursor& cursor) {
+  bool is_some = cursor.read<bool>();
+  if (is_some) {
+    return cursor.read<T>();
+  } else {
+    return std::nullopt;
+  }
+}
+
 void serializeString(Appender& a, StringPiece str) {
   a.write<uint32_t>(str.size());
   a.push(ByteRange(str));