Implement S/G IO for batched sends and eliminate another frame copy (#2874)

src/platform/common.h

@@ -606,15 +606,60 @@ namespace platf {
   void
   restart();
 
-  struct batched_send_info_t {
+  struct buffer_descriptor_t {
     const char *buffer;
-    size_t block_size;
+    size_t size;
+
+    // Constructors required for emplace_back() prior to C++20
+    buffer_descriptor_t(const char *buffer, size_t size):
+        buffer(buffer), size(size) {}
+    buffer_descriptor_t():
+        buffer(nullptr), size(0) {}
+  };
+
+  struct batched_send_info_t {
+    // Optional headers to be prepended to each packet
+    const char *headers;
+    size_t header_size;
+
+    // One or more data buffers to use for the payloads
+    //
+    // NB: Data buffers must be aligned to payload size!
+    std::vector<buffer_descriptor_t> &payload_buffers;
+    size_t payload_size;
+
+    // The offset (in header+payload message blocks) in the header and payload
+    // buffers to begin sending messages from
+    size_t block_offset;
+
+    // The number of header+payload message blocks to send
     size_t block_count;
 
     std::uintptr_t native_socket;
     boost::asio::ip::address &target_address;
     uint16_t target_port;
     boost::asio::ip::address &source_address;
+
+    /**
+     * @brief Returns a payload buffer descriptor for the given payload offset.
+     * @param offset The offset in the total payload data (bytes).
+     * @return Buffer descriptor describing the region at the given offset.
+     */
+    buffer_descriptor_t
+    buffer_for_payload_offset(ptrdiff_t offset) {
+      for (const auto &desc : payload_buffers) {
+        if (offset < desc.size) {
+          return {
+            desc.buffer + offset,
+            desc.size - offset,
+          };
+        }
+        else {
+          offset -= desc.size;
+        }
+      }
+      return {};
+    }
   };
   bool
   send_batch(batched_send_info_t &send_info);