msg/async/ProtocolV2: pass desc as std::string_view to write()

All callers really pass a C string literal, and declaring a
`std::string` parameter will implicitly create two `std::string`
instances: one on the caller's stack, and another one inside write()
as parameter to the continuation lambda.  This causes considerable and
unnecessary overhead.

Signed-off-by: Max Kellermann <[email protected]>

Author: MaxKellermann

src/msg/async/ProtocolV2.cc

@@ -796,7 +796,7 @@ CtPtr ProtocolV2::read(CONTINUATION_RXBPTR_TYPE<ProtocolV2> &next,
 }
 
 template <class F>
-CtPtr ProtocolV2::write(const std::string &desc,
+CtPtr ProtocolV2::write(std::string_view desc,
                         CONTINUATION_TYPE<ProtocolV2> &next,
                         F &frame) {
   ceph::bufferlist bl;
@@ -812,7 +812,7 @@ CtPtr ProtocolV2::write(const std::string &desc,
   return write(desc, next, bl);
 }
 
-CtPtr ProtocolV2::write(const std::string &desc,
+CtPtr ProtocolV2::write(std::string_view desc,
                         CONTINUATION_TYPE<ProtocolV2> &next,
                         ceph::bufferlist &buffer) {
   if (unlikely(pre_auth.enabled)) {

src/msg/async/ProtocolV2.h

@@ -130,10 +130,10 @@ class ProtocolV2 : public Protocol {
   Ct<ProtocolV2> *read(CONTINUATION_RXBPTR_TYPE<ProtocolV2> &next,
                        rx_buffer_t&& buffer);
   template <class F>
-  Ct<ProtocolV2> *write(const std::string &desc,
+  Ct<ProtocolV2> *write(std::string_view desc,
                         CONTINUATION_TYPE<ProtocolV2> &next,
 			F &frame);
-  Ct<ProtocolV2> *write(const std::string &desc,
+  Ct<ProtocolV2> *write(std::string_view desc,
                         CONTINUATION_TYPE<ProtocolV2> &next,
                         ceph::bufferlist &buffer);