CXXCBC-387: Optimising tags for noop_tracer (#461)

* CXXCBC-387: Performance: potential micro-optimisation on tags and local/remote sessions

Profiling indicates that non-trivial time is spent assigning
tags to spans, which is wasted effort if noop_tracer is being
used.

Borrowing an optimisation from the JVM SDKs, where we skip this
assignment in this case.

This optimisation, together with another small one that will be
submitted separately, produced around a 15% performance improvement.

* Clang and sanitizer

core/io/http_command.hxx

@@ -74,17 +74,21 @@ struct http_command : public std::enable_shared_from_this<http_command<Request>>
         if (span_ == nullptr) {
             return;
         }
-        span_->add_tag(tracing::attributes::remote_socket, remote_address);
-        span_->add_tag(tracing::attributes::local_socket, local_address);
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::remote_socket, remote_address);
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::local_socket, local_address);
         span_->end();
         span_ = nullptr;
     }
 
     void start(http_command_handler&& handler)
     {
         span_ = tracer_->start_span(tracing::span_name_for_http_service(request.type), parent_span);
-        span_->add_tag(tracing::attributes::service, tracing::service_name_for_http_service(request.type));
-        span_->add_tag(tracing::attributes::operation_id, client_context_id_);
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::service, tracing::service_name_for_http_service(request.type));
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::operation_id, client_context_id_);
         handler_ = std::move(handler);
         deadline.expires_after(timeout_);
         deadline.async_wait([self = this->shared_from_this()](std::error_code ec) {
@@ -123,7 +127,8 @@ struct http_command : public std::enable_shared_from_this<http_command<Request>>
             return;
         }
         session_ = std::move(session);
-        span_->add_tag(tracing::attributes::local_id, session_->id());
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::local_id, session_->id());
         send();
     }
 

core/io/mcbp_command.hxx

@@ -93,8 +93,10 @@ struct mcbp_command : public std::enable_shared_from_this<mcbp_command<Manager,
     void start(mcbp_command_handler&& handler)
     {
         span_ = manager_->tracer()->start_span(tracing::span_name_for_mcbp_command(encoded_request_type::body_type::opcode), parent_span);
-        span_->add_tag(tracing::attributes::service, tracing::service::key_value);
-        span_->add_tag(tracing::attributes::instance, request.id.bucket());
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::service, tracing::service::key_value);
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::instance, request.id.bucket());
 
         handler_ = std::move(handler);
         deadline.expires_after(timeout_);
@@ -199,7 +201,8 @@ struct mcbp_command : public std::enable_shared_from_this<mcbp_command<Manager,
     {
         opaque_ = session_->next_opaque();
         request.opaque = *opaque_;
-        span_->add_tag(tracing::attributes::operation_id, fmt::format("0x{:x}", request.opaque));
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::operation_id, fmt::format("0x{:x}", request.opaque));
         if (request.id.use_collections() && !request.id.is_collection_resolved()) {
             if (session_->supports_feature(protocol::hello_feature::collections)) {
                 auto collection_id = session_->get_collection_uid(request.id.collection_path());
@@ -249,13 +252,15 @@ struct mcbp_command : public std::enable_shared_from_this<mcbp_command<Manager,
 
               self->retry_backoff.cancel();
               if (ec == asio::error::operation_aborted) {
-                  self->span_->add_tag(tracing::attributes::orphan, "aborted");
+                  if (self->span_->uses_tags())
+                      self->span_->add_tag(tracing::attributes::orphan, "aborted");
                   return self->invoke_handler(make_error_code(self->request.retries.idempotent() ? errc::common::unambiguous_timeout
                                                                                                  : errc::common::ambiguous_timeout));
               }
               if (ec == errc::common::request_canceled) {
                   if (reason == retry_reason::do_not_retry) {
-                      self->span_->add_tag(tracing::attributes::orphan, "canceled");
+                      if (self->span_->uses_tags())
+                          self->span_->add_tag(tracing::attributes::orphan, "canceled");
                       return self->invoke_handler(ec);
                   }
                   return io::retry_orchestrator::maybe_retry(self->manager_, self, reason, ec);
@@ -314,9 +319,12 @@ struct mcbp_command : public std::enable_shared_from_this<mcbp_command<Manager,
             return;
         }
         session_ = std::move(session);
-        span_->add_tag(tracing::attributes::remote_socket, session_->remote_address());
-        span_->add_tag(tracing::attributes::local_socket, session_->local_address());
-        span_->add_tag(tracing::attributes::local_id, session_->id());
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::remote_socket, session_->remote_address());
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::local_socket, session_->local_address());
+        if (span_->uses_tags())
+            span_->add_tag(tracing::attributes::local_id, session_->id());
         send();
     }
 };

core/tracing/noop_tracer.hxx

@@ -38,6 +38,11 @@ class noop_span : public couchbase::tracing::request_span
     {
         /* do nothing */
     }
+
+    bool uses_tags() const override
+    {
+        return false;
+    }
 };
 
 class noop_tracer : public couchbase::tracing::request_tracer

couchbase/tracing/request_span.hxx

@@ -56,6 +56,11 @@ class request_span
         return parent_;
     }
 
+    virtual bool uses_tags() const
+    {
+        return true;
+    }
+
   private:
     std::string name_{};
     std::shared_ptr<request_span> parent_{ nullptr };