From d1ff49a414631a0ec204351350f7be6b61afb1a4 Mon Sep 17 00:00:00 2001
From: Andrew Newdigate <andrew@gitlab.com>
Date: Thu, 28 Feb 2019 20:28:42 +0200
Subject: A distributed tracing to all metric instrumentation

This change adds additional opentracing instrumentation to all
methods instrumented by the `GitLab::Metrics` instrumentation.

This vastly increases the distributed tracing coverage of the ruby
application.
---
 lib/gitlab/metrics/instrumentation.rb | 44 ++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 9 deletions(-)

diff --git a/lib/gitlab/metrics/instrumentation.rb b/lib/gitlab/metrics/instrumentation.rb
index ff3fffe7b95..2dd5cb6a371 100644
--- a/lib/gitlab/metrics/instrumentation.rb
+++ b/lib/gitlab/metrics/instrumentation.rb
@@ -118,7 +118,7 @@ module Gitlab
       # mod  - The module containing the method.
       # name - The name of the method to instrument.
       def self.instrument(type, mod, name)
-        return unless ::Gitlab::Metrics.enabled?
+        return unless ::Gitlab::Metrics.enabled? or ::Gitlab::Tracing.enabled?
 
         name = name.to_sym
         target = type == :instance ? mod : mod.singleton_class
@@ -132,8 +132,7 @@ module Gitlab
           method_name = ".#{name}"
           method = mod.method(name)
         end
-
-        label = "#{mod.name}#{method_name}"
+        method_label = "#{mod.name}#{method_name}"
 
         unless instrumented?(target)
           target.instance_variable_set(PROXY_IVAR, Module.new)
@@ -154,20 +153,47 @@ module Gitlab
             '*args'
           end
 
+        method_body = if ::Gitlab::Metrics.enabled?
+          measure_eval(method_label, mod, method_name)
+        else
+          "super"
+        end
+
+        if ::Gitlab::Tracing.enabled?
+          method_body = trace_eval(method_label, method_body)
+        end
+
         proxy_module.class_eval <<-EOF, __FILE__, __LINE__ + 1
           def #{name}(#{args_signature})
-            if trans = Gitlab::Metrics::Instrumentation.transaction
-              trans.method_call_for(#{label.to_sym.inspect}, #{mod.name.inspect}, "#{method_name}")
-                .measure { super }
-            else
-              super
-            end
+            #{method_body}
           end
         EOF
 
         target.prepend(proxy_module)
       end
 
+      def self.trace_eval(method_label, inner)
+        <<-EOF
+        retval = nil
+        OpenTracing.start_active_span("#{method_label}", { tags: {}}) do |scope|
+          retval = #{inner}
+        end
+
+        retval
+        EOF
+      end
+
+      def self.measure_eval(method_label, mod, method_name)
+        <<-EOF
+        if trans = Gitlab::Metrics::Instrumentation.transaction
+          trans.method_call_for(#{method_label.to_sym.inspect}, #{mod.name.inspect}, "#{method_name}")
+            .measure { super }
+        else
+          super
+        end
+        EOF
+      end
+
       # Small layer of indirection to make it easier to stub out the current
       # transaction.
       def self.transaction
-- 
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