fix(tracing): start the request transaction with the propagation context's span_id

Logs emitted before CaptureExceptions runs carry the propagation context's
span_id. Reuse it for the request's transaction, so those logs point at the
transaction's span.

Author: solnic

sentry-rails/spec/sentry/rails/capture_context_spec.rb

@@ -3,19 +3,25 @@
 require "spec_helper"
 
 RSpec.describe Sentry::Rails::CaptureContext do
-  # Records the current scope's trace_id every time it's called, so specs can
+  # Records the current scope's trace context every time it's called, so specs can
   # compare what a piece of middleware would see at different points in the stack.
   class CaptureContextSpecProbe
     def self.captured_trace_ids
       @captured_trace_ids ||= []
     end
 
+    def self.captured_span_ids
+      @captured_span_ids ||= []
+    end
+
     def initialize(app)
       @app = app
     end
 
     def call(env)
-      self.class.captured_trace_ids << Sentry.get_current_scope.get_trace_context[:trace_id]
+      trace_context = Sentry.get_current_scope.get_trace_context
+      self.class.captured_trace_ids << trace_context[:trace_id]
+      self.class.captured_span_ids << trace_context[:span_id]
       @app.call(env)
     end
   end
@@ -88,6 +94,7 @@ def propagation_context_in_app(env)
   context "when composed with CaptureExceptions", type: :request do
     before do
       CaptureContextSpecProbe.captured_trace_ids.clear
+      CaptureContextSpecProbe.captured_span_ids.clear
     end
 
     context "without tracing enabled" do
@@ -125,6 +132,15 @@ def propagation_context_in_app(env)
         get "/world"
       end
 
+      it "points a span_id captured before CaptureExceptions at the started transaction" do
+        get "/world"
+
+        transaction = transport.events.last
+        early_span_id = CaptureContextSpecProbe.captured_span_ids.first
+
+        expect(early_span_id).to eq(transaction.contexts.dig(:trace, :span_id))
+      end
+
       it "keeps the same trace_id from before CaptureExceptions through the started transaction" do
         get "/world"
 

sentry-ruby/lib/sentry/propagation_context.rb

@@ -165,6 +165,7 @@ def initialize(scope, env = nil)
     def transaction_options
       {
         trace_id: trace_id,
+        span_id: span_id,
         parent_span_id: parent_span_id,
         parent_sampled: parent_sampled,
         baggage: baggage,

sentry-ruby/spec/sentry/propagation_context_spec.rb

@@ -109,6 +109,7 @@
     it "returns the options for continuing a fresh context" do
       expect(subject.transaction_options).to eq({
         trace_id: subject.trace_id,
+        span_id: subject.span_id,
         parent_span_id: nil,
         parent_sampled: nil,
         baggage: nil,
@@ -125,6 +126,7 @@
 
       expect(context.transaction_options).to eq({
         trace_id: "771a43a4192642f0b136d5159a501700",
+        span_id: context.span_id,
         parent_span_id: "7c51afd529da4a2a",
         parent_sampled: true,
         baggage: context.baggage,

sentry-ruby/spec/sentry/rack/capture_exceptions_spec.rb

@@ -138,6 +138,7 @@ def propagation_context_in_app(stack_env)
 
           expect(transaction.type).to eq("transaction")
           expect(transaction.contexts.dig(:trace, :trace_id)).to eq(context_in_app.trace_id)
+          expect(transaction.contexts.dig(:trace, :span_id)).to eq(context_in_app.span_id)
           expect(transaction.contexts.dig(:trace, :parent_span_id)).to be_nil
         end
 
@@ -151,6 +152,7 @@ def propagation_context_in_app(stack_env)
 
           expect(transaction.contexts.dig(:trace, :trace_id)).to eq(external_transaction.trace_id)
           expect(transaction.contexts.dig(:trace, :parent_span_id)).to eq(external_transaction.span_id)
+          expect(transaction.contexts.dig(:trace, :span_id)).to eq(context_in_app.span_id)
         end
       end
     end

sentry-ruby/spec/sentry_spec.rb

@@ -1196,6 +1196,14 @@
         expect(transaction.baggage.mutable).to eq(false)
       end
 
+      it "gives the Transaction the propagation context's span_id" do
+        Sentry.configuration.traces_sample_rate = 1.0
+
+        transaction = described_class.continue_trace(env, name: "foobar")
+
+        expect(transaction.span_id).to eq(Sentry.get_current_scope.propagation_context.span_id)
+      end
+
       describe "sample_rand propagation" do
         before do
           Sentry.configuration.traces_sample_rate = 1.0

spec/features/trace_context_spec.rb

@@ -18,6 +18,13 @@ def request_transaction
       .to eq(request_transaction.dig("contexts", "trace", "trace_id"))
   end
 
+  it "points a log emitted before CaptureExceptions at the transaction's span" do
+    without_trace_propagation { make_request("/trace_context") }
+
+    expect(early_middleware_logs.first["span_id"])
+      .to eq(request_transaction.dig("contexts", "trace", "span_id"))
+  end
+
   it "continues an incoming distributed trace in a log emitted before CaptureExceptions" do
     incoming_trace_id = propagated_trace_id