ddtrace/tracer: add hostname tag to root spans (#437)

This functionality is controlled by the DD_TRACE_REPORT_HOSTNAME
environment variable. When set to "true", the root spans will
automatically have a tag added containing the hostname.

ddtrace/tracer/option.go

@@ -1,6 +1,7 @@
 package tracer
 
 import (
+	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -22,7 +23,7 @@ type config struct {
 	// sampler specifies the sampler that will be used for sampling traces.
 	sampler Sampler
 
-	// agentAddr specifies the hostname and  of the agent where the traces
+	// agentAddr specifies the hostname and port of the agent where the traces
 	// are sent to.
 	agentAddr string
 
@@ -38,6 +39,10 @@ type config struct {
 
 	// httpRoundTripper defines the http.RoundTripper used by the agent transport.
 	httpRoundTripper http.RoundTripper
+
+	// hostname is automatically assigned when the DD_TRACE_REPORT_HOSTNAME is set to true,
+	// and is added as a special tag to the root span of traces.
+	hostname string
 }
 
 // StartOption represents a function that can be provided as a parameter to Start.
@@ -48,6 +53,14 @@ func defaults(c *config) {
 	c.serviceName = filepath.Base(os.Args[0])
 	c.sampler = NewAllSampler()
 	c.agentAddr = defaultAddress
+
+	if os.Getenv("DD_TRACE_REPORT_HOSTNAME") == "true" {
+		var err error
+		c.hostname, err = os.Hostname()
+		if err != nil {
+			log.Printf("%sunable to look up hostname: %v\n", errorPrefix, err)
+		}
+	}
 }
 
 // WithPrioritySampling is deprecated, and priority sampling is enabled by default.

ddtrace/tracer/span.go

@@ -318,4 +318,5 @@ const (
 	keySamplingPriority     = "_sampling_priority_v1"
 	keySamplingPriorityRate = "_sampling_priority_rate_v1"
 	keyOrigin               = "_dd.origin"
+	keyHostname             = "_dd.hostname"
 )

ddtrace/tracer/tracer.go

@@ -273,6 +273,9 @@ func (t *tracer) StartSpan(operationName string, options ...ddtrace.StartSpanOpt
 	if context == nil || context.span == nil {
 		// this is either a root span or it has a remote parent, we should add the PID.
 		span.SetTag(ext.Pid, t.pid)
+		if t.hostname != "" {
+			span.SetTag(keyHostname, t.hostname)
+		}
 	}
 	// add tags from options
 	for k, v := range opts.Tags {

ddtrace/tracer/tracer_test.go

@@ -946,6 +946,47 @@ func TestTracerFlush(t *testing.T) {
 	})
 }
 
+func TestTracerReportsHostname(t *testing.T) {
+	t.Run("enabled", func(t *testing.T) {
+		os.Setenv("DD_TRACE_REPORT_HOSTNAME", "true")
+		defer os.Unsetenv("DD_TRACE_REPORT_HOSTNAME")
+
+		tracer, _, stop := startTestTracer()
+		defer stop()
+
+		root := tracer.StartSpan("root").(*span)
+		child := tracer.StartSpan("child", ChildOf(root.Context())).(*span)
+		child.Finish()
+		root.Finish()
+
+		assert := assert.New(t)
+
+		name, ok := root.Meta[keyHostname]
+		assert.True(ok)
+		assert.Equal(name, tracer.hostname)
+
+		_, ok = child.Meta[keyHostname]
+		assert.False(ok)
+	})
+
+	t.Run("disabled", func(t *testing.T) {
+		tracer, _, stop := startTestTracer()
+		defer stop()
+
+		root := tracer.StartSpan("root").(*span)
+		child := tracer.StartSpan("child", ChildOf(root.Context())).(*span)
+		child.Finish()
+		root.Finish()
+
+		assert := assert.New(t)
+
+		_, ok := root.Meta[keyHostname]
+		assert.False(ok)
+		_, ok = child.Meta[keyHostname]
+		assert.False(ok)
+	})
+}
+
 // BenchmarkConcurrentTracing tests the performance of spawning a lot of
 // goroutines where each one creates a trace with a parent and a child.
 func BenchmarkConcurrentTracing(b *testing.B) {