ddtrace/tracer: make use of DD_AGENT_HOST and DD_TRACE_AGENT_PORT (#362)

This change enables the transport to override host and port information
by reading the environment variables DD_AGENT_HOST and
DD_TRACE_AGENT_PORT.

ddtrace/tracer/transport.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"os"
 	"runtime"
 	"strconv"
 	"strings"
@@ -114,7 +115,8 @@ func (t *httpTransport) send(p *payload) error {
 }
 
 // resolveAddr resolves the given agent address and fills in any missing host
-// and port using the defaults.
+// and port using the defaults. Some environment variable settings will
+// take precedence over configuration.
 func resolveAddr(addr string) string {
 	host, port, err := net.SplitHostPort(addr)
 	if err != nil {
@@ -127,5 +129,11 @@ func resolveAddr(addr string) string {
 	if port == "" {
 		port = defaultPort
 	}
+	if v := os.Getenv("DD_AGENT_HOST"); v != "" {
+		host = v
+	}
+	if v := os.Getenv("DD_TRACE_AGENT_PORT"); v != "" {
+		port = v
+	}
 	return fmt.Sprintf("%s:%s", host, port)
 }

ddtrace/tracer/transport_test.go

@@ -101,16 +101,34 @@ func TestTracesAgentIntegration(t *testing.T) {
 
 func TestResolveAddr(t *testing.T) {
 	for _, tt := range []struct {
-		in, out string
+		in, envHost, envPort, out string
 	}{
-		{"host", fmt.Sprintf("host:%s", defaultPort)},
-		{"www.my-address.com", fmt.Sprintf("www.my-address.com:%s", defaultPort)},
-		{"localhost", fmt.Sprintf("localhost:%s", defaultPort)},
-		{":1111", fmt.Sprintf("%s:1111", defaultHostname)},
-		{"", defaultAddress},
-		{"custom:1234", "custom:1234"},
+		{"host", "", "", fmt.Sprintf("host:%s", defaultPort)},
+		{"www.my-address.com", "", "", fmt.Sprintf("www.my-address.com:%s", defaultPort)},
+		{"localhost", "", "", fmt.Sprintf("localhost:%s", defaultPort)},
+		{":1111", "", "", fmt.Sprintf("%s:1111", defaultHostname)},
+		{"", "", "", defaultAddress},
+		{"custom:1234", "", "", "custom:1234"},
+		{"", "", "", defaultAddress},
+		{"", "ip.local", "", fmt.Sprintf("ip.local:%s", defaultPort)},
+		{"", "", "1234", fmt.Sprintf("%s:1234", defaultHostname)},
+		{"", "ip.local", "1234", "ip.local:1234"},
+		{"ip.other", "ip.local", "", fmt.Sprintf("ip.local:%s", defaultPort)},
+		{"ip.other:1234", "ip.local", "", "ip.local:1234"},
+		{":8888", "", "1234", fmt.Sprintf("%s:1234", defaultHostname)},
+		{"ip.other:8888", "", "1234", "ip.other:1234"},
+		{"ip.other", "ip.local", "1234", "ip.local:1234"},
+		{"ip.other:8888", "ip.local", "1234", "ip.local:1234"},
 	} {
-		t.Run(tt.in, func(t *testing.T) {
+		t.Run("", func(t *testing.T) {
+			if tt.envHost != "" {
+				os.Setenv("DD_AGENT_HOST", tt.envHost)
+				defer os.Unsetenv("DD_AGENT_HOST")
+			}
+			if tt.envPort != "" {
+				os.Setenv("DD_TRACE_AGENT_PORT", tt.envPort)
+				defer os.Unsetenv("DD_TRACE_AGENT_PORT")
+			}
 			assert.Equal(t, resolveAddr(tt.in), tt.out)
 		})
 	}