feat: debug connection reuse (#323)

Add a new SENTRYGODEBUG option httptrace to print DSN resolution and TCP
connection information, including whether connections are being reused.

Rename internal debug option dumphttp -> httpdump for symmetry with
httptrace (that derives from the net/http/httptrace package name).

client.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"reflect"
 	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -212,10 +213,26 @@ func NewClient(options ClientOptions) (*Client, error) {
 		options.Environment = os.Getenv("SENTRY_ENVIRONMENT")
 	}
 
-	if env := os.Getenv("SENTRYGODEBUG"); env == "dumphttp=1" {
+	// SENTRYGODEBUG is a comma-separated list of key=value pairs (similar
+	// to GODEBUG). It is not a supported feature: recognized debug options
+	// may change any time.
+	//
+	// The intended public is SDK developers. It is orthogonal to
+	// options.Debug, which is also available for SDK users.
+	dbg := strings.Split(os.Getenv("SENTRYGODEBUG"), ",")
+	sort.Strings(dbg)
+	// dbgOpt returns true when the given debug option is enabled, for
+	// example SENTRYGODEBUG=someopt=1.
+	dbgOpt := func(opt string) bool {
+		s := opt + "=1"
+		return dbg[sort.SearchStrings(dbg, s)%len(dbg)] == s
+	}
+	if dbgOpt("httpdump") || dbgOpt("httptrace") {
 		options.HTTPTransport = &debug.Transport{
 			RoundTripper: http.DefaultTransport,
 			Output:       os.Stderr,
+			Dump:         dbgOpt("httpdump"),
+			Trace:        dbgOpt("httptrace"),
 		}
 	}
 

internal/debug/transport.go

@@ -1,38 +1,72 @@
 package debug
 
 import (
+	"bytes"
+	"fmt"
 	"io"
 	"net/http"
+	"net/http/httptrace"
 	"net/http/httputil"
 )
 
 // Transport implements http.RoundTripper and can be used to wrap other HTTP
-// transports to dump request and responses for debugging.
+// transports for debugging, normally http.DefaultTransport.
 type Transport struct {
 	http.RoundTripper
 	Output io.Writer
+	// Dump controls whether to dump HTTP request and responses.
+	Dump bool
+	// Trace enables usage of net/http/httptrace.
+	Trace bool
 }
 
 func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
-	b, err := httputil.DumpRequestOut(req, true)
-	if err != nil {
-		return nil, err
+	var buf bytes.Buffer
+	if t.Dump {
+		b, err := httputil.DumpRequestOut(req, true)
+		if err != nil {
+			panic(err)
+		}
+		_, err = buf.Write(ensureTrailingNewline(b))
+		if err != nil {
+			panic(err)
+		}
 	}
-	_, err = t.Output.Write(ensureTrailingNewline(b))
-	if err != nil {
-		return nil, err
+	if t.Trace {
+		trace := &httptrace.ClientTrace{
+			DNSDone: func(di httptrace.DNSDoneInfo) {
+				fmt.Fprintf(&buf, "* DNS %v → %v\n", req.Host, di.Addrs)
+			},
+			GotConn: func(ci httptrace.GotConnInfo) {
+				fmt.Fprintf(&buf, "* Connection local=%v remote=%v", ci.Conn.LocalAddr(), ci.Conn.RemoteAddr())
+				if ci.Reused {
+					fmt.Fprint(&buf, " (reused)")
+				}
+				if ci.WasIdle {
+					fmt.Fprintf(&buf, " (idle %v)", ci.IdleTime)
+				}
+				fmt.Fprintln(&buf)
+			},
+		}
+		req = req.WithContext(httptrace.WithClientTrace(req.Context(), trace))
 	}
 	resp, err := t.RoundTripper.RoundTrip(req)
 	if err != nil {
 		return nil, err
 	}
-	b, err = httputil.DumpResponse(resp, true)
-	if err != nil {
-		return nil, err
+	if t.Dump {
+		b, err := httputil.DumpResponse(resp, true)
+		if err != nil {
+			panic(err)
+		}
+		_, err = buf.Write(ensureTrailingNewline(b))
+		if err != nil {
+			panic(err)
+		}
 	}
-	_, err = t.Output.Write(ensureTrailingNewline(b))
+	_, err = io.Copy(t.Output, &buf)
 	if err != nil {
-		return nil, err
+		panic(err)
 	}
 	return resp, nil
 }