4 files changed, 157 insertions, 0 deletions

diff --git a/internal/trace/trace.go b/internal/trace/trace.go
new file mode 100644
index 0000000..3e15c5b
--- /dev/null
+++ b/internal/trace/trace.go
@@ -0,0 +1,55 @@
+package trace
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"sync/atomic"
+)
+
+var (
+	// logger is the logger to use for tracing.
+	logger = newLogger()
+
+	// current is the targets that are enabled for tracing.
+	current atomic.Int32
+)
+
+func newLogger() *log.Logger {
+	return log.New(os.Stderr, "", log.Ltime|log.Lmicroseconds|log.Lshortfile)
+}
+
+// Target is a tracing target.
+type Target int32
+
+const (
+	// General traces general operations.
+	General Target = 1 << iota
+
+	// Packet traces git packets.
+	Packet
+)
+
+// SetTarget sets the tracing targets.
+func SetTarget(target Target) {
+	current.Store(int32(target))
+}
+
+// SetLogger sets the logger to use for tracing.
+func SetLogger(l *log.Logger) {
+	logger = l
+}
+
+// Print prints the given message only if the target is enabled.
+func (t Target) Print(args ...interface{}) {
+	if int32(t)&current.Load() != 0 {
+		logger.Output(2, fmt.Sprint(args...)) // nolint: errcheck
+	}
+}
+
+// Printf prints the given message only if the target is enabled.
+func (t Target) Printf(format string, args ...interface{}) {
+	if int32(t)&current.Load() != 0 {
+		logger.Output(2, fmt.Sprintf(format, args...)) // nolint: errcheck
+	}
+}
diff --git a/internal/trace/trace_test.go b/internal/trace/trace_test.go
new file mode 100644
index 0000000..6f8f140
--- /dev/null
+++ b/internal/trace/trace_test.go
@@ -0,0 +1,95 @@
+package trace
+
+import (
+	"bytes"
+	"io"
+	"log"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	defer SetLogger(newLogger())
+	if code := m.Run(); code != 0 {
+		panic(code)
+	}
+}
+
+func setUpTest(t testing.TB, buf *bytes.Buffer) {
+	t.Cleanup(func() {
+		if buf != nil {
+			buf.Reset()
+		}
+		SetTarget(0)
+	})
+	w := io.Discard
+	if buf != nil {
+		w = buf
+	}
+	SetLogger(log.New(w, "", 0))
+}
+
+func TestEmpty(t *testing.T) {
+	var buf bytes.Buffer
+	setUpTest(t, &buf)
+	General.Print("test")
+	if buf.String() != "" {
+		t.Error("expected empty string")
+	}
+}
+
+func TestOneTarget(t *testing.T) {
+	var buf bytes.Buffer
+	setUpTest(t, &buf)
+	SetTarget(General)
+	General.Print("test")
+	if buf.String() != "test\n" {
+		t.Error("expected 'test'")
+	}
+}
+
+func TestMultipleTargets(t *testing.T) {
+	var buf bytes.Buffer
+	setUpTest(t, &buf)
+	SetTarget(General | Packet)
+	General.Print("a")
+	Packet.Print("b")
+	if buf.String() != "a\nb\n" {
+		t.Error("expected 'a\nb\n'")
+	}
+}
+
+func TestPrintf(t *testing.T) {
+	var buf bytes.Buffer
+	setUpTest(t, &buf)
+	SetTarget(General)
+	General.Printf("a %d", 1)
+	if buf.String() != "a 1\n" {
+		t.Error("expected 'a 1\n'")
+	}
+}
+
+func TestDisabledMultipleTargets(t *testing.T) {
+	var buf bytes.Buffer
+	setUpTest(t, &buf)
+	SetTarget(General)
+	General.Print("a")
+	Packet.Print("b")
+	if buf.String() != "a\n" {
+		t.Error("expected 'a\n'")
+	}
+}
+
+func BenchmarkDisabledTarget(b *testing.B) {
+	setUpTest(b, nil)
+	for i := 0; i < b.N; i++ {
+		General.Print("test")
+	}
+}
+
+func BenchmarkEnabledTarget(b *testing.B) {
+	setUpTest(b, nil)
+	SetTarget(General)
+	for i := 0; i < b.N; i++ {
+		General.Print("test")
+	}
+}
diff --git a/plumbing/format/pktline/encoder.go b/plumbing/format/pktline/encoder.go
index 6d40979..1b9b84e 100644
--- a/plumbing/format/pktline/encoder.go
+++ b/plumbing/format/pktline/encoder.go
@@ -7,6 +7,8 @@ import (
 	"errors"
 	"fmt"
 	"io"
+
+	"github.com/go-git/go-git/v5/internal/trace"
 )
 
 // An Encoder writes pkt-lines to an output stream.
@@ -43,6 +45,7 @@ func NewEncoder(w io.Writer) *Encoder {
 
 // Flush encodes a flush-pkt to the output stream.
 func (e *Encoder) Flush() error {
+	defer trace.Packet.Print("packet: > 0000")
 	_, err := e.w.Write(FlushPkt)
 	return err
 }
@@ -70,6 +73,7 @@ func (e *Encoder) encodeLine(p []byte) error {
 	}
 
 	n := len(p) + 4
+	defer trace.Packet.Printf("packet: > %04x %s", n, p)
 	if _, err := e.w.Write(asciiHex16(n)); err != nil {
 		return err
 	}
diff --git a/plumbing/format/pktline/scanner.go b/plumbing/format/pktline/scanner.go
index 99aab46..86068af 100644
--- a/plumbing/format/pktline/scanner.go
+++ b/plumbing/format/pktline/scanner.go
@@ -3,6 +3,8 @@ package pktline
 import (
 	"errors"
 	"io"
+
+	"github.com/go-git/go-git/v5/internal/trace"
 )
 
 const (
@@ -65,6 +67,7 @@ func (s *Scanner) Scan() bool {
 		return false
 	}
 	s.payload = s.payload[:l]
+	trace.Packet.Printf("packet: < %04x %s", l, s.payload)
 
 	return true
 }