cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality (#12945)

cmd/k8s-operator,k8s-operator/sessionrecording,sessionrecording,ssh/tailssh: refactor session recording functionality

Refactor SSH session recording functionality (mostly the bits related to
Kubernetes API server proxy 'kubectl exec' session recording):

- move the session recording bits used by both Tailscale SSH
and the Kubernetes API server proxy into a shared sessionrecording package,
to avoid having the operator to import ssh/tailssh

- move the Kubernetes API server proxy session recording functionality
into a k8s-operator/sessionrecording package, add some abstractions
in preparation for adding support for a second streaming protocol (WebSockets)

Updates tailscale/corp#19821

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

k8s-operator/sessionrecording/fakes/fakes.go

@@ -0,0 +1,118 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !plan9
+
+// Package fakes contains mocks used for testing 'kubectl exec' session
+// recording functionality.
+package fakes
+
+import (
+	"bytes"
+	"encoding/json"
+	"net"
+	"sync"
+	"testing"
+
+	"tailscale.com/sessionrecording"
+	"tailscale.com/tstime"
+)
+
+func New(conn net.Conn, wb bytes.Buffer, rb bytes.Buffer, closed bool) net.Conn {
+	return &TestConn{
+		Conn:     conn,
+		writeBuf: wb,
+		readBuf:  rb,
+		closed:   closed,
+	}
+}
+
+type TestConn struct {
+	net.Conn
+	// writeBuf contains whatever was send to the conn via Write.
+	writeBuf bytes.Buffer
+	// readBuf contains whatever was sent to the conn via Read.
+	readBuf      bytes.Buffer
+	sync.RWMutex // protects the following
+	closed       bool
+}
+
+var _ net.Conn = &TestConn{}
+
+func (tc *TestConn) Read(b []byte) (int, error) {
+	return tc.readBuf.Read(b)
+}
+
+func (tc *TestConn) Write(b []byte) (int, error) {
+	return tc.writeBuf.Write(b)
+}
+
+func (tc *TestConn) Close() error {
+	tc.Lock()
+	defer tc.Unlock()
+	tc.closed = true
+	return nil
+}
+
+func (tc *TestConn) IsClosed() bool {
+	tc.Lock()
+	defer tc.Unlock()
+	return tc.closed
+}
+
+func (tc *TestConn) WriteBufBytes() []byte {
+	return tc.writeBuf.Bytes()
+}
+
+func (tc *TestConn) ResetReadBuf() {
+	tc.readBuf.Reset()
+}
+
+func (tc *TestConn) WriteReadBufBytes(b []byte) error {
+	_, err := tc.readBuf.Write(b)
+	return err
+}
+
+type TestSessionRecorder struct {
+	// buf holds data that was sent to the session recorder.
+	buf bytes.Buffer
+}
+
+func (t *TestSessionRecorder) Write(b []byte) (int, error) {
+	return t.buf.Write(b)
+}
+
+func (t *TestSessionRecorder) Close() error {
+	t.buf.Reset()
+	return nil
+}
+
+func (t *TestSessionRecorder) Bytes() []byte {
+	return t.buf.Bytes()
+}
+
+func CastLine(t *testing.T, p []byte, clock tstime.Clock) []byte {
+	t.Helper()
+	j, err := json.Marshal([]any{
+		clock.Now().Sub(clock.Now()).Seconds(),
+		"o",
+		string(p),
+	})
+	if err != nil {
+		t.Fatalf("error marshalling cast line: %v", err)
+	}
+	return append(j, '\n')
+}
+
+func AsciinemaResizeMsg(t *testing.T, width, height int) []byte {
+	t.Helper()
+	ch := sessionrecording.CastHeader{
+		Width:  width,
+		Height: height,
+	}
+	bs, err := json.Marshal(ch)
+	if err != nil {
+		t.Fatalf("error marshalling CastHeader: %v", err)
+	}
+	return append(bs, '\n')
+}