Add dial-stdio command

This allows the buildx CLI to act a proxy to the configured instance.
It allows external code to use buildx itself as a driver for connecting
to buildkitd instances.

Instance and node selection should follow the same semantics as as
`buildx build`, including taking into account the `BUILDX_BUILDER` env
var and the `--builder` global flag.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

tests/dialstdio.go

@@ -0,0 +1,124 @@
+package tests
+
+import (
+	"bytes"
+	"context"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/docker/buildx/util/progress"
+	"github.com/moby/buildkit/client"
+	"github.com/moby/buildkit/client/llb"
+	gwclient "github.com/moby/buildkit/frontend/gateway/client"
+	"github.com/moby/buildkit/util/progress/progressui"
+	"github.com/moby/buildkit/util/testutil/integration"
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+var dialstdioTests = []func(t *testing.T, sb integration.Sandbox){
+	testDialStdio,
+}
+
+func testDialStdio(t *testing.T, sb integration.Sandbox) {
+	do := func(t *testing.T, pipe func(t *testing.T, cmd *exec.Cmd) net.Conn) {
+		errBuf := bytes.NewBuffer(nil)
+		defer func() {
+			if t.Failed() {
+				t.Log(errBuf.String())
+			}
+		}()
+		var cmd *exec.Cmd
+		c, err := client.New(sb.Context(), "", client.WithContextDialer(func(ctx context.Context, _ string) (net.Conn, error) {
+			cmd = buildxCmd(sb, withArgs("dial-stdio", "--progress", "auto"))
+			conn := pipe(t, cmd)
+			cmd.Stderr = errBuf
+			if err := cmd.Start(); err != nil {
+				return nil, errors.Wrap(err, errBuf.String())
+			}
+
+			return conn, nil
+		}))
+		require.NoError(t, err)
+
+		defer func() {
+			c.Close()
+			// Since the client is closed (and as such the connection shutdown), the buildx command should exit cleanly.
+			chErr := make(chan error, 1)
+			go func() {
+				chErr <- cmd.Wait()
+			}()
+			select {
+			case <-time.After(10 * time.Second):
+				t.Error("timeout waiting for buildx command to exit")
+			case <-chErr:
+				assert.NoError(t, err)
+			}
+		}()
+
+		_, err = c.Info(sb.Context())
+		require.NoError(t, err)
+
+		require.Contains(t, errBuf.String(), "builder: "+sb.Address())
+
+		dir := t.TempDir()
+
+		f, err := os.CreateTemp(dir, "log")
+		require.NoError(t, err)
+		defer f.Close()
+
+		defer func() {
+			if t.Failed() {
+				dt, _ := os.ReadFile(f.Name())
+				t.Log(string(dt))
+			}
+		}()
+
+		p, err := progress.NewPrinter(sb.Context(), f, progressui.AutoMode)
+		require.NoError(t, err)
+
+		ch, chDone := progress.NewChannel(p)
+		done := func() {
+			select {
+			case <-sb.Context().Done():
+			case <-chDone:
+			}
+		}
+
+		_, err = c.Build(sb.Context(), client.SolveOpt{
+			Exports: []client.ExportEntry{
+				{Type: "local", OutputDir: dir},
+			},
+		}, "", func(ctx context.Context, gwc gwclient.Client) (*gwclient.Result, error) {
+			def, err := llb.Scratch().File(llb.Mkfile("hello", 0o600, []byte("world"))).Marshal(ctx)
+			if err != nil {
+				return nil, err
+			}
+
+			return gwc.Solve(ctx, gwclient.SolveRequest{
+				Definition: def.ToPB(),
+			})
+		}, ch)
+		done()
+		require.NoError(t, err)
+
+		dt, err := os.ReadFile(filepath.Join(dir, "hello"))
+		require.NoError(t, err)
+		require.Equal(t, "world", string(dt))
+	}
+
+	t.Run("conn=netpipe", func(t *testing.T) {
+		t.Parallel()
+		do(t, func(t *testing.T, cmd *exec.Cmd) net.Conn {
+			c1, c2 := net.Pipe()
+			cmd.Stdin = c1
+			cmd.Stdout = c1
+			return c2
+		})
+	})
+}