monitor: Move commands to a separated package

This commit moves monitor commands to `monior/commands` package.
Commands still need access to the `monitor` object and buildx controller so this
commit enables this via `Monitor` interface stored in `monitor/types`.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

monitor/commands/attach.go

@@ -0,0 +1,71 @@
+package commands
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/docker/buildx/monitor/types"
+	"github.com/pkg/errors"
+)
+
+type AttachCmd struct {
+	m types.Monitor
+
+	stdout io.WriteCloser
+}
+
+func NewAttachCmd(m types.Monitor, stdout io.WriteCloser) types.Command {
+	return &AttachCmd{m, stdout}
+}
+
+func (cm *AttachCmd) Info() types.CommandInfo {
+	return types.CommandInfo{HelpMessage: "attach to a buildx server or a process in the container"}
+}
+
+func (cm *AttachCmd) Exec(ctx context.Context, args []string) error {
+	if len(args) < 2 {
+		return errors.Errorf("server name must be passed")
+	}
+	ref := args[1]
+	var id string
+
+	isProcess, err := isProcessID(ctx, cm.m, ref)
+	if err == nil && isProcess {
+		cm.m.Attach(ctx, ref)
+		id = ref
+	}
+	if id == "" {
+		refs, err := cm.m.List(ctx)
+		if err != nil {
+			return errors.Errorf("failed to get the list of sessions: %v", err)
+		}
+		found := false
+		for _, s := range refs {
+			if s == ref {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return errors.Errorf("unknown ID: %q", ref)
+		}
+		cm.m.Detach() // Finish existing attach
+		cm.m.AttachSession(ref)
+	}
+	fmt.Fprintf(cm.stdout, "Attached to process %q. Press Ctrl-a-c to switch to the new container\n", id)
+	return nil
+}
+
+func isProcessID(ctx context.Context, c types.Monitor, ref string) (bool, error) {
+	infos, err := c.ListProcesses(ctx)
+	if err != nil {
+		return false, err
+	}
+	for _, p := range infos {
+		if p.ProcessID == ref {
+			return true, nil
+		}
+	}
+	return false, nil
+}