build: add experimental support for print flag

Print flag can be used to make additional information
requests about the build and print their results.

Currently Dockerfile supports: outline, targets, subrequests.describe

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/frontend/subrequests/targets/targets.go

@@ -0,0 +1,84 @@
+package targets
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"text/tabwriter"
+
+	"github.com/moby/buildkit/frontend/gateway/client"
+	"github.com/moby/buildkit/frontend/subrequests"
+	"github.com/moby/buildkit/solver/pb"
+)
+
+const RequestTargets = "frontend.targets"
+
+var SubrequestsTargetsDefinition = subrequests.Request{
+	Name:        RequestTargets,
+	Version:     "1.0.0",
+	Type:        subrequests.TypeRPC,
+	Description: "List all targets current build supports",
+	Opts:        []subrequests.Named{},
+	Metadata: []subrequests.Named{
+		{Name: "result.json"},
+		{Name: "result.txt"},
+	},
+}
+
+type List struct {
+	Targets []Target `json:"targets"`
+	Sources [][]byte `json:"sources"`
+}
+
+func (l List) ToResult() (*client.Result, error) {
+	res := client.NewResult()
+	dt, err := json.MarshalIndent(l, "", "  ")
+	if err != nil {
+		return nil, err
+	}
+	res.AddMeta("result.json", dt)
+
+	b := bytes.NewBuffer(nil)
+	if err := PrintTargets(dt, b); err != nil {
+		return nil, err
+	}
+	res.AddMeta("result.txt", b.Bytes())
+
+	res.AddMeta("version", []byte(SubrequestsTargetsDefinition.Version))
+	return res, nil
+}
+
+type Target struct {
+	Name        string       `json:"name,omitempty"`
+	Default     bool         `json:"default,omitempty"`
+	Description string       `json:"description,omitempty"`
+	Base        string       `json:"base,omitempty"`
+	Platform    string       `json:"platform,omitempty"`
+	Location    *pb.Location `json:"location,omitempty"`
+}
+
+func PrintTargets(dt []byte, w io.Writer) error {
+	var l List
+
+	if err := json.Unmarshal(dt, &l); err != nil {
+		return err
+	}
+
+	tw := tabwriter.NewWriter(w, 0, 0, 1, ' ', 0)
+	fmt.Fprintf(tw, "TARGET\tDESCRIPTION\n")
+
+	for _, t := range l.Targets {
+		name := t.Name
+		if name == "" && t.Default {
+			name = "(default)"
+		} else {
+			if t.Default {
+				name = fmt.Sprintf("%s (default)", name)
+			}
+		}
+		fmt.Fprintf(tw, "%s\t%s\n", name, t.Description)
+	}
+
+	return tw.Flush()
+}