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/describe.go

@@ -0,0 +1,81 @@
+package subrequests
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+	"text/tabwriter"
+
+	"github.com/moby/buildkit/frontend/gateway/client"
+	gwpb "github.com/moby/buildkit/frontend/gateway/pb"
+	"github.com/moby/buildkit/solver/errdefs"
+	"github.com/pkg/errors"
+)
+
+const RequestSubrequestsDescribe = "frontend.subrequests.describe"
+
+var SubrequestsDescribeDefinition = Request{
+	Name:        RequestSubrequestsDescribe,
+	Version:     "1.0.0",
+	Type:        TypeRPC,
+	Description: "List available subrequest types",
+	Metadata: []Named{
+		{Name: "result.json"},
+		{Name: "result.txt"},
+	},
+}
+
+func Describe(ctx context.Context, c client.Client) ([]Request, error) {
+	gwcaps := c.BuildOpts().Caps
+
+	if err := (&gwcaps).Supports(gwpb.CapFrontendCaps); err != nil {
+		return nil, errdefs.NewUnsupportedSubrequestError(RequestSubrequestsDescribe)
+	}
+
+	res, err := c.Solve(ctx, client.SolveRequest{
+		FrontendOpt: map[string]string{
+			"requestid":     RequestSubrequestsDescribe,
+			"frontend.caps": "moby.buildkit.frontend.subrequests",
+		},
+		Frontend: "dockerfile.v0",
+	})
+	if err != nil {
+		var reqErr *errdefs.UnsupportedSubrequestError
+		if errors.As(err, &reqErr) {
+			return nil, err
+		}
+		var capErr *errdefs.UnsupportedFrontendCapError
+		if errors.As(err, &capErr) {
+			return nil, errdefs.NewUnsupportedSubrequestError(RequestSubrequestsDescribe)
+		}
+		return nil, err
+	}
+
+	dt, ok := res.Metadata["result.json"]
+	if !ok {
+		return nil, errors.Errorf("no result.json metadata in response")
+	}
+
+	var reqs []Request
+	if err := json.Unmarshal(dt, &reqs); err != nil {
+		return nil, errors.Wrap(err, "failed to parse describe result")
+	}
+	return reqs, nil
+}
+
+func PrintDescribe(dt []byte, w io.Writer) error {
+	var d []Request
+	if err := json.Unmarshal(dt, &d); err != nil {
+		return err
+	}
+
+	tw := tabwriter.NewWriter(w, 0, 0, 1, ' ', 0)
+	fmt.Fprintf(tw, "NAME\tVERSION\tDESCRIPTION\n")
+
+	for _, r := range d {
+		fmt.Fprintf(tw, "%s\t%s\t%s\n", strings.TrimPrefix(r.Name, "frontend."), r.Version, r.Description)
+	}
+	return tw.Flush()
+}