Add Solve API for performing a build on a build definition

This commit adds Solve API to the controller. This receives a build definition,
performs that build using ResultHandler held by that session. After Solve
completes, the client can debug the result using other APIs including Invoke.
Note that the ResultHandle provided by Solve overwrites the ResultHandle
previously stored in that session (possibly generated by the past Build or Solve
API call).

Using this API, user can perform build-and-debugging loop on the same session.

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

controller/local/controller.go

@@ -15,6 +15,7 @@ import (
 	"github.com/docker/buildx/util/progress"
 	"github.com/docker/cli/cli/command"
 	"github.com/moby/buildkit/client"
+	solverpb "github.com/moby/buildkit/solver/pb"
 	"github.com/pkg/errors"
 )
 
@@ -40,6 +41,8 @@ type localController struct {
 	processes   *processes.Manager
 
 	buildOnGoing atomic.Bool
+
+	originalResult *build.ResultHandle
 }
 
 func (b *localController) Build(ctx context.Context, options controllerapi.BuildOptions, in io.ReadCloser, progress progress.Writer) (string, *client.SolveResponse, error) {
@@ -55,6 +58,7 @@ func (b *localController) Build(ctx context.Context, options controllerapi.Build
 			resultCtx:    res,
 			buildOptions: &options,
 		}
+		b.originalResult = res
 		if buildErr != nil {
 			buildErr = controllererrors.WrapBuild(buildErr, b.ref)
 		}
@@ -144,3 +148,20 @@ func (b *localController) Inspect(ctx context.Context, ref string) (*controllera
 	}
 	return &controllerapi.InspectResponse{Options: b.buildConfig.buildOptions}, nil
 }
+
+func (b *localController) Solve(ctx context.Context, ref string, target *solverpb.Definition, progress progress.Writer) error {
+	if ref != b.ref {
+		return errors.Errorf("unknown ref %q", ref)
+	}
+	if b.originalResult == nil {
+		return errors.Errorf("no build has been called")
+	}
+	res, err := build.SolveWithResultHandler(ctx, "buildx", b.originalResult, target, progress)
+	if err == nil {
+		b.buildConfig.resultCtx = res
+		if se := res.SolveError(); se != nil {
+			err = errors.Errorf("failed solve: %v", se)
+		}
+	}
+	return err
+}