builder: enhance driver factory logic when loading drivers

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

builder/builder.go

@@ -4,10 +4,12 @@ import (
 	"context"
 	"os"
 	"sort"
+	"sync"
 
 	"github.com/docker/buildx/driver"
 	"github.com/docker/buildx/store"
 	"github.com/docker/buildx/store/storeutil"
+	"github.com/docker/buildx/util/dockerutil"
 	"github.com/docker/buildx/util/imagetools"
 	"github.com/docker/buildx/util/progress"
 	"github.com/docker/cli/cli/command"
@@ -18,6 +20,7 @@ import (
 // Builder represents an active builder object
 type Builder struct {
 	*store.NodeGroup
+	driverFactory driverFactory
 	nodes         []Node
 	opts          builderOpts
 	err           error
@@ -201,6 +204,45 @@ func (b *Builder) Err() error {
 	return b.err
 }
 
+type driverFactory struct {
+	driver.Factory
+	once sync.Once
+}
+
+// Factory returns the driver factory.
+func (b *Builder) Factory(ctx context.Context) (_ driver.Factory, err error) {
+	b.driverFactory.once.Do(func() {
+		if b.Driver != "" {
+			b.driverFactory.Factory, err = driver.GetFactory(b.Driver, true)
+			if err != nil {
+				return
+			}
+		} else {
+			// empty driver means nodegroup was implicitly created as a default
+			// driver for a docker context and allows falling back to a
+			// docker-container driver for older daemon that doesn't support
+			// buildkit (< 18.06).
+			ep := b.nodes[0].Endpoint
+			var dockerapi *dockerutil.ClientAPI
+			dockerapi, err = dockerutil.NewClientAPI(b.opts.dockerCli, b.nodes[0].Endpoint)
+			if err != nil {
+				return
+			}
+			// check if endpoint is healthy is needed to determine the driver type.
+			// if this fails then can't continue with driver selection.
+			if _, err = dockerapi.Ping(ctx); err != nil {
+				return
+			}
+			b.driverFactory.Factory, err = driver.GetDefaultFactory(ctx, ep, dockerapi, false)
+			if err != nil {
+				return
+			}
+			b.Driver = b.driverFactory.Factory.Name()
+		}
+	})
+	return b.driverFactory.Factory, err
+}
+
 // GetBuilders returns all builders
 func GetBuilders(dockerCli command.Cli, txn *store.Txn) ([]*Builder, error) {
 	storeng, err := txn.List()

builder/node.go

@@ -46,34 +46,10 @@ func (b *Builder) LoadNodes(ctx context.Context, withData bool) (_ []Node, err e
 		}
 	}()
 
-	// TODO: factory should be lazy and a dedicated method in Builder object
+	factory, err := b.Factory(ctx)
-	var factory driver.Factory
-	if b.NodeGroup.Driver != "" {
-		factory, err = driver.GetFactory(b.NodeGroup.Driver, true)
 	if err != nil {
 		return nil, err
 	}
-	} else {
-		// empty driver means nodegroup was implicitly created as a default
-		// driver for a docker context and allows falling back to a
-		// docker-container driver for older daemon that doesn't support
-		// buildkit (< 18.06).
-		ep := b.NodeGroup.Nodes[0].Endpoint
-		dockerapi, err := dockerutil.NewClientAPI(b.opts.dockerCli, b.NodeGroup.Nodes[0].Endpoint)
-		if err != nil {
-			return nil, err
-		}
-		// check if endpoint is healthy is needed to determine the driver type.
-		// if this fails then can't continue with driver selection.
-		if _, err = dockerapi.Ping(ctx); err != nil {
-			return nil, err
-		}
-		factory, err = driver.GetDefaultFactory(ctx, ep, dockerapi, false)
-		if err != nil {
-			return nil, err
-		}
-		b.NodeGroup.Driver = factory.Name()
-	}
 
 	imageopt, err := b.ImageOpt()
 	if err != nil {