frontend: cache spec during route registration

Add a cachedSpecClient wrapper that implements gwutil.SpecLoader and
uses sync.Once to cache the spec loaded by LoadSpecFromClient. This
eliminates redundant spec loads during route registration where each
distro's Routes() independently calls LoadSpecFromClient (14+ times
per build request).

LoadSpecFromClient now checks for the SpecLoader interface before
falling back to the full dockerui load path. NewRouter wraps the
client internally so the cache is scoped to registration only.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Author: cpuguy83

frontend/router.go

@@ -19,6 +19,7 @@ import (
 	"github.com/moby/buildkit/util/bklog"
 	"github.com/pkg/errors"
 	"github.com/project-dalec/dalec"
+	"github.com/project-dalec/dalec/internal/gwutil"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/exp/maps"
 )
@@ -385,6 +386,10 @@ func topLevelKey(routePath string) string {
 // context. Build args are substituted with WithAllowAnyArg so that
 // unresolved args don't cause errors during route setup.
 func LoadSpecFromClient(ctx context.Context, client gwclient.Client) (*dalec.Spec, error) {
+	if loader, ok := client.(gwutil.SpecLoader); ok {
+		return loader.LoadSpec(ctx)
+	}
+
 	dc, err := dockerui.NewClient(client)
 	if err != nil {
 		return nil, err

internal/frontendapi/router.go

@@ -3,17 +3,21 @@ package frontendapi
 import (
 	"context"
 	"fmt"
+	"sync"
 
 	"github.com/containerd/plugin"
 	gwclient "github.com/moby/buildkit/frontend/gateway/client"
+	"github.com/project-dalec/dalec"
 	"github.com/project-dalec/dalec/frontend"
 	"github.com/project-dalec/dalec/frontend/debug"
+	"github.com/project-dalec/dalec/internal/gwutil"
 	"github.com/project-dalec/dalec/internal/plugins"
 	_ "github.com/project-dalec/dalec/targets/plugin"
 )
 
 // NewRouter creates a flat Router with all routes registered eagerly.
 func NewRouter(ctx context.Context, client gwclient.Client) (*frontend.Router, error) {
+	client = newCachedSpecClient(client)
 	r := &frontend.Router{}
 
 	// Register debug routes.
@@ -67,3 +71,29 @@ func loadRouteProviders(ctx context.Context, client gwclient.Client, r *frontend
 
 	return nil
 }
+
+// cachedSpecClient wraps a gwclient.Client and caches the spec loaded by
+// LoadSpecFromClient so that it is only loaded once.
+type cachedSpecClient struct {
+	gwclient.Client
+
+	loadOnce sync.Once
+	spec     *dalec.Spec
+	err      error
+}
+
+// Compile-time check that cachedSpecClient implements gwutil.SpecLoader.
+var _ gwutil.SpecLoader = (*cachedSpecClient)(nil)
+
+func newCachedSpecClient(client gwclient.Client) gwclient.Client {
+	return gwutil.WithCurrentFrontend(client, &cachedSpecClient{
+		Client: client,
+	})
+}
+
+func (c *cachedSpecClient) LoadSpec(ctx context.Context) (*dalec.Spec, error) {
+	c.loadOnce.Do(func() {
+		c.spec, c.err = frontend.LoadSpecFromClient(ctx, c.Client)
+	})
+	return c.spec, c.err
+}