Allow spec loaders to inject allowed args

This makes it so frontends can have their own args that the dalec core
does not need to know about.

This also moves the custom args for windowscross local to that
implementation.

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

frontend/build.go

@@ -15,7 +15,39 @@ import (
 	"github.com/pkg/errors"
 )
 
-func LoadSpec(ctx context.Context, client *dockerui.Client, platform *ocispecs.Platform) (*dalec.Spec, error) {
+type LoadConfig struct {
+	SubstituteOpts []dalec.SubstituteOpt
+}
+
+type LoadOpt func(*LoadConfig)
+
+func WithAllowArgs(args ...string) LoadOpt {
+	return func(cfg *LoadConfig) {
+		set := make(map[string]struct{}, len(args))
+		for _, arg := range args {
+			set[arg] = struct{}{}
+		}
+		cfg.SubstituteOpts = append(cfg.SubstituteOpts, func(cfg *dalec.SubstituteConfig) {
+			orig := cfg.AllowArg
+
+			cfg.AllowArg = func(key string) bool {
+				if orig != nil && orig(key) {
+					return true
+				}
+				_, ok := set[key]
+				return ok
+			}
+		})
+	}
+}
+
+func LoadSpec(ctx context.Context, client *dockerui.Client, platform *ocispecs.Platform, opts ...LoadOpt) (*dalec.Spec, error) {
+	cfg := LoadConfig{}
+
+	for _, o := range opts {
+		o(&cfg)
+	}
+
 	src, err := client.ReadEntrypoint(ctx, "Dockerfile")
 	if err != nil {
 		return nil, fmt.Errorf("could not read spec file: %w", err)
@@ -35,7 +67,7 @@ func LoadSpec(ctx context.Context, client *dockerui.Client, platform *ocispecs.P
 	fillPlatformArgs("TARGET", args, *platform)
 	fillPlatformArgs("BUILD", args, client.BuildPlatforms[0])
 
-	if err := spec.SubstituteArgs(args); err != nil {
+	if err := spec.SubstituteArgs(args, cfg.SubstituteOpts...); err != nil {
 		return nil, errors.Wrap(err, "error resolving build args")
 	}
 	return spec, nil

frontend/mux.go

@@ -204,7 +204,14 @@ func (m *BuildMux) loadSpec(ctx context.Context, client gwclient.Client) (*dalec
 	}
 
 	// Note: this is not suitable for passing to builds since it does not have platform information
-	spec, err := LoadSpec(ctx, dc, nil)
+	spec, err := LoadSpec(ctx, dc, nil, func(cfg *LoadConfig) {
+		cfg.SubstituteOpts = append(cfg.SubstituteOpts, func(cfg *dalec.SubstituteConfig) {
+			// Allow any args here since we aren't trying to validate the spec at this point.
+			cfg.AllowArg = func(string) bool {
+				return true
+			}
+		})
+	})
 	if err != nil {
 		return nil, err
 	}