[variants] Support component-level exclusion

whereby excluded components are simply not present and all dependencies
to their packages are silently removed.

Author: Christian Weichel

pkg/leeway/package.go

@@ -502,6 +502,9 @@ type packageVariantInternal struct {
 		Include []string `yaml:"include"`
 		Exclude []string `yaml:"exclude"`
 	} `yaml:"srcs"`
+	Components struct {
+		Exclude []string `yaml:"exclude"`
+	} `yaml:"components"`
 	Environment []string                  `yaml:"env"`
 	RawConfig   map[PackageType]yaml.Node `yaml:"config"`
 }
@@ -551,6 +554,16 @@ func (v *PackageVariant) Config(t PackageType) (cfg PackageConfig, ok bool) {
 	return
 }
 
+// ExcludeComponent returns true if this variants excludes the component
+func (v *PackageVariant) ExcludeComponent(name string) bool {
+	for _, c := range v.Components.Exclude {
+		if c == name {
+			return true
+		}
+	}
+	return false
+}
+
 // ResolveSources lists all files which are explicitely included or excluded by this variant.
 // Inclusion takes precedence over exclusion.
 func (v *PackageVariant) ResolveSources(workspace *Workspace, loc string) (incl []string, excl []string, err error) {

pkg/leeway/workspace.go

@@ -481,6 +481,11 @@ func discoverComponents(ctx context.Context, workspace *Workspace, args Argument
 		defer wg.Done()
 
 		for c := range cchan {
+			// filter variant-excluded components and all their packages
+			if filterExcludedComponents(variant, c) {
+				continue
+			}
+
 			comps = append(comps, c)
 		}
 	}()
@@ -494,6 +499,33 @@ func discoverComponents(ctx context.Context, workspace *Workspace, args Argument
 	return comps, nil
 }
 
+// filterExcludedComponents returns true if the component is excluded by the variant.
+// This function also removes all dependencies to excluded components.
+func filterExcludedComponents(variant *PackageVariant, c *Component) (ignoreComponent bool) {
+	if variant == nil {
+		return false
+	}
+	if variant.ExcludeComponent(c.Name) {
+		log.WithField("component", c.Name).Debug("selected variant excludes this component")
+		return true
+	}
+
+	for _, p := range c.Packages {
+		for i, dep := range p.Dependencies {
+			segs := strings.Split(dep, ":")
+			if len(segs) != 2 {
+				continue
+			}
+
+			if variant.ExcludeComponent(segs[0]) {
+				p.Dependencies[i] = p.Dependencies[len(p.Dependencies)-1]
+				p.Dependencies = p.Dependencies[:len(p.Dependencies)-1]
+			}
+		}
+	}
+	return false
+}
+
 // loadComponent loads a component from a BUILD.yaml file
 func loadComponent(ctx context.Context, workspace *Workspace, path string, args Arguments, variant *PackageVariant) (c Component, err error) {
 	defer trace.StartRegion(context.Background(), "loadComponent").End()