feat(runtime-templates): add support for runtime templates in bind mounts (#412)

pkg/api/runtime_template.go

@@ -0,0 +1,90 @@
+package api
+
+import (
+	"bytes"
+	"fmt"
+	"text/template"
+)
+
+// RuntimeTemplateContext contains the data available to runtime templates in a service spec.
+type RuntimeTemplateContext struct {
+	Container RuntimeTemplateContainerContext
+}
+
+// RuntimeTemplateContainerContext contains container data available to runtime templates in a service spec.
+type RuntimeTemplateContainerContext struct {
+	Name string
+}
+
+// RenderRuntimeTemplates returns a copy of the service spec with supported runtime template expressions rendered
+// using metadata from ctx. Runtime templates use Go template expressions such as {{.Container.Name}} and are supported
+// in bind volume host paths and volume mount container paths. The original service spec is not modified.
+func (s *ServiceSpec) RenderRuntimeTemplates(ctx RuntimeTemplateContext) (ServiceSpec, error) {
+	if ctx.Container.Name == "" {
+		return ServiceSpec{}, fmt.Errorf("container name must not be empty")
+	}
+
+	spec := s.Clone()
+
+	for i := range spec.Volumes {
+		volume := &spec.Volumes[i]
+		if volume.Type != VolumeTypeBind {
+			continue
+		}
+		if err := volume.Validate(); err != nil {
+			return ServiceSpec{}, fmt.Errorf("invalid bind volume '%s': %w", volume.Name, err)
+		}
+
+		hostPath, err := renderRuntimeTemplate(volume.BindOptions.HostPath, ctx)
+		if err != nil {
+			return ServiceSpec{}, fmt.Errorf("render runtime template in bind volume '%s' host path: %w",
+				volume.Name, err)
+		}
+		volume.BindOptions.HostPath = hostPath
+
+		if err = volume.Validate(); err != nil {
+			return ServiceSpec{}, fmt.Errorf("invalid rendered bind volume '%s': %w", volume.Name, err)
+		}
+	}
+
+	for i := range spec.Container.VolumeMounts {
+		volumeMount := &spec.Container.VolumeMounts[i]
+		if err := volumeMount.Validate(); err != nil {
+			return ServiceSpec{}, fmt.Errorf("invalid volume mount '%s': %w", volumeMount.VolumeName, err)
+		}
+
+		containerPath, err := renderRuntimeTemplate(volumeMount.ContainerPath, ctx)
+		if err != nil {
+			return ServiceSpec{}, fmt.Errorf(
+				"render runtime template in volume '%s' container path: %w", volumeMount.VolumeName, err)
+		}
+		volumeMount.ContainerPath = containerPath
+
+		if err = volumeMount.Validate(); err != nil {
+			return ServiceSpec{}, fmt.Errorf("invalid rendered volume mount '%s': %w", volumeMount.VolumeName, err)
+		}
+	}
+
+	return spec, nil
+}
+
+func renderRuntimeTemplate(value string, ctx RuntimeTemplateContext) (string, error) {
+	tmpl, err := template.New("runtime template").Option("missingkey=error").Parse(value)
+	if err != nil {
+		return "", fmt.Errorf("parse runtime template: %w", err)
+	}
+
+	var rendered bytes.Buffer
+	if err = tmpl.Execute(&rendered, ctx); err != nil {
+		return "", fmt.Errorf("execute runtime template: %w", err)
+	}
+
+	return rendered.String(), nil
+}
+
+func validateRuntimeTemplates(spec *ServiceSpec) error {
+	_, err := spec.RenderRuntimeTemplates(RuntimeTemplateContext{
+		Container: RuntimeTemplateContainerContext{Name: "validation-container-name"},
+	})
+	return err
+}