Add a plugin mechanism to provide custom locator URL schemes

The template reader will attempt to call `limactl-url-$SCHEME` to
rewrite a custom URL as either a local filename, or a URL with a
supported URL like https.

Signed-off-by: Jan Dubois <jan.dubois@suse.com>

Author: jandubois

cmd/limactl/main.go

@@ -254,7 +254,10 @@ func addPluginCommands(rootCmd *cobra.Command) {
 				// unreachable
 			},
 		}
-
+		// Don't show the url scheme helper in the help output.
+		if strings.HasPrefix(plugin.Name, "url-") {
+			pluginCmd.Hidden = true
+		}
 		rootCmd.AddCommand(pluginCmd)
 	}
 }

cmd/limactl/template.go

@@ -40,6 +40,7 @@ func newTemplateCommand() *cobra.Command {
 		newTemplateCopyCommand(),
 		newTemplateValidateCommand(),
 		newTemplateYQCommand(),
+		newTemplateURLCommand(),
 	)
 	return templateCommand
 }
@@ -281,3 +282,22 @@ func templateValidateAction(cmd *cobra.Command, args []string) error {
 
 	return nil
 }
+
+func newTemplateURLCommand() *cobra.Command {
+	templateURLCommand := &cobra.Command{
+		Use:   "url CUSTOM_URL",
+		Short: "Transform custom template URLs to regular file or https URLs",
+		Args:  WrapArgsError(cobra.ExactArgs(1)),
+		RunE:  templateURLAction,
+	}
+	return templateURLCommand
+}
+
+func templateURLAction(cmd *cobra.Command, args []string) error {
+	url, err := limatmpl.TransformCustomURL(cmd.Context(), args[0])
+	if err != nil {
+		return err
+	}
+	_, err = fmt.Fprintln(cmd.OutOrStdout(), url)
+	return err
+}

pkg/limatmpl/locator.go

@@ -5,11 +5,13 @@ package limatmpl
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"net/url"
 	"os"
+	"os/exec"
 	"path"
 	"path/filepath"
 	"regexp"
@@ -22,18 +24,23 @@ import (
 	"github.com/lima-vm/lima/v2/pkg/ioutilx"
 	"github.com/lima-vm/lima/v2/pkg/limatype"
 	"github.com/lima-vm/lima/v2/pkg/limayaml"
+	"github.com/lima-vm/lima/v2/pkg/plugins"
 	"github.com/lima-vm/lima/v2/pkg/templatestore"
 )
 
 const yBytesLimit = 4 * 1024 * 1024 // 4MiB
 
 func Read(ctx context.Context, name, locator string) (*Template, error) {
-	var err error
 	tmpl := &Template{
 		Name:    name,
 		Locator: locator,
 	}
 
+	locator, err := TransformCustomURL(ctx, locator)
+	if err != nil {
+		return nil, err
+	}
+
 	if imageTemplate(tmpl, locator) {
 		return tmpl, nil
 	}
@@ -241,7 +248,10 @@ func SeemsTemplateURL(arg string) (isTemplate bool, templateName string) {
 		return false, ""
 	}
 	if u.Scheme == "template" {
-		return true, path.Join(u.Host, u.Path)
+		if u.Opaque == "" {
+			return true, path.Join(u.Host, u.Path)
+		}
+		return true, u.Opaque
 	}
 	return false, ""
 }
@@ -285,3 +295,55 @@ func InstNameFromYAMLPath(yamlPath string) (string, error) {
 	}
 	return s, nil
 }
+
+func TransformCustomURL(ctx context.Context, locator string) (string, error) {
+	u, err := url.Parse(locator)
+	if err != nil || len(u.Scheme) <= 1 {
+		return locator, nil
+	}
+
+	if u.Scheme == "template" {
+		if u.Opaque != "" {
+			return locator, nil
+		}
+		// Fix malformed "template://" URLs.
+		newLocator := "template:" + path.Join(u.Host, u.Path)
+		logrus.Warnf("Template locator %q should be written %q", locator, newLocator)
+		return newLocator, nil
+	}
+
+	plugin, err := plugins.Find("url-" + u.Scheme)
+	if err != nil {
+		return "", err
+	}
+	if plugin == nil {
+		return locator, nil
+	}
+
+	currentPath := os.Getenv("PATH")
+	defer os.Setenv("PATH", currentPath)
+	err = plugins.UpdatePath()
+	if err != nil {
+		return "", err
+	}
+
+	cmd := exec.CommandContext(ctx, plugin.Path, strings.TrimPrefix(u.String(), u.Scheme+":"))
+	cmd.Env = os.Environ()
+
+	stdout, err := cmd.Output()
+	if err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) {
+			stderrMsg := string(exitErr.Stderr)
+			if stderrMsg != "" {
+				return "", fmt.Errorf("command %q failed: %s", cmd.String(), strings.TrimSpace(stderrMsg))
+			}
+		}
+		return "", fmt.Errorf("command %q failed: %w", cmd.String(), err)
+	}
+	newLocator := strings.TrimSpace(string(stdout))
+	if newLocator != locator {
+		logrus.Debugf("Custom locator %q replaced with %q", locator, newLocator)
+	}
+	return newLocator, nil
+}

pkg/plugins/plugins.go

@@ -192,6 +192,21 @@ func extractDescFromScript(path string) string {
 	return desc
 }
 
+// Find locates a plugin by name and returns a pointer to a copy.
+func Find(name string) (*Plugin, error) {
+	allPlugins, err := Discover()
+	if err != nil {
+		return nil, err
+	}
+	for _, plugin := range allPlugins {
+		if name == plugin.Name {
+			pluginCopy := plugin
+			return &pluginCopy, nil
+		}
+	}
+	return nil, nil
+}
+
 func UpdatePath() error {
 	pluginDirs := getPluginDirectories()
 	newPath := strings.Join(pluginDirs, string(filepath.ListSeparator))