report invalid global expressions clearly

__tests__/meta.test.ts

@@ -76,6 +76,27 @@ describe('isRawStatement', () => {
   });
 });
 
+describe('global expressions', () => {
+  test('throws for unknown global expression', async () => {
+    process.env = dotenv.parse(fs.readFileSync(path.join(__dirname, 'fixtures', 'event_push_master.env')));
+    const toolkit = new Toolkit();
+    const repo = await toolkit.github.repoData();
+    const context = await getContext(ContextSource.workflow, toolkit);
+
+    expect(() => {
+      new Meta(
+        {
+          ...getInputs(),
+          images: ['user/app'],
+          tags: [`type=raw,value=latest,enable={{is_prerelease}}`]
+        },
+        context,
+        repo
+      );
+    }).toThrow('{{is_prerelease}} is not a valid global expression');
+  });
+});
+
 const tagsLabelsTest = async (name: string, envFile: string, inputs: Inputs, exVersion: Version, exTags: Array<string>, exLabels: Array<string>, exAnnotations: Array<string> | undefined) => {
   process.env = dotenv.parse(fs.readFileSync(path.join(__dirname, 'fixtures', envFile)));
   const toolkit = new Toolkit();

src/meta.ts

@@ -400,7 +400,7 @@ export class Meta {
     const context = this.context;
     const currentDate = this.date;
     const commitDate = this.context.commitDate;
-    return handlebars.compile(val)({
+    const globalExp = {
       branch: function () {
         if (!/^refs\/heads\//.test(context.ref)) {
           return '';
@@ -480,7 +480,19 @@ export class Meta {
         });
         return m.tz(tz).format(format);
       }
-    });
+    };
+    const expressions = Object.keys(globalExp);
+    const template = handlebars.parseWithoutProcessing(val);
+    for (const node of template.body) {
+      const statement = node as {type: string; path?: {type: string; original: string}};
+      if (statement.type !== 'MustacheStatement' || statement.path?.type !== 'PathExpression') {
+        continue;
+      }
+      if (!expressions.includes(statement.path.original)) {
+        throw new Error(`{{${statement.path.original}}} is not a valid global expression`);
+      }
+    }
+    return handlebars.compile(val)(globalExp);
   }
 
   private getImageNames(): Array<string> {