test(Variables): Cover custom variable sources

test/fixtures/programmatic/plugin/custom-variable-source.js

@@ -0,0 +1,23 @@
+'use strict';
+
+module.exports = class CustomVariableSourcePlugin {
+  constructor(serverless, options, utils) {
+    this.serverless = serverless;
+    this.options = options;
+    this.utils = utils;
+    this.configurationVariablesSources = {
+      other: {
+        async resolve({ address }) {
+          // `address` contains the name of the variable to resolve:
+          // In `${foo:some-variable}`, address will contain `some-variable`.
+
+          // Resolver is expected to return an object with the value in the `value` property:
+          return {
+            //
+            value: `Resolving variable ${address}`,
+          };
+        },
+      },
+    };
+  }
+};

test/unit/scripts/serverless.test.js

@@ -225,6 +225,20 @@ describe('test/unit/scripts/serverless.test.js', () => {
     }
   });
 
+  it('should support custom variable soruces', async () => {
+    const { servicePath: serviceDir } = await programmaticFixturesEngine.setup('plugin', {
+      configExt: {
+        custom: {
+          otherVar: '${other:addressValue}',
+        },
+        plugins: ['./custom-variable-source'],
+      },
+    });
+    expect(
+      String((await spawn('node', [serverlessPath, 'print'], { cwd: serviceDir })).stdoutBuffer)
+    ).to.include('otherVar: Resolving variable addressValue');
+  });
+
   it('should reject unresolved "plugins" property', async () => {
     try {
       await spawn('node', [serverlessPath, 'print'], {