Add warning for multiple functions having same handler

lib/classes/Service.js

@@ -208,6 +208,28 @@ class Service {
       }
     });
 
+    const warnOnDuplicateHandlers = () => {
+      const functionObjs = this.getAllFunctions()
+        .map(name => ({
+          name,
+          body: this.getFunction(name),
+        }))
+        .filter(func => _.isString(func.body.handler));
+      const groupedByHandler = _.groupBy(functionObjs, func => func.body.handler);
+      _.entriesIn(groupedByHandler)
+        .filter(arg => arg[1].length >= 2)
+        .forEach(arg => {
+          const handlerName = arg[0];
+          const functionGroup = arg[1].map(func => func.name);
+          const warnMessage = [
+            `Warning: A handler "${handlerName}" is duplicated in functions: `,
+            `${functionGroup.join(', ')}.`,
+          ].join('');
+          this.serverless.cli.log(warnMessage);
+        });
+    };
+    warnOnDuplicateHandlers();
+
     return this;
   }
 

lib/classes/Service.test.js

@@ -9,6 +9,7 @@ const Service = require('../../lib/classes/Service');
 const Utils = require('../../lib/classes/Utils');
 const Serverless = require('../../lib/Serverless');
 const testUtils = require('../../tests/utils');
+const CLI = require('../../lib/classes/CLI');
 
 // Configure chai
 chai.use(require('chai-as-promised'));
@@ -722,6 +723,46 @@ describe('Service', () => {
           .to.throw('Events for "functionA" must be an array, not an string');
       });
     });
+
+    it('should warn if multiple functions have same handler', () => {
+      const SUtils = new Utils();
+      const serverlessYml = {
+        service: 'testService',
+        provider: 'testProvider',
+        functions: {
+          functionA: {
+            handler: 'foo.functionA',
+            events: [],
+          },
+          functionB: {
+            handler: 'foo.functionA',
+            events: [],
+          },
+          function123: {
+            handler: 'foo.function123',
+            events: [],
+          },
+          function456: {
+            handler: 'foo.function123',
+            events: [],
+          },
+        },
+      };
+      SUtils.writeFileSync(path.join(tmpDirPath, 'serverless.yml'),
+        YAML.dump(serverlessYml));
+
+      const serverless = new Serverless({ servicePath: tmpDirPath });
+
+      class CustomCLI extends CLI {}
+      const consoleLogStub = sinon.stub(CustomCLI.prototype, 'log');
+      serverless.cli = new CustomCLI(serverless);
+
+      return expect(serverless.service.load()).to.eventually.be.fulfilled
+      .then(() => {
+        serverless.service.validate();
+        expect(consoleLogStub.callCount).to.equal(2);
+      });
+    });
   });
 
   describe('#mergeArrays', () => {