build: fail mocha tests on (forgotten) console logs (#1058)

Modify the command "lb-mocha" to load a helper file that detects
calls of console.log/console.error/console.warn APIs and fails
the test suite in such case.

This forces developers to write tests in such way that the console
output remain clean, uncluttered by random logs.

packages/build/bin/run-mocha.js

@@ -37,6 +37,19 @@ function run(argv, dryRun) {
     );
     mochaOpts.unshift('--require', sourceMapRegisterPath);
   }
+
+  const allowConsoleLogsIx = mochaOpts.indexOf('--allow-console-logs');
+  if (allowConsoleLogsIx === -1) {
+    // Fail any tests that are printing to console.
+    mochaOpts.unshift(
+      '--require',
+      require.resolve('../src/fail-on-console-logs')
+    );
+  } else {
+    // Allow tests to print to console, remove --allow-console-logs argument
+    mochaOpts.splice(allowConsoleLogsIx, 1);
+  }
+
   const args = [...mochaOpts];
 
   return utils.runCLI('mocha/bin/mocha', args, dryRun);

packages/build/src/fail-on-console-logs.js

@@ -0,0 +1,60 @@
+// Copyright IBM Corp. 2017,2018. All Rights Reserved.
+// Node module: @loopback/build
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+'use strict';
+
+const util = require('util');
+
+const originalConsole = {
+  log: console.log,
+  error: console.error,
+  warn: console.warn,
+};
+
+console.log = recordForbiddenCall('log');
+console.warn = recordForbiddenCall('warn');
+console.error = recordForbiddenCall('error');
+
+const problems = [];
+
+function recordForbiddenCall(methodName) {
+  return function recordForbiddenConsoleUsage(...args) {
+    // Print the original message
+    originalConsole[methodName](...args);
+
+    // Find out who called us.
+    // The first line is the error message,
+    // the second line points to this very function.
+    const stack = new Error().stack.split(/\n/).slice(2);
+
+    // Mocha reporters are allowed to call console functions
+    if (/[\/\\]node_modules[\/\\]mocha[\/\\]/.test(stack[0])) {
+      return;
+    }
+
+    // Record the problem otherwise
+    const msg = util.format(...args);
+    problems.push({msg, stack});
+  }
+}
+
+process.on('exit', (code) => {
+  if (!problems.length) return;
+  const log = originalConsole.log;
+
+  log('\n=== ATTENTION - INVALID USAGE OF CONSOLE LOGS DETECTED ===\n');
+
+  for (const p of problems) {
+    // print the first line of the console log
+    log(p.msg.split(/\n/)[0]);
+    // print the stack trace
+    log(p.stack.join('\n'));
+    // add an empty line as a delimiter
+    log('\n');
+  }
+
+  // ensure the process returns non-zero exit code to indicate test failure
+  process.exitCode = code || 10;
+});