Wrap calls to format.transform with try / catch. (winstonjs#1347)

* [fix] Wrap calls to `format.transform` with try / catch. Potential fix for winstonjs#1261.

* [test] Test error trapping for user-defined formats.

lib/winston/logger.js

@@ -216,9 +216,17 @@ class Logger extends stream.Transform {
     }
 
     // Here we write to the `format` pipe-chain, which on `readable` above will
-    // push the formatted `info` Object onto the buffer for this instance.
-    this.push(this.format.transform(info, this.format.options));
-    callback();
+    // push the formatted `info` Object onto the buffer for this instance. We trap
+    // (and re-throw) any errors generated by the user-provided format, but also
+    // guarantee that the streams callback is invoked so that we can continue flowing.
+    try {
+      this.push(this.format.transform(info, this.format.options));
+    } catch (ex) {
+      throw ex;
+    } finally {
+      // eslint-disable-next-line callback-return
+      callback();
+    }
   }
 
   /**

test/logger.test.js

@@ -352,6 +352,41 @@ describe('Logger (stream semantics)', function () {
 
     setImmediate(() => logger.end());
   });
+
+  it(`rethrows errors from user-defined formats`, function () {
+    stdMocks.use();
+    const logger = winston.createLogger( {
+      transports: [new winston.transports.Console()],
+      format: winston.format.printf((info) => {
+        // Set a trap.
+        if (info.message === 'ENDOR') {
+          throw new Error('ITS A TRAP!');
+        }
+
+        return info.message;
+      })
+    });
+
+    // Trigger the trap.  Swallow the error so processing continues.
+    try {
+      logger.info('ENDOR');
+    } catch (err) {
+      assume(err.message).equals('ITS A TRAP!');
+    }
+
+    const expected = [
+      'Now witness the power of the fully armed and operational logger',
+      'Consider the philosophical and metaphysical – BANANA BANANA BANANA',
+      'I was god once. I saw – you were doing well until everyone died.'
+    ];
+
+    expected.forEach(msg => logger.info(msg));
+
+    stdMocks.restore();
+    const actual = stdMocks.flush();
+    assume(actual.stdout).deep.equals(expected.map(msg => `${msg}\n`));
+    assume(actual.stderr).deep.equals([]);
+  });
 });
 
 describe('Logger (logging exotic data types)', function () {