Returning undefined from a formatter sometimes results in transport exception.

[djencks]

From the documentation I suspect this might belong in winston core, but the exception is showing up in transport.

With this configuration:

const winston = require('winston')
const logger = winston.loggers.get('Antora')
...
  logger.configure({
    format: winston.format((info, opts) => {
      messages.push(info)
      return undefined
    }
    )(),
    transports: [new winston.transports.Console({ level: 'error' })],
  })

there is one circumstance out of 143 in which I get:

     TypeError: Cannot read property 'exception' of undefined
      at Console._write (node_modules/winston-transport/index.js:69:28)
      at doWrite (node_modules/readable-stream/lib/_stream_writable.js:428:64)
      at writeOrBuffer (node_modules/readable-stream/lib/_stream_writable.js:417:5)
      at Console.Writable.write (node_modules/readable-stream/lib/_stream_writable.js:334:11)
      at DerivedLogger.ondata (node_modules/winston/node_modules/readable-stream/lib/_stream_readable.js:662:20)
      at addChunk (node_modules/winston/node_modules/readable-stream/lib/_stream_readable.js:297:12)
      at readableAddChunk (node_modules/winston/node_modules/readable-stream/lib/_stream_readable.js:279:11)
      at DerivedLogger.Readable.push (node_modules/winston/node_modules/readable-stream/lib/_stream_readable.js:240:10)
      at DerivedLogger.Transform.push (node_modules/winston/node_modules/readable-stream/lib/_stream_transform.js:139:32)
      at DerivedLogger._transform (node_modules/winston/lib/winston/logger.js:305:12)
      at DerivedLogger.Transform._read (node_modules/winston/node_modules/readable-stream/lib/_stream_transform.js:177:10)
      at DerivedLogger.Transform._write (node_modules/winston/node_modules/readable-stream/lib/_stream_transform.js:164:83)
      at doWrite (node_modules/winston/node_modules/readable-stream/lib/_stream_writable.js:405:139)
      at writeOrBuffer (node_modules/winston/node_modules/readable-stream/lib/_stream_writable.js:394:5)
      at DerivedLogger.Writable.write (node_modules/winston/node_modules/readable-stream/lib/_stream_writable.js:303:11)
      at DerivedLogger.log (node_modules/winston/lib/winston/logger.js:210:12)
      at Object.add (packages/asciidoc-loader/lib/load-asciidoc.js:51:19)
      at Object.<anonymous> (node_modules/asciidoctor.js/dist/node/asciidoctor.js:19896:33)
      at Opal.send (node_modules/opal-runtime/src/opal.js:1660:19)
      at Object.$$error (node_modules/asciidoctor.js/dist/node/asciidoctor.js:1458:14)
      at $PreprocessorReader.$$resolve_include_path (node_modules/asciidoctor.js/dist/node/asciidoctor.js:13638:28)
      at $PreprocessorReader.$$preprocess_include_directive (node_modules/asciidoctor.js/dist/node/asciidoctor.js:13292:21)
      at $PreprocessorReader.$$process_line (node_modules/asciidoctor.js/dist/node/asciidoctor.js:13092:37)
      at $PreprocessorReader.$$peek_line (node_modules/asciidoctor.js/dist/node/asciidoctor.js:12562:41)
      at Opal.send (node_modules/opal-runtime/src/opal.js:1660:19)
      at $PreprocessorReader.$$peek_line (node_modules/asciidoctor.js/dist/node/asciidoctor.js:13149:29)
      at $PreprocessorReader.$$skip_blank_lines (node_modules/asciidoctor.js/dist/node/asciidoctor.js:12683:42)
      at Function.$$next_section (node_modules/asciidoctor.js/dist/node/asciidoctor.js:9146:35)
      at Function.$$parse (node_modules/asciidoctor.js/dist/node/asciidoctor.js:8760:23)
      at $Document.$$parse (node_modules/asciidoctor.js/dist/node/asciidoctor.js:7357:34)
      at /Users/david/projects/antora/antora/node_modules/asciidoctor.js/dist/node/asciidoctor.js:19550:66
      at Function.$$load (node_modules/asciidoctor.js/dist/node/asciidoctor.js:19551:28)
      at Function.$$convert (node_modules/asciidoctor.js/dist/node/asciidoctor.js:19673:20)
      at Function.Asciidoctor.convert (node_modules/asciidoctor.js/dist/node/asciidoctor.js:20027:21)
      at captureStderr (packages/asciidoc-loader/test/load-asciidoc-test.js:107:62)
      at captureStderr (packages/asciidoc-loader/test/load-asciidoc-test.js:48:23)
      at Context.it (packages/asciidoc-loader/test/load-asciidoc-test.js:107:30)

From the documentation, I expect that since the formatter returns 'undefined', a falsy value, nothing further would happen with the message.

Returning any of the other falsy values, null, NaN, 0, '', false, all work as expected.

Changing line 69ff to

TransportStream.prototype._write = function _write(info, enc, callback) {
  if (this.silent || (info && info.exception === true && !this.handleExceptions)) {
    return callback(null);
  }

ought to fix the issue, but I suspect this is not the correct solution.