Ensure stream for type='stream' stream is a writable stream :)

Fixes #332

lib/bunyan.js

@@ -84,6 +84,7 @@ if (runtimeEnv === 'browser') {
 var util = require('util');
 var assert = require('assert');
 var EventEmitter = require('events').EventEmitter;
+var stream = require('stream');
 
 try {
     var safeJsonStringify = require('safe-json-stringify');
@@ -303,6 +304,13 @@ function resolveLevel(nameOrNum) {
 }
 
 
+function isWritable(obj) {
+    if (obj instanceof stream.Writable) {
+        return true;
+    }
+    return typeof (obj.write) === 'function';
+}
+
 
 //---- Logger class
 
@@ -568,6 +576,9 @@ Logger.prototype.addStream = function addStream(s, defaultLevel) {
 
     switch (s.type) {
     case 'stream':
+        assert.ok(isWritable(s.stream),
+                  '"stream" stream is not writable: ' + util.inspect(s.stream));
+
         if (!s.closeOnExit) {
             s.closeOnExit = false;
         }

test/add-stream.test.js

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2016 Trent Mick. All rights reserved.
+ *
+ * Test stream adding.
+ */
+
+var bunyan = require('../lib/bunyan');
+
+// node-tap API
+if (require.cache[__dirname + '/tap4nodeunit.js'])
+        delete require.cache[__dirname + '/tap4nodeunit.js'];
+var tap4nodeunit = require('./tap4nodeunit.js');
+var test = tap4nodeunit.test;
+
+
+test('non-writables passed as stream', function (t) {
+    var things = ['process.stdout', {}];
+    things.forEach(function (thing) {
+        function createLogger() {
+            bunyan.createLogger({
+                name: 'foo',
+                stream: thing
+            });
+        }
+        t.throws(createLogger,
+            /stream is not writable/,
+            '"stream" stream is not writable');
+    })
+    t.end();
+});
+
+test('proper stream', function (t) {
+    var log = bunyan.createLogger({
+        name: 'foo',
+        stream: process.stdout
+    });
+    t.ok('should not throw');
+    t.end();
+});