Require "remove_tag_prefix" to be specified in configs. (#78)

Without this option specified, the output of the plugin will be fed back to
itself, causing infinite recursion and errors that can be hard to understand.

README.rdoc

@@ -42,19 +42,23 @@ will also install and configure the gem.
 
 The plugin supports the following parameters:
 
+=== Required
+
+[remove_tag_prefix]  The prefix to remove from the input tag when outputting
+                     a record. A prefix has to be a complete tag part.
+                     Example: If remove_tag_prefix is set to 'foo', the input
+                     tag foo.bar.baz is transformed to bar.baz and the input tag
+                     'foofoo.bar' is not modified.
+
+=== Optional
+
 [message]  Name of the field in the JSON record that contains the
            single-line log messages that shall be scanned for exceptions.
            If this is set to '', the plugin will try 'message' and 'log',
            in that order.
            This parameter is only applicable to structured (JSON) log streams.
            Default: ''.
 
-[remove_tag_prefix]  The prefix to remove from the input tag when outputting
-                     a record. A prefix has to be a complete tag part.
-                     Example: If remove_tag_prefix is set to 'foo', the input
-                     tag foo.bar.baz is transformed to bar.baz and the input tag
-                     'foofoo.bar' is not modified. Default: empty string.
-
 [languages]  A list of language for which exception stack traces shall be
              detected. The values in the list can be separated by commas or
              written as JSON list.

lib/fluent/plugin/out_detect_exceptions.rb

@@ -22,10 +22,10 @@ module Fluent
   # an exception stack trace, they forwarded as a single, combined JSON
   # object. Otherwise, the input log data is forwarded as is.
   class DetectExceptionsOutput < Output
+    desc 'The prefix to be removed from the input tag when outputting a record.'
+    config_param :remove_tag_prefix, :string
     desc 'The field which contains the raw message text in the input JSON data.'
     config_param :message, :string, default: ''
-    desc 'The prefix to be removed from the input tag when outputting a record.'
-    config_param :remove_tag_prefix, :string, default: ''
     desc 'The interval of flushing the buffer for multiline format.'
     config_param :multiline_flush_interval, :time, default: nil
     desc 'Programming languages for which to detect exceptions. Default: all.'

test/plugin/test_out_detect_exceptions.rb

@@ -28,6 +28,8 @@ def setup
 
   DEFAULT_TAG = 'prefix.test.tag'.freeze
 
+  DEFAULT_TAG_STRIPPED = 'test.tag'.freeze
+
   ARBITRARY_TEXT = 'This line is not an exception.'.freeze
 
   JAVA_EXC = <<END.freeze
@@ -125,7 +127,9 @@ def test_ignore_nested_exceptions
     }
 
     test_cases.each do |language, exception|
-      cfg = "languages #{language}"
+      cfg = %(
+#{CONFIG}
+languages #{language})
       d = create_driver(cfg)
       t = Time.now.to_i
 
@@ -156,12 +160,12 @@ def test_ignore_nested_exceptions
 
       # Validate that each line received is emitted separately as expected.
       router_mock.should_receive(:emit)
-                 .once.with(DEFAULT_TAG, Integer,
+                 .once.with(DEFAULT_TAG_STRIPPED, Integer,
                             'message' => json_line_with_exception,
                             'count' => 0)
 
       router_mock.should_receive(:emit)
-                 .once.with(DEFAULT_TAG, Integer,
+                 .once.with(DEFAULT_TAG_STRIPPED, Integer,
                             'message' => json_line_without_exception,
                             'count' => 1)
 
@@ -174,7 +178,9 @@ def test_ignore_nested_exceptions
   end
 
   def test_single_language_config
-    cfg = 'languages java'
+    cfg = %(
+#{CONFIG}
+languages java)
     d = create_driver(cfg)
     t = Time.now.to_i
     d.run do
@@ -185,7 +191,9 @@ def test_single_language_config
   end
 
   def test_multi_language_config
-    cfg = 'languages python, java'
+    cfg = %(
+#{CONFIG}
+languages python, java)
     d = create_driver(cfg)
     t = Time.now.to_i
     d.run do
@@ -196,7 +204,9 @@ def test_multi_language_config
   end
 
   def test_split_exception_after_timeout
-    cfg = 'multiline_flush_interval 1'
+    cfg = %(
+#{CONFIG}
+multiline_flush_interval 1)
     d = create_driver(cfg)
     t1 = 0
     t2 = 0
@@ -227,6 +237,12 @@ def test_do_not_split_exception_after_pause
     assert_equal(make_logs(t1, JAVA_EXC + "  at x\n  at y\n"), d.events)
   end
 
+  def test_remove_tag_prefix_is_required
+    cfg = ''
+    e = assert_raises(Fluent::ConfigError) { create_driver(cfg) }
+    assert_match(/remove_tag_prefix/, e.message)
+  end
+
   def get_out_tags(remove_tag_prefix, original_tag)
     cfg = "remove_tag_prefix #{remove_tag_prefix}"
     d = create_driver(cfg, original_tag)
@@ -244,7 +260,9 @@ def test_remove_tag_prefix
   end
 
   def test_flush_after_max_lines
-    cfg = 'max_lines 2'
+    cfg = %(
+#{CONFIG}
+max_lines 2)
     d = create_driver(cfg)
     t = Time.now.to_i
     d.run do
@@ -263,7 +281,9 @@ def test_flush_after_max_lines
   end
 
   def test_separate_streams
-    cfg = 'stream stream'
+    cfg = %(
+#{CONFIG}
+stream stream)
     d = create_driver(cfg)
     t = Time.now.to_i
     d.run do