in_tail: fix error when files without read permission are included in glob patterns (#5161)

**Which issue(s) this PR fixes**:
Fixes #5152

**What this PR does / why we need it**:
It is referencing an uninitialized instance variable for Linux
Capability feature, causing an error.
This PR will fix the bug.

**Docs Changes**:
N/A

**Release Note**:
* in_tail: fix error preventing the use of Linux Capability feature

---------

Signed-off-by: Shizuo Fujita <[email protected]>
Signed-off-by: Kentaro Hayashi <[email protected]>

Author: Watson1978

lib/fluent/plugin/in_tail.rb

@@ -59,6 +59,7 @@ def initialize
       @shutdown_start_time = nil
       @metrics = nil
       @startup = true
+      @capability = Fluent::Capability.new(:current_process)
     end
 
     desc 'The paths to read. Multiple paths can be specified, separated by comma.'
@@ -195,7 +196,6 @@ def configure(conf)
       @dir_perm = system_config.dir_permission || Fluent::DEFAULT_DIR_PERMISSION
       # parser is already created by parser helper
       @parser = parser_create(usage: parser_config['usage'] || @parser_configs.first.usage)
-      @capability = Fluent::Capability.new(:current_process)
       if @read_bytes_limit_per_second > 0
         if !@enable_watch_timer
           raise Fluent::ConfigError, "Need to enable watch timer when using log throttling feature"

test/plugin/test_in_tail.rb

@@ -2648,6 +2648,22 @@ def test_no_warn_with_directory_permission
     end
   end
 
+  def test_other_user_owned_files
+    # https://github.com/fluent/fluentd/issues/5141
+    omit "Cannot test with root user" if Process::UID.eid == 0
+    omit "NTFS doesn't support UNIX like permissions" if Fluent.windows?
+
+    config = config_element('', '', {
+      'tag' => "tail",
+      'path' => "/var/log/*.log",
+      'format' => 'none',
+      "pos_file" => "#{@tmp_dir}/tail.pos",
+    })
+    assert_nothing_raised do
+      create_driver(config, false)
+    end
+  end
+
   def test_shutdown_timeout
     Fluent::FileWrapper.open("#{@tmp_dir}/tail.txt", "wb") do |f|
       # Should be large enough to take too long time to consume