out_file: add symlink_path_use_relative option to use relative path instead of absolute path in symlink_path (#4904)

**Which issue(s) this PR fixes**: 
None.

**What this PR does / why we need it**: 
This PR moves from an absolute link to a relative link for
`symlink_path` parameter. Otherwise, this does not work in a dockerized
environment where the path is wired to the the outside of the container.
I can also add an additional parameter to switch between absolute and
relative linking if necessary and/or desired.

**Docs Changes**:
TODO https://docs.fluentd.org/input/tail

**Release Note**: 
Same as the title.

---------

Signed-off-by: Florian Dobener <florian.dobener@dwd.de>
Signed-off-by: domna <florian.dobener@physik.hu-berlin.de>

Author: domna

lib/fluent/plugin/out_file.rb

@@ -17,6 +17,7 @@
 require 'fileutils'
 require 'zlib'
 require 'time'
+require 'pathname'
 
 require 'fluent/plugin/output'
 require 'fluent/config/error'
@@ -54,6 +55,8 @@ class FileOutput < Output
     config_param :recompress, :bool, default: false
     desc "Create symlink to temporary buffered file when buffer_type is file (disabled on Windows)."
     config_param :symlink_path, :string, default: nil
+    desc "Use relative path for symlink target (default: false)"
+    config_param :symlink_path_use_relative, :bool, default: false
 
     config_section :format do
       config_set_default :@type, 'out_file'
@@ -97,7 +100,12 @@ def generate_chunk(metadata)
         if chunk.metadata == @latest_metadata
           sym_path = @_output_plugin_for_symlink.extract_placeholders(@_symlink_path, chunk)
           FileUtils.mkdir_p(File.dirname(sym_path), mode: @_output_plugin_for_symlink.dir_perm)
-          FileUtils.ln_sf(chunk.path, sym_path)
+          if @_output_plugin_for_symlink.symlink_path_use_relative
+            relative_path = Pathname.new(chunk.path).relative_path_from(Pathname.new(File.dirname(sym_path)))
+            FileUtils.ln_sf(relative_path, sym_path)
+          else
+            FileUtils.ln_sf(chunk.path, sym_path)
+          end
         end
         chunk
       end

test/plugin/test_out_file.rb

@@ -779,7 +779,24 @@ def parse_system(text)
       end
     end
 
-    def run_and_check(d, symlink_path)
+    test 'relative symlink' do
+      omit "Windows doesn't support symlinks" if Fluent.windows?
+
+      conf = CONFIG + %[
+        symlink_path #{SYMLINK_PATH}
+        symlink_path_use_relative true
+      ]
+      symlink_path = "#{SYMLINK_PATH}"
+
+      d = create_driver(conf)
+      begin
+        run_and_check(d, symlink_path, relative_symlink=true)
+      ensure
+        FileUtils.rm_rf(symlink_path)
+      end
+    end
+
+    def run_and_check(d, symlink_path, relative_symlink=false)
       d.run(default_tag: 'tag') do
         es = Fluent::OneEventStream.new(event_time("2011-01-02 13:14:15 UTC"), {"a"=>1})
         d.feed(es)
@@ -794,7 +811,14 @@ def run_and_check(d, symlink_path)
         assert File.exist?(symlink_path)
 
         meta = d.instance.metadata('tag', event_time("2011-01-03 14:15:16 UTC"), {})
-        assert_equal d.instance.buffer.instance_eval{ @stage[meta].path }, File.readlink(symlink_path)
+        if relative_symlink
+          target_path = d.instance.buffer.instance_eval{ @stage[meta].path }
+          link_target = File.readlink(symlink_path)
+          expected_path = Pathname.new(target_path).relative_path_from(Pathname.new(File.dirname(symlink_path))).to_s
+          assert_equal expected_path, link_target
+        else
+          assert_equal d.instance.buffer.instance_eval{ @stage[meta].path }, File.readlink(symlink_path)
+        end
       end
     end
   end