Guard launching duplicated fluentd with same configuration

Before:

  If you launch multiple Fluentd instance with same
  configuration file, it causes a disaster with inconsistency
  processed buffer or pos file.

After:

  Detect fluentd service's main process and fetch FLUENT_CONF.
  if configuration is same as spawned process, abort it.

  It can block the following conditions are met:

  * fluentd is launched via systemd (fluent-package)
    configuration file is specified via FLUENT_CONF.
  * manually try to launch fluentd  with same configuration file as
    fluentd user with -c option.

  Thus running fluentd service and manually try to launch normal
  user case can't be detected.

NOTE: Windows is out of scope in this PR.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

lib/fluent/command/fluentd.rb

@@ -20,6 +20,7 @@
 require 'fluent/log'
 require 'fluent/env'
 require 'fluent/version'
+require 'fluent/process'
 
 $fluentdargv = Marshal.load(Marshal.dump(ARGV))
 
@@ -340,6 +341,16 @@
   early_exit = true
 end
 
+begin
+  running_fluentd_conf = Fluent::ProcessDetector.running_fluentd_conf
+  if opts[:config_path] and opts[:config_path] == running_fluentd_conf
+    puts "Error: can't start duplicate Fluentd instance with same #{opts[:config_path]}"
+    exit 2
+  end
+rescue Errno::EACCES
+  # e.g. unprivileged access error, can't detect duplicated instance from command line parameter.
+end
+
 if start_service
   Service.start(opts[:winsvc_name])
 end

lib/fluent/process.rb

@@ -15,8 +15,32 @@
 #
 
 require 'fluent/compat/detach_process_mixin'
+require 'fluent/env'
 
 module Fluent
   DetachProcessMixin = Fluent::Compat::DetachProcessMixin
   DetachMultiProcessMixin = Fluent::Compat::DetachMultiProcessMixin
+
+  class ProcessDetector
+    def self.running_fluentd_conf
+      # Detect if same configuration is used
+      unless Fluent.windows?
+        IO.popen(["/usr/bin/ps", "-e", "-o", "uid,pid,ppid,cmd"]) do |_io|
+          _io.readlines.each do |line|
+            uid, pid, ppid, cmd = line.split(' ', 4)
+            # skip self and supervisor process
+            next if Process.pid == pid.to_i or Process.pid == ppid.to_i
+            if cmd and cmd.chomp.include?("fluentd") and ppid.to_i == 1
+              # under systemd control
+              File.open("/proc/#{pid.to_i}/environ") do |file|
+                conf = file.read.split("\u0000").select { |entry| entry.include?("FLUENT_CONF") }
+                return conf.first.split('=').last unless conf.empty?
+              end
+            end
+          end
+        end
+      end
+      return nil
+    end
+  end
 end