USR2 will now reopen *all* log files (like Unicorn). Also, the logfile will be reopened using IO#reopen rather than creating a new Logger object, reducing the possibility of having a stale logger object

lib/sidekiq/cli.rb

@@ -108,7 +108,7 @@ module Sidekiq
       when 'USR2'
         if Sidekiq.options[:logfile]
           Sidekiq.logger.info "Received USR2, reopening log file"
-          initialize_logger
+          Sidekiq::Logging.reopen_logs
         end
       when 'TTIN'
         Thread.list.each do |thread|

lib/sidekiq/logging.rb

@@ -42,6 +42,54 @@ module Sidekiq
       @logger = (log ? log : Logger.new('/dev/null'))
     end
 
+    # This reopens ALL logfiles in the process that have been rotated
+    # using logrotate(8) (without copytruncate) or similar tools.
+    # A +File+ object is considered for reopening if it is:
+    #   1) opened with the O_APPEND and O_WRONLY flags
+    #   2) the current open file handle does not match its original open path
+    #   3) unbuffered (as far as userspace buffering goes, not O_SYNC)
+    # Returns the number of files reopened
+    def self.reopen_logs
+      to_reopen = []
+      nr = 0
+      ObjectSpace.each_object(File) { |fp| is_log?(fp) and to_reopen << fp }
+
+      to_reopen.each do |fp|
+        orig_st = begin
+          fp.stat
+        rescue IOError, Errno::EBADF
+          next
+        end
+
+        begin
+          b = File.stat(fp.path)
+          next if orig_st.ino == b.ino && orig_st.dev == b.dev
+        rescue Errno::ENOENT
+        end
+
+        begin
+          File.open(fp.path, 'a') { |tmpfp| fp.reopen(tmpfp) }
+          fp.sync = true
+
+          nr += 1
+        rescue IOError, Errno::EBADF
+          # not much we can do...
+        end
+      end
+      nr
+    end
+
+    def self.is_log?(fp)
+      append_flags = File::WRONLY | File::APPEND
+
+      ! fp.closed? &&
+        fp.stat.file? &&
+        fp.sync &&
+        (fp.fcntl(Fcntl::F_GETFL) & append_flags) == append_flags
+    rescue IOError, Errno::EBADF
+      false
+    end
+
     def logger
       Sidekiq::Logging.logger
     end