今天早上遇到这个情况，我在提交一个最普通的查询form的时候， 报错。
这个一看就是rails 日志，在记录每个请求的时候，只要参数中有一些 对号 这样的东西，就会有问题。
/managements/members?utf8=✓&email=22&commit=查询
33 2019-05-07 07:25:49 +0800: Rack app error handling request { GET /managements/members }
34 #
35 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/iooutputter.rb:42:in `write'
36 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/iooutputter.rb:42:in `print'
37 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/iooutputter.rb:42:in `write'
38 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/datefileoutputter.rb:80:in `write'
39 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/outputter.rb:110:in `block in canonical_log'
40 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/outputter.rb:130:in `block in synch'
41 /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/monitor.rb:226:in `mon_synchronize'
42 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/outputter.rb:130:in `synch'
43 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/outputter.rb:110:in `canonical_log'
44 (eval):3:in `info'
45 (eval):8:in `block in info'
46 (eval):8:in `each'
47 (eval):8:in `info'
48 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-4.2.10/lib/rails/rack/logger.rb:37:in `call_app'
49 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-4.2.10/lib/rails/rack/logger.rb:22:in `call'
50 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/actionpack-4.2.10/lib/action_dispatch/middleware/request_id.rb:21:in `call'
51 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/rack-1.6.8/lib/rack/methodoverride.rb:22:in `call'
52 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/rack-1.6.8/lib/rack/runtime.rb:18:in `call'
53 /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/actionpack-4.2.10/lib/action_dispatch/middleware/static.rb:120:in `call'
解决办法：
修改log4r 中的 rb文件
# ~/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/log4r-1.1.9/lib/log4r/outputter/iooutputter.rb
# perform the write
def write(data)
begin
@out.print data
@out.flush
rescue IOError => ioe # recover from this instead of crash
Logger.log_internal {"IOError in Outputter '#{@name}'!"}
Logger.log_internal {ioe}
close
rescue NameError => ne
Logger.log_internal {"Outputter '#{@name}' IO is #{@out.class}!"}
Logger.log_internal {ne}
close
# 增加这句话就可以了。
rescue Encoding::UndefinedConversionError => e
puts "==!!! by dashi , log4r utf error ? ... #{e}"
close
end
end