Exceptions when sending to closed connections on an HTTP::Server bound to a UNIX socket

[j8r]

While running an HTTP::Server bound to a UNIX socket, trying to send a response after the client closed the connection prints exceptions.
This lets me think it is a bug since this behavior doesn't happen when using HTTP::Server#bind_tcp, which doesn't print any exception after a connection has been closed.

To reproduce it:

1. Run the below code snippet
2. On a second terminal, make a HTTP call: curl --unix-socket /tmp/server.sock 'http://localhost/'
3. When seeing "Receiving request", CTRL+C the curl command
4. Exceptions will be printed on the STDOUT.

Here is the code to reproduce:

require "http/server"

http_server = HTTP::Server.new do |context|
  puts "Receiving request"
  sleep 5.seconds
  context.response.puts "hello"
end
# http_server.bind_tcp 8080
http_server.bind_unix "/tmp/server.sock"
Signal::INT.trap { http_server.close }
http_server.listen

The exceptions:

Unhandled exception in spawn: Error while flushing data to the client (HTTP::Server::ClientError)
  from /usr/lib/crystal/lib/http/server/response.cr:328:9 in 'unbuffered_flush'
  from /usr/lib/crystal/lib/io/buffered.cr:251:5 in 'flush'
  from /usr/lib/crystal/lib/io/buffered.cr:258:5 in 'close'
  from /usr/lib/crystal/lib/http/server/response.cr:298:9 in 'close'
  from /usr/lib/crystal/lib/http/server/request_processor.cr:63:11 in 'process'
  from /usr/lib/crystal/lib/http/server.cr:521:5 in 'handle_client'
  from /usr/lib/crystal/lib/http/server.cr:451:5 in '->'
  from /usr/lib/crystal/lib/fiber.cr:143:11 in 'run'
  from /usr/lib/crystal/lib/fiber.cr:95:34 in '->'
  from ???
Caused by: Error writing to socket (#<UNIXSocket:0x7f4feac85e00>): Broken pipe (IO::Error)
  from /usr/lib/crystal/lib/crystal/system/unix/event_loop_libevent.cr:227:11 in 'write'
  from /usr/lib/crystal/lib/crystal/system/socket.cr:89:5 in 'system_write'
  from /usr/lib/crystal/lib/socket.cr:451:16 in 'unbuffered_write'
  from /usr/lib/crystal/lib/io/buffered.cr:250:5 in 'flush'
  from /usr/lib/crystal/lib/http/server/response.cr:325:9 in 'unbuffered_flush'
  from /usr/lib/crystal/lib/io/buffered.cr:251:5 in 'flush'
  from /usr/lib/crystal/lib/io/buffered.cr:258:5 in 'close'
  from /usr/lib/crystal/lib/http/server/response.cr:298:9 in 'close'
  from /usr/lib/crystal/lib/http/server/request_processor.cr:63:11 in 'process'
  from /usr/lib/crystal/lib/http/server.cr:521:5 in 'handle_client'
  from /usr/lib/crystal/lib/http/server.cr:451:5 in '->'
  from /usr/lib/crystal/lib/fiber.cr:143:11 in 'run'
  from /usr/lib/crystal/lib/fiber.cr:95:34 in '->'
  from ???

[straight-shoota]

The error is triggered when flushing the buffer in Response::Output#close.
Apparently the unix socket is buffered while tcp socket is unbuffered. That makes it flush the buffer on close, which is usually not observed with tcp sockets.
And the problem is that we rescue errors from @handler.call but not from response.output.close:

crystal/src/http/server/request_processor.cr

Lines 50 to 64 in cd2dd0e

	Log.with_context do
	@handler.call(context)
	rescue ex : ClientError
	Log.debug(exception: ex.cause) { ex.message }
	rescue ex
	Log.error(exception: ex) { "Unhandled exception on HTTP::Handler" }
	unless response.closed?
	unless response.wrote_headers?
	response.respond_with_status(:internal_server_error)
	end
	end
	return
	ensure
	response.output.close
	end