Impossible to run Kemal with reuse_port: true

[vlazar]

Description

Due to changes in HTTP::Server API in Crystal 0.25.0 and the way Kemal 0.23.0 starts server it's now impossible to run Kemal with reuse_port: true.

Ref: https://github.com/kemalcr/kemal/pull/452/files#diff-e5663d7073109a4b406e96582b8cb5c2R71

The problem is listen now also does bind_tcp:

Ref: https://github.com/crystal-lang/crystal/blob/7fb783f7afec0a199d6f22ef78aae0f45517068a/src/http/server.cr#L216-L220

Steps to Reproduce

# reuse_port_issue.cr
require "kemal"

get "/plaintext" do |env|
  env.response.content_type = "text/plain"
  "Hello, World!"
end

Kemal.config do |cfg|
  cfg.serve_static = false
  cfg.logging = false
end

Kemal.run do |cfg|
  cfg.server.not_nil!.bind_tcp(cfg.port, reuse_port: true)
end

$ crystal reuse_port_issue.cr

Expected behavior: It is possible to run Kemal with reuse_port: true

Actual behavior: Unhandled exception: bind: Address already in use (Errno)

Reproduces how often: Always

Versions

Kemal 0.23.0

$ crystal --version
Crystal 0.25.0 (2018-06-20)

LLVM: 5.0.1
Default target: x86_64-apple-macosx

Additional Information

Trying to update TFB to latest Crystal and frameworks here TechEmpower/FrameworkBenchmarks@e6c2f95#diff-7a0f270376d33ada73bacaf95cf345fbL116

TechEmpower/FrameworkBenchmarks#3884

[waghanza]

@vlazar Same issue if port is 3000

[waghanza]

using listen it works

[vlazar]

@waghanza I'm not sure what do you mean by listen works. Can you share the code?

[waghanza]

@vlazar sorry, I think I have to give more details

Kemal.run { |cfg| cfg.server.not_nil!.bind_tcp("0.0.0.0",3000, reuse_port: true) }

seems not to work for me, but

Kemal.run { |cfg| cfg.server.not_nil!.listen("0.0.0.0",3000, reuse_port: true) }

works

[straight-shoota]

Yes, this works because listen blocks the current fiber and the code won't reach the second listen call until the server is stopped. It's not nice but fine as a quick workaround.

[waghanza]

@straight-shoota thanks for explanation

[vlazar]

@straight-shoota Thanks for explanation! TIL. I thought with @waghanza example two server.listen calls were actually made and created 2 sockets internally. It turns out that's not the case.