Add RunFd

[yyoshiki41]

I use gin http.Server under circus which is a socket manager.
I need to run http.Server through a file descriptor, so implemented RunFd method.

When using other socket managers (e.g. einhorn), maybe work well.

Usage

package main

import (
    "flag"
    "github.com/yyoshiki41/gin"
)

var (
    fd = flag.Int("fd", -1, "")
)

func main() {
    flag.Parse()

    r := gin.Default()
    // Ping test
    r.GET("/ping", func(c *gin.Context) {
        c.String(200, "pong")
    })
    // Method arg is the file descriptor number(int).
    r.RunFd(*fd)
}

and, circus.ini example is below.

[circus]
statsd = 1

[watcher:webapp]
cmd = ./main -fd $(circus.sockets.web)
numprocesses = 1
use_sockets = True

[socket:web]
host = 127.0.0.1
port = 8080

Demo

$ circusd circus.ini
2016-02-03 11:07:13 circus[46347] [INFO] Starting master on pid 46347
2016-02-03 11:07:13 circus[46347] [INFO] sockets started
2016-02-03 11:07:13 circus[46347] [INFO] Arbiter now waiting for commands
2016-02-03 11:07:13 circus[46347] [INFO] webapp started
2016-02-03 11:07:13 circus[46347] [INFO] circusd-stats started
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
 - using env:   export GIN_MODE=release
 - using code:  gin.SetMode(gin.ReleaseMode)

[GIN-debug] GET    /ping                     --> main.main.func1 (3 handlers)
[GIN-debug] Listening and serving HTTP on fd@5
2016-02-03 11:07:13 circus[46354] [INFO] Starting the stats streamer

# Ping test
$ curl localhost:8080/ping
pong

# Other terminal
[GIN] 2016/02/03 - 11:08:11 | 200 |    2.571629ms | 127.0.0.1 |   GET     /ping

thx ☀️

[gotokatsuya]

+1

@manucorporat

[se77en]

+1
@yyoshiki41 Hope you can write the tests.

[coveralls]

Coverage decreased (-0.6%) to 93.445% when pulling d277f9d on yyoshiki41:listen-fd into 542be2f on gin-gonic:develop.

[yyoshiki41]

@manucorporat @se77en
sorry for late reply.

Added the tests for RunFd.
plz, review this pr.

[coveralls]

Coverage increased (+0.04%) to 94.125% when pulling 48a0ff6 on yyoshiki41:listen-fd into 542be2f on gin-gonic:develop.

[coveralls]

Coverage increased (+0.04%) to 94.125% when pulling f26b613 on yyoshiki41:listen-fd into f931d1e on gin-gonic:develop.

[thinkerou]

@yyoshiki41 please re-commit the pull request to master branch, thanks!

[yyoshiki41]

@thinkerou

thanks for your reply.
Close this and reopened a new pr forked master.
#1609

thanks.