02.diff

--- 00config/output.txt	2020-02-19 20:01:01.000000000 +0900
+++ 01overwrite-config/output.txt	2020-02-19 20:01:01.000000000 +0900
@@ -1,10 +1,10 @@
-conf.Config{
+&conf.Config{
   Version:  "3.7",
   Services: conf.Services{
     Wordpress: conf.Wordpress{
       Image: "wordpress",
       Ports: []string{
-        "8080:80",
+        "9090:80",
       },
       Networks: []string{
         "overlay",
@@ -25,7 +25,7 @@
       },
       Deploy: conf.Deploy{
         Mode:         "replicated",
-        Replicas:     2,
+        Replicas:     5,
         EndpointMode: "dnsrr",
       },
     },

config.go

package conf

// https://docs.docker.com/compose/compose-file/
// https://mholt.github.io/json-to-go/

type Config struct {
	Version  string   `json:"version"`
	Services Services `json:"services"`
	Volumes  Volumes  `json:"volumes"`
	Networks Networks `json:"networks"`
}
type Deploy struct {
	Mode         string `json:"mode"`
	Replicas     int    `json:"replicas"`
	EndpointMode string `json:"endpoint_mode"`
}
type Wordpress struct {
	Image    string   `json:"image"`
	Ports    []string `json:"ports"`
	Networks []string `json:"networks"`
	Deploy   Deploy   `json:"deploy"`
}
type Mysql struct {
	Image    string   `json:"image"`
	Volumes  []string `json:"volumes"`
	Networks []string `json:"networks"`
	Deploy   Deploy   `json:"deploy"`
}
type Services struct {
	Wordpress Wordpress `json:"wordpress"`
	Mysql     Mysql     `json:"mysql"`
}
type Volumes struct {
	DbData interface{} `json:"db-data"`
}
type Networks struct {
	Overlay interface{} `json:"overlay"`
}

config.json

{
  "version": "3.7",
  "services": {
    "wordpress": {
      "image": "wordpress",
      "ports": [
        "8080:80"
      ],
      "networks": [
        "overlay"
      ],
      "deploy": {
        "mode": "replicated",
        "replicas": 2,
        "endpoint_mode": "vip"
      }
    },
    "mysql": {
      "image": "mysql",
      "volumes": [
        "db-data:/var/lib/mysql/data"
      ],
      "networks": [
        "overlay"
      ],
      "deploy": {
        "mode": "replicated",
        "replicas": 2,
        "endpoint_mode": "dnsrr"
      }
    }
  },
  "volumes": {
    "db-data": null
  },
  "networks": {
    "overlay": null
  }
}

go.mod

module m

go 1.13

require (
	github.com/imdario/mergo v0.3.8 // indirect
	github.com/k0kubun/pp v3.0.1+incompatible // indirect
	github.com/mattn/go-colorable v0.1.4 // indirect
	github.com/spf13/pflag v1.0.5 // indirect
)

go.sum

github.com/imdario/mergo v0.3.8 h1:CGgOkSJeqMRmt0D9XLWExdT4m4F1vd3FV3VPt+0VxkQ=
github.com/imdario/mergo v0.3.8/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=
github.com/k0kubun/pp v3.0.1+incompatible h1:3tqvf7QgUnZ5tXO6pNAZlrvHgl6DvifjDrd9g2S9Z40=
github.com/k0kubun/pp v3.0.1+incompatible/go.mod h1:GWse8YhT0p8pT4ir3ZgBbfZild3tgzSScAn6HmfYukg=
github.com/mattn/go-colorable v0.1.4 h1:snbPLB8fVfU9iwbbo30TPtbLRzwWu6aJS6Xh4eaaviA=
github.com/mattn/go-colorable v0.1.4/go.mod h1:U0ppj6V5qS13XJ6of8GYAs25YV2eR4EVcfRqFIhoBtE=
github.com/mattn/go-isatty v0.0.8 h1:HLtExJ+uU2HOZ+wI0Tt5DtUDrx8yhUqDcp7fYERX4CE=
github.com/mattn/go-isatty v0.0.8/go.mod h1:Iq45c/XA43vh69/j3iqttzPXn0bhXyGjM0Hdxcsrc5s=
github.com/spf13/pflag v1.0.5 h1:iy+VFUOCP1a+8yFto/drg2CJ5u0yRoB7fZw3DKv/JXA=
github.com/spf13/pflag v1.0.5/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
golang.org/x/sys v0.0.0-20190222072716-a9d3bda3a223/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=

main.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"m/conf"
	"os"

	"github.com/imdario/mergo"
	"github.com/k0kubun/pp"
	"github.com/spf13/pflag"
)

// JSONLoadFile ...
func JSONLoadFile(filename string, c interface{}) error {
	f, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer f.Close()
	decoder := json.NewDecoder(f)
	return decoder.Decode(c)
}

// LoadConfig ...
func LoadConfig(filename string) (*conf.Config, error) {
	var c conf.Config
	if err := JSONLoadFile(filename, &c); err != nil {
		return nil, err
	}

	overwritefile := os.Getenv("OVERWRITE_CONFIG")
	if overwritefile == "" {
		return &c, nil
	}

	fmt.Fprintf(os.Stderr, "***** OVERWRITE CONFIG by %q *****\n", overwritefile)

	var c2 conf.Config
	if err := JSONLoadFile(overwritefile, &c2); err != nil {
		return &c, err
	}
	if err := mergo.Merge(&c2, &c); err != nil {
		return &c, err
	}
	return &c2, nil
}

var (
	// Cmd ...
	Cmd    = pflag.CommandLine
	config = Cmd.String("config", "", "config")
)

func main() {
	Cmd.Parse(os.Args[1:])
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func run() error {
	config := *config

	c, err := LoadConfig(config)
	if err != nil {
		return err
	}

	pp.ColoringEnabled = false
	pp.Println(c)
	return nil
}

Makefile

00:
	go run $(shell echo $@*/)main.go --config config.json | tee $(shell echo $@*/)output.txt

# overwrite
01:
	OVERWRITE_CONFIG=overwrite.json go run $(shell echo $@*/)main.go --config config.json | tee $(shell echo $@*/)output.txt

02:
	diff -u $(shell echo 00*/)output.txt $(shell echo 01*/)output.txt > $@.diff || exit 0

# config (python)
03:
	python $(shell echo $@*/)main.py --config config.json | tee $(shell echo $@*/)output.txt

# overwrite (python)
04:
	OVERWRITE_CONFIG=overwrite.json python $(shell echo $@*/)main.py --config config.json | tee $(shell echo $@*/)output.txt

05:
	diff -u $(shell echo 03*/)output.txt $(shell echo 04*/)output.txt > $@.diff || exit 0

output.txt

&conf.Config{
  Version:  "3.7",
  Services: conf.Services{
    Wordpress: conf.Wordpress{
      Image: "wordpress",
      Ports: []string{
        "9090:80",
      },
      Networks: []string{
        "overlay",
      },
      Deploy: conf.Deploy{
        Mode:         "replicated",
        Replicas:     2,
        EndpointMode: "vip",
      },
    },
    Mysql: conf.Mysql{
      Image:   "mysql",
      Volumes: []string{
        "db-data:/var/lib/mysql/data",
      },
      Networks: []string{
        "overlay",
      },
      Deploy: conf.Deploy{
        Mode:         "replicated",
        Replicas:     5,
        EndpointMode: "dnsrr",
      },
    },
  },
  Volumes: conf.Volumes{
    DbData: nil,
  },
  Networks: conf.Networks{
    Overlay: nil,
  },
}

overwrite.json

{
  "services": {
    "wordpress": {
      "ports": [
        "9090:80"
      ]
    },
    "mysql": {
      "deploy": {
        "replicas": 5
      }
    }
  }
}