Move go version check into main() (#3734)

Previously the check was done in init().  This patch moves into main
for unit testable friendly function.

main.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ * Minio Cloud Storage, (C) 2016,2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,11 +23,47 @@
 package main // import "github.com/minio/minio"
 
 import (
+	"fmt"
 	"os"
+	"runtime"
 
+	version "github.com/hashicorp/go-version"
+	"github.com/minio/mc/pkg/console"
 	minio "github.com/minio/minio/cmd"
 )
 
+const (
+	// Minio requires at least Go v1.7
+	minGoVersion        = "1.7"
+	goVersionConstraint = ">= " + minGoVersion
+)
+
+// Check if this binary is compiled with at least minimum Go version.
+func checkGoVersion(goVersionStr string) error {
+	constraint, err := version.NewConstraint(goVersionConstraint)
+	if err != nil {
+		return fmt.Errorf("'%s': %s", goVersionConstraint, err)
+	}
+
+	goVersion, err := version.NewVersion(goVersionStr)
+	if err != nil {
+		return err
+	}
+
+	if !constraint.Check(goVersion) {
+		return fmt.Errorf("Minio is not compiled by Go %s.  Please recompile accordingly.",
+			goVersionConstraint)
+	}
+
+	return nil
+}
+
 func main() {
+	// When `go get` is used minimum Go version check is not triggered but it would have compiled it successfully.
+	// However such binary will fail at runtime, hence we also check Go version at runtime.
+	if err := checkGoVersion(runtime.Version()[2:]); err != nil {
+		console.Fatalln("Go runtime version check failed.", err)
+	}
+
 	minio.Main(os.Args, os.Exit)
 }