Bring semantic versioning to provide for rolling upgrades (#5495)

This PR brings semver capabilities in our RPC layer to
ensure that we can upgrade the servers in rolling fashion
while keeping I/O in progress. This is only a framework change
the functionality remains the same as such and we do not
have any special API changes for now. But in future when
we bring in API changes we will be able to upgrade servers
without a downtime.

Additional change in this PR is to not abort when serverVersions
mismatch in a distributed cluster, instead wait for the quorum
treat the situation as if the server is down. This allows
for administrator to properly upgrade all the servers in the cluster.

Fixes #5393

cmd/rpc-common.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
+ * Minio Cloud Storage, (C) 2016, 2017, 2018 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"strconv"
 	"time"
 
 	"github.com/minio/dsync"
@@ -33,10 +34,51 @@ func isRequestTimeAllowed(requestTime time.Time) bool {
 		utcNow.Sub(requestTime) > rpcSkewTimeAllowed)
 }
 
+// semVersion - RPC semantic versioning.
+type semVersion struct {
+	Major uint64
+	Minor uint64
+	Patch uint64
+}
+
+// semver comparator implementation based on the semver 2.0.0 https://semver.org/.
+func (v semVersion) Compare(o semVersion) int {
+	if v.Major != o.Major {
+		if v.Major > o.Major {
+			return 1
+		}
+		return -1
+	}
+	if v.Minor != o.Minor {
+		if v.Minor > o.Minor {
+			return 1
+		}
+		return -1
+	}
+	if v.Patch != o.Patch {
+		if v.Patch > o.Patch {
+			return 1
+		}
+		return -1
+	}
+	return 0
+}
+
+func (v semVersion) String() string {
+	b := make([]byte, 0, 5)
+	b = strconv.AppendUint(b, v.Major, 10)
+	b = append(b, '.')
+	b = strconv.AppendUint(b, v.Minor, 10)
+	b = append(b, '.')
+	b = strconv.AppendUint(b, v.Patch, 10)
+	return string(b)
+}
+
 // AuthRPCArgs represents minimum required arguments to make any authenticated RPC call.
 type AuthRPCArgs struct {
 	// Authentication token to be verified by the server for every RPC call.
 	AuthToken string
+	Version   semVersion
 }
 
 // SetAuthToken - sets the token to the supplied value.
@@ -44,8 +86,21 @@ func (args *AuthRPCArgs) SetAuthToken(authToken string) {
 	args.AuthToken = authToken
 }
 
+// SetRPCAPIVersion - sets the rpc version to the supplied value.
+func (args *AuthRPCArgs) SetRPCAPIVersion(version semVersion) {
+	args.Version = version
+}
+
 // IsAuthenticated - validated whether this auth RPC args are already authenticated or not.
 func (args AuthRPCArgs) IsAuthenticated() error {
+	// checks if rpc Version is not equal to current server rpc version.
+	// this is fine for now, but in future when we add backward compatible
+	// APIs we need to make sure to allow lesser versioned clients to
+	// talk over RPC, until then we are fine with this check.
+	if args.Version.Compare(globalRPCAPIVersion) != 0 {
+		return errRPCAPIVersionUnsupported
+	}
+
 	// Check whether the token is valid
 	if !isAuthTokenValid(args.AuthToken) {
 		return errInvalidToken
@@ -61,15 +116,18 @@ type AuthRPCReply struct{}
 // LoginRPCArgs - login username and password for RPC.
 type LoginRPCArgs struct {
 	AuthToken   string
-	Version     string
+	Version     semVersion
 	RequestTime time.Time
 }
 
 // IsValid - validates whether this LoginRPCArgs are valid for authentication.
 func (args LoginRPCArgs) IsValid() error {
-	// Check if version matches.
-	if args.Version != Version {
-		return errServerVersionMismatch
+	// checks if rpc Version is not equal to current server rpc version.
+	// this is fine for now, but in future when we add backward compatible
+	// APIs we need to make sure to allow lesser versioned clients to
+	// talk over RPC, until then we are fine with this check.
+	if args.Version.Compare(globalRPCAPIVersion) != 0 {
+		return errRPCAPIVersionUnsupported
 	}
 
 	if !isRequestTimeAllowed(args.RequestTime) {