Protect map from concurrent access (Fixes #2915) (#2916)

Protects the Peers RPC clients map from concurrent access to fix a data race condition.

cmd/s3-peer-client.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"fmt"
 	"path"
+	"sync"
 	"time"
 
 	"github.com/minio/minio-go/pkg/set"
@@ -26,22 +27,30 @@ import (
 
 type s3Peers struct {
 	// A map of peer server address (in `host:port` format) to RPC
-	// client connections
-	rpcClient map[string]*AuthRPCClient
+	// client connections.
+	rpcClients map[string]*AuthRPCClient
 
-	// slice of all peer addresses (in `host:port` format)
+	mutex *sync.RWMutex
+
+	// Slice of all peer addresses (in `host:port` format).
 	peers []string
 }
 
 func initGlobalS3Peers(disks []string) {
-	// get list of de-duplicated peers
+	// Get list of de-duplicated peers.
 	peers := getAllPeers(disks)
-	globalS3Peers = s3Peers{make(map[string]*AuthRPCClient), nil}
+
+	// Initialize global state.
+	globalS3Peers = s3Peers{
+		rpcClients: make(map[string]*AuthRPCClient),
+		mutex:      &sync.RWMutex{},
+	}
+	// Initialize each peer connection.
 	for _, peer := range peers {
 		globalS3Peers.InitS3PeerClient(peer)
 	}
 
-	// Additionally setup a local peer if one does not exist
+	// Additionally setup a local peer if one does not exist.
 	if globalS3Peers.GetPeerClient(globalMinioAddr) == nil {
 		globalS3Peers.InitS3PeerClient(globalMinioAddr)
 		peers = append(peers, globalMinioAddr)
@@ -55,16 +64,23 @@ func (s3p *s3Peers) GetPeers() []string {
 }
 
 func (s3p *s3Peers) GetPeerClient(peer string) *AuthRPCClient {
-	return s3p.rpcClient[peer]
+	// Take a read lock
+	s3p.mutex.RLock()
+	defer s3p.mutex.RUnlock()
+	return s3p.rpcClients[peer]
 }
 
 // Initializes a new RPC connection (or closes and re-opens if it
 // already exists) to a peer. Note that peer address is in `host:port`
 // format.
 func (s3p *s3Peers) InitS3PeerClient(peer string) {
-	if s3p.rpcClient[peer] != nil {
-		s3p.rpcClient[peer].Close()
-		delete(s3p.rpcClient, peer)
+	// Take a write lock
+	s3p.mutex.Lock()
+	defer s3p.mutex.Unlock()
+
+	if s3p.rpcClients[peer] != nil {
+		_ = s3p.rpcClients[peer].Close()
+		delete(s3p.rpcClients, peer)
 	}
 	authCfg := &authConfig{
 		accessKey:   serverConfig.GetCredential().AccessKeyID,
@@ -73,16 +89,20 @@ func (s3p *s3Peers) InitS3PeerClient(peer string) {
 		path:        path.Join(reservedBucket, s3Path),
 		loginMethod: "S3.LoginHandler",
 	}
-	s3p.rpcClient[peer] = newAuthClient(authCfg)
+	s3p.rpcClients[peer] = newAuthClient(authCfg)
 }
 
 func (s3p *s3Peers) Close() error {
-	for _, v := range s3p.rpcClient {
+	// Take a write lock
+	s3p.mutex.Lock()
+	defer s3p.mutex.Unlock()
+
+	for _, v := range s3p.rpcClients {
 		if err := v.Close(); err != nil {
 			return err
 		}
 	}
-	s3p.rpcClient = nil
+	s3p.rpcClients = nil
 	s3p.peers = nil
 	return nil
 }
@@ -121,33 +141,48 @@ func (s3p *s3Peers) SendRPC(peers []string, method string, args interface {
 	SetToken(token string)
 	SetTimestamp(tstamp time.Time)
 }) map[string]error {
-	// result type
+	// Take read lock for rpcClient map
+	s3p.mutex.RLock()
+	defer s3p.mutex.RUnlock()
+
+	// Result type
 	type callResult struct {
 		target string
 		err    error
 	}
-	// channel to collect results from goroutines
+
+	// Channel to collect results from goroutines
 	resChan := make(chan callResult)
-	// closure to make a single request.
+
+	// Closure to make a single request.
 	callTarget := func(target string) {
 		reply := &GenericReply{}
-		err := s3p.rpcClient[target].Call(method, args, reply)
+		client, ok := s3p.rpcClients[target]
+		var err error
+		if !ok {
+			err = fmt.Errorf("Requested client was not initialized - %v",
+				target)
+		} else {
+			err = client.Call(method, args, reply)
+		}
 		resChan <- callResult{target, err}
 	}
-	// map of errors
+
+	// Map of errors
 	errsMap := make(map[string]error)
 	// make network calls in parallel
 	for _, target := range peers {
 		go callTarget(target)
 	}
-	// wait on channel and collect all results
+	// Wait on channel and collect all results
 	for range peers {
 		res := <-resChan
 		if res.err != nil {
 			errsMap[res.target] = res.err
 		}
 	}
-	// return errors map
+
+	// Return errors map
 	return errsMap
 }