storage: Implement Close() in REST client (#6826)

Calling /minio/prometheuses/metrics calls xlSets.StorageInfo() which creates a new
storage REST client and closes it. However, currently, closing does nothing
to the underlying opened http client.

This commit introduces a closing behavior by calling CloseIdleConnections
provided by http.Transport upon the initialization of this latter.

cmd/rest/client.go

@@ -36,6 +36,7 @@ const DefaultRESTTimeout = 1 * time.Minute
 // Client - http based RPC client.
 type Client struct {
 	httpClient          *http.Client
+	httpIdleConnsCloser func()
 	url                 *url.URL
 	newAuthToken        func() string
 }
@@ -67,6 +68,13 @@ func (c *Client) Call(method string, values url.Values, body io.Reader) (reply i
 	return resp.Body, nil
 }
 
+// Close closes all idle connections of the underlying http client
+func (c *Client) Close() {
+	if c.httpIdleConnsCloser != nil {
+		c.httpIdleConnsCloser()
+	}
+}
+
 func newCustomDialContext(timeout time.Duration) func(ctx context.Context, network, addr string) (net.Conn, error) {
 	return func(ctx context.Context, network, addr string) (net.Conn, error) {
 		dialer := &net.Dialer{
@@ -84,13 +92,11 @@ func newCustomDialContext(timeout time.Duration) func(ctx context.Context, netwo
 	}
 }
 
-// NewClient - returns new RPC client.
+// NewClient - returns new REST client.
 func NewClient(url *url.URL, tlsConfig *tls.Config, timeout time.Duration, newAuthToken func() string) *Client {
-	return &Client{
-		httpClient: &http.Client{
 	// Transport is exactly same as Go default in https://golang.org/pkg/net/http/#RoundTripper
 	// except custom DialContext and TLSClientConfig.
-			Transport: &http.Transport{
+	tr := &http.Transport{
 		Proxy:                 http.ProxyFromEnvironment,
 		DialContext:           newCustomDialContext(timeout),
 		MaxIdleConnsPerHost:   4096,
@@ -100,8 +106,11 @@ func NewClient(url *url.URL, tlsConfig *tls.Config, timeout time.Duration, newAu
 		ExpectContinueTimeout: 1 * time.Second,
 		TLSClientConfig:       tlsConfig,
 		DisableCompression:    true,
-			},
+	}
-		},
+
+	return &Client{
+		httpClient:          &http.Client{Transport: tr},
+		httpIdleConnsCloser: tr.CloseIdleConnections,
 		url:                 url,
 		newAuthToken:        newAuthToken,
 	}

cmd/storage-rest-client.go

@@ -322,6 +322,7 @@ func (client *storageRESTClient) RenameFile(srcVolume, srcPath, dstVolume, dstPa
 // Close - marks the client as closed.
 func (client *storageRESTClient) Close() error {
 	client.connected = false
+	client.restClient.Close()
 	return nil
 }