Unwrap network errors (#10934)

Alternative to #10927

Instead of having an upstream fix, do unwrap when checking network errors.

'As' will also work when destination is an interface as checked by the tests.

cmd/rest/client_test.go

@@ -0,0 +1,68 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package rest
+
+import (
+	"errors"
+	"net"
+	"net/url"
+	"testing"
+)
+
+func TestNetworkError_Unwrap(t *testing.T) {
+	tests := []struct {
+		name   string
+		err    error
+		target interface{}
+		want   bool
+	}{
+		{
+			name:   "url.Error",
+			err:    &url.Error{Op: "PUT", URL: "http://localhost/1234", Err: restError("remote server offline")},
+			target: &url.Error{},
+			want:   true,
+		},
+		{
+			name: "net.Error",
+			err:  &url.Error{Op: "PUT", URL: "http://localhost/1234", Err: restError("remote server offline")},
+			want: true,
+		},
+		{
+			name: "net.Error-unmatched",
+			err:  errors.New("something"),
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Wrap error
+			n := &NetworkError{
+				Err: tt.err,
+			}
+			if tt.target == nil {
+				var netErrInterface net.Error
+				if errors.As(n, &netErrInterface) != tt.want {
+					t.Errorf("errors.As(n, &tt.target) != tt.want, n: %#v, target: %#v, want:%v, got: %v", n, tt.target, tt.want, !tt.want)
+				}
+			} else {
+				if errors.As(n, &tt.target) != tt.want {
+					t.Errorf("errors.As(n, &tt.target) != tt.want, n: %#v, target: %#v, want:%v, got: %v", n, tt.target, tt.want, !tt.want)
+				}
+			}
+		})
+	}
+}