auth: add method for merging two AuthConfigurations instances

auth.go

@@ -55,10 +55,23 @@ func (c AuthConfigurations) isEmpty() bool {
 	return len(c.Configs) == 0
 }
 
-func (c AuthConfigurations) headerKey() string {
+func (AuthConfigurations) headerKey() string {
 	return "X-Registry-Config"
 }
 
+// merge updates the configuration. If a key is defined in both maps, the one
+// in c.Configs takes precedence.
+func (c *AuthConfigurations) merge(other AuthConfigurations) {
+	for k, v := range other.Configs {
+		if c.Configs == nil {
+			c.Configs = make(map[string]AuthConfiguration)
+		}
+		if _, ok := c.Configs[k]; !ok {
+			c.Configs[k] = v
+		}
+	}
+}
+
 // AuthConfigurations119 is used to serialize a set of AuthConfigurations
 // for Docker API >= 1.19.
 type AuthConfigurations119 map[string]AuthConfiguration
@@ -119,6 +132,13 @@ func NewAuthConfigurationsFromDockerCfg() (*AuthConfigurations, error) {
 	var err error
 	for _, path := range pathsToTry {
 		auths, err = NewAuthConfigurationsFromFile(path)
+		if err != nil {
+			continue
+		}
+
+		if !auths.isEmpty() {
+			return auths, nil
+		}
 		if err == nil {
 			return auths, nil
 		}

auth_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"os"
 	"path"
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -237,3 +238,95 @@ func TestAuthCheck(t *testing.T) {
 		t.Fatal("expected failure from unauthorized auth")
 	}
 }
+
+func TestAuthConfigurationsMerge(t *testing.T) {
+	t.Parallel()
+	var tests = []struct {
+		name     string
+		left     AuthConfigurations
+		right    AuthConfigurations
+		expected AuthConfigurations
+	}{
+		{
+			name:     "empty configs",
+			expected: AuthConfigurations{},
+		},
+		{
+			name: "empty left config",
+			right: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+				},
+			},
+			expected: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+				},
+			},
+		},
+		{
+			name: "empty right config",
+			left: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+				},
+			},
+			expected: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+				},
+			},
+		},
+		{
+			name: "no conflicts",
+			left: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+				},
+			},
+			right: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"us.gcr.io": {Email: "[email protected]"},
+				},
+			},
+			expected: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+					"us.gcr.io": {Email: "[email protected]"},
+				},
+			},
+		},
+		{
+			name: "no conflicts",
+			left: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+					"us.gcr.io": {Email: "[email protected]"},
+				},
+			},
+			right: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"us.gcr.io": {Email: "[email protected]"},
+				},
+			},
+			expected: AuthConfigurations{
+				Configs: map[string]AuthConfiguration{
+					"docker.io": {Email: "[email protected]"},
+					"us.gcr.io": {Email: "[email protected]"},
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			test.left.merge(test.right)
+
+			if !reflect.DeepEqual(test.left, test.expected) {
+				t.Errorf("wrong configuration map after merge\nwant %#v\ngot  %#v", test.expected, test.left)
+			}
+		})
+	}
+}