chore: improves auth plugin resolution.

plugins/rest/rest.go

@@ -10,6 +10,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"reflect"
@@ -65,23 +66,33 @@ func (c *Config) Equal(other *Config) bool {
 
 func (c *Config) authPlugin(authPluginLookup func(string) HTTPAuthPlugin) (HTTPAuthPlugin, error) {
 	var candidate HTTPAuthPlugin
-	if c.Credentials.Plugin != nil && authPluginLookup != nil {
+	if c.Credentials.Plugin != nil {
+		if authPluginLookup == nil {
+			// if no authPluginLookup function is passed we can't resolve the plugin
+			return nil, errors.New("missing auth plugin lookup function")
+		}
+
 		candidate := authPluginLookup(*c.Credentials.Plugin)
-		if candidate != nil {
-			return candidate, nil
+		if candidate == nil {
+			return nil, fmt.Errorf("auth plugin %q not found", *c.Credentials.Plugin)
 		}
+
+		return candidate, nil
 	}
 	// reflection avoids need for this code to change as auth plugins are added
 	s := reflect.ValueOf(c.Credentials)
 	for i := 0; i < s.NumField(); i++ {
 		if s.Field(i).IsNil() {
 			continue
 		}
+
 		if candidate != nil {
 			return nil, errors.New("a maximum one credential method must be specified")
 		}
+
 		candidate = s.Field(i).Interface().(HTTPAuthPlugin)
 	}
+
 	if candidate == nil {
 		return &defaultAuthPlugin{}, nil
 	}

plugins/rest/rest_test.go

@@ -41,8 +41,31 @@ import (
 
 const keyID = "key1"
 
-func TestNew(t *testing.T) {
+func TestAuthPluginWithNoAuthPluginLookup(t *testing.T) {
+	authPlugin := "anything"
+	cfg := Config{
+		Credentials: struct {
+			Bearer               *bearerAuthPlugin                  `json:"bearer,omitempty"`
+			OAuth2               *oauth2ClientCredentialsAuthPlugin `json:"oauth2,omitempty"`
+			ClientTLS            *clientTLSAuthPlugin               `json:"client_tls,omitempty"`
+			S3Signing            *awsSigningAuthPlugin              `json:"s3_signing,omitempty"`
+			GCPMetadata          *gcpMetadataAuthPlugin             `json:"gcp_metadata,omitempty"`
+			AzureManagedIdentity *azureManagedIdentitiesAuthPlugin  `json:"azure_managed_identity,omitempty"`
+			Plugin               *string                            `json:"plugin,omitempty"`
+		}{
+			Plugin: &authPlugin,
+		},
+	}
+	_, err := cfg.authPlugin(nil)
+	if err == nil {
+		t.Error("Expected error but got nil")
+	}
+	if want, have := "missing auth plugin lookup function", err.Error(); want != have {
+		t.Errorf("Unexpected error, want %q, have %q", want, have)
+	}
+}
 
+func TestNew(t *testing.T) {
 	tests := []struct {
 		name    string
 		input   string
@@ -623,6 +646,17 @@ func TestNew(t *testing.T) {
         }
 			}`,
 		},
+		{
+			name: "Unknown plugin",
+			input: `{
+				"name": "foo",
+				"url": "http://localhost",
+				"credentials": {
+					"plugin": "unknown_plugin"
+        }
+			}`,
+			wantErr: true,
+		},
 	}
 
 	var results []Client