plugins/discovery: Improve error message about prohibited config

Previously the discovery plugin would just log a generic error if any
plugins were enabled. With this commit, it will log an error that
mentions which plugins were enabled, making the source of the problem
a bit more obvious.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

config/config.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
 
 	"github.com/open-policy-agent/opa/ast"
 	"github.com/open-policy-agent/opa/internal/ref"
@@ -44,7 +45,27 @@ func ParseConfig(raw []byte, id string) (*Config, error) {
 	return &result, result.validateAndInjectDefaults(id)
 }
 
+// PluginNames returns a sorted list of names of enabled plugins.
+func (c Config) PluginNames() (result []string) {
+	if c.Bundle != nil || c.Bundles != nil {
+		result = append(result, "bundles")
+	}
+	if c.Status != nil {
+		result = append(result, "status")
+	}
+	if c.DecisionLogs != nil {
+		result = append(result, "decision_logs")
+	}
+	for name := range c.Plugins {
+		result = append(result, name)
+	}
+	sort.Strings(result)
+	return result
+}
+
 // PluginsEnabled returns true if one or more plugin features are enabled.
+//
+// Deprecated. Use PluginNames instead.
 func (c Config) PluginsEnabled() bool {
 	return c.Bundle != nil || c.Bundles != nil || c.DecisionLogs != nil || c.Status != nil || len(c.Plugins) > 0
 }

config/config_test.go

@@ -16,6 +16,74 @@ import (
 	"github.com/open-policy-agent/opa/version"
 )
 
+func TestConfigPluginNames(t *testing.T) {
+	tests := []struct {
+		name     string
+		conf     Config
+		expected []string
+	}{
+		{
+			name:     "empty config",
+			conf:     Config{},
+			expected: nil,
+		},
+		{
+			name: "bundle",
+			conf: Config{
+				Bundle: []byte(`{"bundle": {"name": "test-bundle"}}`),
+			},
+			expected: []string{"bundles"},
+		},
+		{
+			name: "bundles",
+			conf: Config{
+				Bundles: []byte(`{"bundles": {"test-bundle": {}}`),
+			},
+			expected: []string{"bundles"},
+		},
+		{
+			name: "decision_logs",
+			conf: Config{
+				DecisionLogs: []byte(`{decision_logs: {}}`),
+			},
+			expected: []string{"decision_logs"},
+		},
+		{
+			name: "status",
+			conf: Config{
+				Status: []byte(`{status: {}}`),
+			},
+			expected: []string{"status"},
+		},
+		{
+			name: "plugins",
+			conf: Config{
+				Plugins: map[string]json.RawMessage{
+					"some-plugin": {},
+				},
+			},
+			expected: []string{"some-plugin"},
+		},
+		{
+			name: "sorted",
+			conf: Config{
+				DecisionLogs: []byte(`{decision_logs: {}}`),
+				Status:       []byte(`{status: {}}`),
+			},
+			expected: []string{"decision_logs", "status"},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			actual := test.conf.PluginNames()
+			if !reflect.DeepEqual(actual, test.expected) {
+				t.Errorf("Expected %v but got %v", test.expected, actual)
+			}
+		})
+	}
+}
+
 func TestConfigPluginsEnabled(t *testing.T) {
 	tests := []struct {
 		name     string

plugins/discovery/discovery.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"strings"
 	"sync"
 
 	"github.com/open-policy-agent/opa/ast"
@@ -84,8 +85,8 @@ func New(manager *plugins.Manager, opts ...func(*Discovery)) (*Discovery, error)
 		return result, nil
 	}
 
-	if manager.Config.PluginsEnabled() {
-		return nil, fmt.Errorf("plugins cannot be specified in the bootstrap configuration when discovery enabled")
+	if names := manager.Config.PluginNames(); len(names) > 0 {
+		return nil, fmt.Errorf("discovery prohibits manual configuration of %v", strings.Join(names, " and "))
 	}
 
 	result.config = config