feat(config): default configuration with merge support for downstream overrides (#396)

* feat(config): default configuration with merge support for downstream overrides

Creates the required infrastructure for downstream forks to be able to provide
default config overrides without modifying the original source code.

Downstream forks should be able to create merge/rebase scripts that
automatically accepted downstream merge conflicts in config_default_overrides.go
since this file will never change upstream.

Example usage for downstream forks:

To customize defaults, simply populate fields in the returned StaticConfig:

```go
func defaultOverrides() *StaticConfig {
  return &StaticConfig{
    ListOutput: "json",           // Override default list output format
    Toolsets:   []string{"core"}, // Override default enabled toolsets
    Port:       "9000",           // Override default port
  }
}
```

Any fields specified here will override the base defaults defined in config_default.go.
Fields not specified will preserve their base default values.

Signed-off-by: Marc Nuri <[email protected]>

* test: skip downstream toolset (full) tests

Skips toolset metadata tests in case there are config overrides.
This is useful in downstream forks where the default toolsets
might be different to those upstream.

Signed-off-by: Marc Nuri <[email protected]>

---------

Signed-off-by: Marc Nuri <[email protected]>

Author: manusa

pkg/config/config.go

@@ -19,7 +19,7 @@ const (
 type StaticConfig struct {
 	DeniedResources []GroupVersionKind `toml:"denied_resources"`
 
-	LogLevel   int    `toml:"log_level,omitempty"`
+	LogLevel   int    `toml:"log_level,omitzero"`
 	Port       string `toml:"port,omitempty"`
 	SSEBaseURL string `toml:"sse_base_url,omitempty"`
 	KubeConfig string `toml:"kubeconfig,omitempty"`
@@ -70,13 +70,6 @@ type StaticConfig struct {
 	parsedClusterProviderConfigs map[string]ProviderConfig
 }
 
-func Default() *StaticConfig {
-	return &StaticConfig{
-		ListOutput: "table",
-		Toolsets:   []string{"core", "config", "helm"},
-	}
-}
-
 type GroupVersionKind struct {
 	Group   string `toml:"group"`
 	Version string `toml:"version"`

pkg/config/config_default.go

@@ -0,0 +1,43 @@
+package config
+
+import (
+	"bytes"
+
+	"github.com/BurntSushi/toml"
+)
+
+func Default() *StaticConfig {
+	defaultConfig := StaticConfig{
+		ListOutput: "table",
+		Toolsets:   []string{"core", "config", "helm"},
+	}
+	overrides := defaultOverrides()
+	mergedConfig := mergeConfig(defaultConfig, overrides)
+	return &mergedConfig
+}
+
+// HasDefaultOverrides indicates whether the internal defaultOverrides function
+// provides any overrides or an empty StaticConfig.
+func HasDefaultOverrides() bool {
+	overrides := defaultOverrides()
+	var buf bytes.Buffer
+	if err := toml.NewEncoder(&buf).Encode(overrides); err != nil {
+		// If marshaling fails, assume no overrides
+		return false
+	}
+	return len(bytes.TrimSpace(buf.Bytes())) > 0
+}
+
+// mergeConfig applies non-zero values from override to base using TOML serialization
+// and returns the merged StaticConfig.
+// In case of any error during marshalling or unmarshalling, it returns the base config unchanged.
+func mergeConfig(base, override StaticConfig) StaticConfig {
+	var overrideBuffer bytes.Buffer
+	if err := toml.NewEncoder(&overrideBuffer).Encode(override); err != nil {
+		// If marshaling fails, return base unchanged
+		return base
+	}
+
+	_, _ = toml.NewDecoder(&overrideBuffer).Decode(&base)
+	return base
+}

pkg/config/config_default_overrides.go

@@ -0,0 +1,8 @@
+package config
+
+func defaultOverrides() StaticConfig {
+	return StaticConfig{
+		// IMPORTANT: this file is used to override default config values in downstream builds.
+		// This is intentionally left blank.
+	}
+}

pkg/config/config_test.go

@@ -174,6 +174,49 @@ func (s *ConfigSuite) TestReadConfigValidPreservesDefaultsForMissingFields() {
 	})
 }
 
+func (s *ConfigSuite) TestMergeConfig() {
+	base := StaticConfig{
+		ListOutput: "table",
+		Toolsets:   []string{"core", "config", "helm"},
+		Port:       "8080",
+	}
+	s.Run("merges override values on top of base", func() {
+		override := StaticConfig{
+			ListOutput: "json",
+			Port:       "9090",
+		}
+
+		result := mergeConfig(base, override)
+
+		s.Equal("json", result.ListOutput, "ListOutput should be overridden")
+		s.Equal("9090", result.Port, "Port should be overridden")
+	})
+
+	s.Run("preserves base values when override is empty", func() {
+		override := StaticConfig{}
+
+		result := mergeConfig(base, override)
+
+		s.Equal("table", result.ListOutput, "ListOutput should be preserved from base")
+		s.Equal([]string{"core", "config", "helm"}, result.Toolsets, "Toolsets should be preserved from base")
+		s.Equal("8080", result.Port, "Port should be preserved from base")
+	})
+
+	s.Run("handles partial overrides", func() {
+		override := StaticConfig{
+			Toolsets: []string{"custom"},
+			ReadOnly: true,
+		}
+
+		result := mergeConfig(base, override)
+
+		s.Equal("table", result.ListOutput, "ListOutput should be preserved from base")
+		s.Equal([]string{"custom"}, result.Toolsets, "Toolsets should be overridden")
+		s.Equal("8080", result.Port, "Port should be preserved from base since override doesn't specify it")
+		s.True(result.ReadOnly, "ReadOnly should be overridden to true")
+	})
+}
+
 func TestConfig(t *testing.T) {
 	suite.Run(t, new(ConfigSuite))
 }

pkg/mcp/toolsets_test.go

@@ -65,6 +65,9 @@ func (s *ToolsetsSuite) TestNoToolsets() {
 }
 
 func (s *ToolsetsSuite) TestDefaultToolsetsTools() {
+	if configuration.HasDefaultOverrides() {
+		s.T().Skip("Skipping test because default configuration overrides are present (this is a downstream fork)")
+	}
 	s.Run("Default configuration toolsets", func() {
 		s.InitMcpClient()
 		tools, err := s.ListTools(s.T().Context(), mcp.ListToolsRequest{})
@@ -82,6 +85,9 @@ func (s *ToolsetsSuite) TestDefaultToolsetsTools() {
 }
 
 func (s *ToolsetsSuite) TestDefaultToolsetsToolsInOpenShift() {
+	if configuration.HasDefaultOverrides() {
+		s.T().Skip("Skipping test because default configuration overrides are present (this is a downstream fork)")
+	}
 	s.Run("Default configuration toolsets in OpenShift", func() {
 		s.Handle(&test.InOpenShiftHandler{})
 		s.InitMcpClient()
@@ -100,6 +106,9 @@ func (s *ToolsetsSuite) TestDefaultToolsetsToolsInOpenShift() {
 }
 
 func (s *ToolsetsSuite) TestDefaultToolsetsToolsInMultiCluster() {
+	if configuration.HasDefaultOverrides() {
+		s.T().Skip("Skipping test because default configuration overrides are present (this is a downstream fork)")
+	}
 	s.Run("Default configuration toolsets in multi-cluster (with 11 clusters)", func() {
 		kubeconfig := s.Kubeconfig()
 		for i := 0; i < 10; i++ {
@@ -123,6 +132,9 @@ func (s *ToolsetsSuite) TestDefaultToolsetsToolsInMultiCluster() {
 }
 
 func (s *ToolsetsSuite) TestDefaultToolsetsToolsInMultiClusterEnum() {
+	if configuration.HasDefaultOverrides() {
+		s.T().Skip("Skipping test because default configuration overrides are present (this is a downstream fork)")
+	}
 	s.Run("Default configuration toolsets in multi-cluster (with 2 clusters)", func() {
 		kubeconfig := s.Kubeconfig()
 		// Add additional cluster to force multi-cluster behavior with enum parameter