fix: handle null Docker runtimes

pkg/config/engine/docker/docker.go

@@ -74,8 +74,8 @@ func (c *Config) AddRuntime(name string, path string, setAsDefault bool) error {
 
 	// Read the existing runtimes
 	runtimes := make(map[string]any)
-	if _, exists := config["runtimes"]; exists {
-		runtimes = config["runtimes"].(map[string]any)
+	if rt, ok := config["runtimes"].(map[string]any); ok {
+		runtimes = rt
 	}
 
 	// Add / update the runtime definitions
@@ -128,16 +128,13 @@ func (c *Config) RemoveRuntime(name string) error {
 	}
 	config := *c
 
-	if _, exists := config["default-runtime"]; exists {
-		defaultRuntime := config["default-runtime"].(string)
+	if defaultRuntime, ok := config["default-runtime"].(string); ok {
 		if defaultRuntime == name {
 			config["default-runtime"] = defaultDockerRuntime
 		}
 	}
 
-	if _, exists := config["runtimes"]; exists {
-		runtimes := config["runtimes"].(map[string]any)
-
+	if runtimes, ok := config["runtimes"].(map[string]any); ok {
 		delete(runtimes, name)
 
 		if len(runtimes) == 0 {
@@ -171,8 +168,7 @@ func (c *Config) UpdateDefaultRuntime(name string, action string) error {
 	if action == engine.UpdateActionSet {
 		config["default-runtime"] = name
 	} else {
-		if _, exists := config["default-runtime"]; exists {
-			defaultRuntime := config["default-runtime"].(string)
+		if defaultRuntime, ok := config["default-runtime"].(string); ok {
 			if defaultRuntime == name {
 				config["default-runtime"] = defaultDockerRuntime
 			}
@@ -202,11 +198,9 @@ func (c *Config) GetRuntimeConfig(name string) (engine.RuntimeConfig, error) {
 
 	cfg := *c
 
-	var runtimes map[string]any
-	if _, ok := cfg["runtimes"]; ok {
-		runtimes = cfg["runtimes"].(map[string]any)
-		if r, ok := runtimes[name]; ok {
-			dr := dockerRuntime(r.(map[string]any))
+	if runtimes, ok := cfg["runtimes"].(map[string]any); ok {
+		if r, ok := runtimes[name].(map[string]any); ok {
+			dr := dockerRuntime(r)
 			return &dr, nil
 		}
 	}

pkg/config/engine/docker/docker_test.go

@@ -24,6 +24,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/NVIDIA/nvidia-container-toolkit/pkg/config/engine"
 )
 
 func TestUpdateConfigDefaultRuntime(t *testing.T) {
@@ -250,6 +252,88 @@ func TestGetRuntimeConfig(t *testing.T) {
 	}
 }
 
+func TestNullRuntimesFromFile(t *testing.T) {
+	for _, action := range []string{"add", "remove", "get"} {
+		t.Run(action, func(t *testing.T) {
+			configPath := filepath.Join(t.TempDir(), "daemon.json")
+			require.NoError(t, os.WriteFile(configPath, []byte(`{"runtimes":null,"log-driver":"json-file"}`), 0600))
+			config, err := New(WithPath(configPath))
+			require.NoError(t, err)
+
+			switch action {
+			case "add":
+				require.NoError(t, config.AddRuntime("nvidia", "/usr/bin/nvidia-container-runtime", false))
+				runtime, err := config.GetRuntimeConfig("nvidia")
+				require.NoError(t, err)
+				require.Equal(t, "/usr/bin/nvidia-container-runtime", runtime.GetBinaryPath())
+			case "remove":
+				require.NoError(t, config.RemoveRuntime("nvidia"))
+			case "get":
+				runtime, err := config.GetRuntimeConfig("nvidia")
+				require.NoError(t, err)
+				require.Empty(t, runtime.GetBinaryPath())
+			}
+
+			_, err = config.Save(configPath)
+			require.NoError(t, err)
+			contents, err := os.ReadFile(configPath)
+			require.NoError(t, err)
+			var saved map[string]any
+			require.NoError(t, json.Unmarshal(contents, &saved))
+			require.Equal(t, "json-file", saved["log-driver"])
+		})
+	}
+}
+
+func TestNullRuntimeFieldsFromFile(t *testing.T) {
+	testCases := map[string]struct {
+		input    string
+		action   string
+		expected string
+	}{
+		"remove with null default runtime": {
+			input:    `{"default-runtime":null,"runtimes":{"nvidia":{"path":"nvidia-container-runtime"}},"log-driver":"json-file"}`,
+			action:   "remove",
+			expected: `{"default-runtime":null,"log-driver":"json-file"}`,
+		},
+		"unset null default runtime": {
+			input:    `{"default-runtime":null,"runtimes":{"nvidia":{"path":"nvidia-container-runtime"}},"log-driver":"json-file"}`,
+			action:   "unset",
+			expected: `{"default-runtime":null,"runtimes":{"nvidia":{"path":"nvidia-container-runtime"}},"log-driver":"json-file"}`,
+		},
+		"get null runtime definition": {
+			input:    `{"runtimes":{"nvidia":null},"log-driver":"json-file"}`,
+			action:   "get",
+			expected: `{"runtimes":{"nvidia":null},"log-driver":"json-file"}`,
+		},
+	}
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			configPath := filepath.Join(t.TempDir(), "daemon.json")
+			require.NoError(t, os.WriteFile(configPath, []byte(tc.input), 0o600))
+			config, err := New(WithPath(configPath))
+			require.NoError(t, err)
+
+			switch tc.action {
+			case "remove":
+				require.NoError(t, config.RemoveRuntime("nvidia"))
+			case "unset":
+				require.NoError(t, config.UpdateDefaultRuntime("nvidia", engine.UpdateActionUnset))
+			case "get":
+				runtime, err := config.GetRuntimeConfig("nvidia")
+				require.NoError(t, err)
+				require.Empty(t, runtime.GetBinaryPath())
+			}
+
+			_, err = config.Save(configPath)
+			require.NoError(t, err)
+			contents, err := os.ReadFile(configPath)
+			require.NoError(t, err)
+			require.JSONEq(t, tc.expected, string(contents))
+		})
+	}
+}
+
 func TestEnableCDIPreservesFeaturesFromFile(t *testing.T) {
 	tests := []struct {
 		name     string