fix(urunc config): Ensure default values for monitor configuration

pkg/unikontainers/urunc_config.go

@@ -78,13 +78,18 @@ func defaultTimestampsConfig() UruncTimestamps {
 	}
 }
 
+const (
+	defaultMonitorMemoryMB uint = 256
+	defaultMonitorVCPUs    uint = 1
+)
+
 func defaultMonitorsConfig() map[string]types.MonitorConfig {
 	return map[string]types.MonitorConfig{
-		"qemu":             {DefaultMemoryMB: 256, DefaultVCPUs: 1},
-		"hvt":              {DefaultMemoryMB: 256, DefaultVCPUs: 1},
-		"spt":              {DefaultMemoryMB: 256, DefaultVCPUs: 1},
-		"firecracker":      {DefaultMemoryMB: 256, DefaultVCPUs: 1},
-		"cloud-hypervisor": {DefaultMemoryMB: 256, DefaultVCPUs: 1},
+		"qemu":             {DefaultMemoryMB: defaultMonitorMemoryMB, DefaultVCPUs: defaultMonitorVCPUs},
+		"hvt":              {DefaultMemoryMB: defaultMonitorMemoryMB, DefaultVCPUs: defaultMonitorVCPUs},
+		"spt":              {DefaultMemoryMB: defaultMonitorMemoryMB, DefaultVCPUs: defaultMonitorVCPUs},
+		"firecracker":      {DefaultMemoryMB: defaultMonitorMemoryMB, DefaultVCPUs: defaultMonitorVCPUs},
+		"cloud-hypervisor": {DefaultMemoryMB: defaultMonitorMemoryMB, DefaultVCPUs: defaultMonitorVCPUs},
 	}
 }
 
@@ -108,11 +113,24 @@ func defaultUruncConfig() *UruncConfig {
 func LoadUruncConfig(path string) (*UruncConfig, error) {
 	cfg := defaultUruncConfig()
 	_, err := toml.DecodeFile(path, cfg)
-	if err == nil {
-		return cfg, nil
+	if err != nil {
+		uniklog.Warnf("Failed to load urunc config from %s: %v. Using default configuration.", path, err)
+		return defaultUruncConfig(), err
 	}
-	uniklog.Warnf("Failed to load urunc config from %s: %v. Using default configuration.", path, err)
-	return defaultUruncConfig(), err
+	// Decoding a partially-specified [monitors.<name>] section (e.g. one that
+	// only sets binary_path) zeroes the fields absent from the file, dropping
+	// the seeded defaults. Re-apply them for any zero value.
+	for name, mon := range cfg.Monitors {
+		if mon.DefaultMemoryMB == 0 {
+			mon.DefaultMemoryMB = defaultMonitorMemoryMB
+		}
+		if mon.DefaultVCPUs == 0 {
+			mon.DefaultVCPUs = defaultMonitorVCPUs
+		}
+		cfg.Monitors[name] = mon
+	}
+
+	return cfg, nil
 }
 
 func (p *UruncConfig) Map() map[string]string {

pkg/unikontainers/urunc_config_test.go

@@ -15,6 +15,8 @@
 package unikontainers
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -537,3 +539,82 @@ func TestDefaultConfigs(t *testing.T) {
 		assert.Equal(t, testTimestampsPath, config.Timestamps.Destination)
 	})
 }
+
+func writeTestConfig(t *testing.T, content string) string {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "config.toml")
+	if err := os.WriteFile(path, []byte(content), 0o600); err != nil {
+		t.Fatalf("failed to write test config: %v", err)
+	}
+
+	return path
+}
+
+func TestLoadUruncConfig(t *testing.T) {
+	t.Run("partial monitor section keeps default memory and vcpus", func(t *testing.T) {
+		t.Parallel()
+		path := writeTestConfig(t, `
+[monitors.qemu]
+path = "/opt/urunc/bin/qemu-system-x86_64"
+data_path = "/opt/urunc"
+`)
+		config, err := LoadUruncConfig(path)
+		assert.NoError(t, err)
+
+		qemu := config.Monitors["qemu"]
+		assert.Equal(t, defaultMonitorMemoryMB, qemu.DefaultMemoryMB)
+		assert.Equal(t, defaultMonitorVCPUs, qemu.DefaultVCPUs)
+		assert.Equal(t, "/opt/urunc/bin/qemu-system-x86_64", qemu.BinaryPath)
+		assert.Equal(t, "/opt/urunc", qemu.DataPath)
+	})
+
+	t.Run("explicit values are honored", func(t *testing.T) {
+		t.Parallel()
+		path := writeTestConfig(t, `
+[monitors.qemu]
+default_memory_mb = 512
+default_vcpus = 4
+`)
+		config, err := LoadUruncConfig(path)
+		assert.NoError(t, err)
+
+		qemu := config.Monitors["qemu"]
+		assert.Equal(t, uint(512), qemu.DefaultMemoryMB)
+		assert.Equal(t, uint(4), qemu.DefaultVCPUs)
+	})
+
+	t.Run("monitors absent from the file keep their defaults", func(t *testing.T) {
+		t.Parallel()
+		path := writeTestConfig(t, `
+[monitors.qemu]
+default_memory_mb = 512
+`)
+		config, err := LoadUruncConfig(path)
+		assert.NoError(t, err)
+
+		assert.Equal(t, uint(512), config.Monitors["qemu"].DefaultMemoryMB)
+		assert.Equal(t, defaultMonitorMemoryMB, config.Monitors["hvt"].DefaultMemoryMB)
+		assert.Equal(t, defaultMonitorVCPUs, config.Monitors["hvt"].DefaultVCPUs)
+	})
+
+	t.Run("custom monitor gets defaults for omitted fields", func(t *testing.T) {
+		t.Parallel()
+		path := writeTestConfig(t, `
+[monitors.mon]
+path = "/usr/bin/mon"
+`)
+		config, err := LoadUruncConfig(path)
+		assert.NoError(t, err)
+
+		mon := config.Monitors["mon"]
+		assert.Equal(t, defaultMonitorMemoryMB, mon.DefaultMemoryMB)
+		assert.Equal(t, defaultMonitorVCPUs, mon.DefaultVCPUs)
+	})
+
+	t.Run("missing file returns defaults", func(t *testing.T) {
+		t.Parallel()
+		config, err := LoadUruncConfig(filepath.Join(t.TempDir(), "does-not-exist.toml"))
+		assert.Error(t, err)
+		assert.Equal(t, defaultMonitorsConfig(), config.Monitors)
+	})
+}