runtime: Add framework to manipulate config structs via reflection

For 'tomlConfig' substructures stored in Golang maps - 'hypervisor' and
'agent' - BurntSushi doesn't preserve their previous contents as it does
for substructures stored directly (e.g. 'runtime').  We use reflection
to work around this.

This commit adds three primitive operations to work with struct fields
identified by their `toml:"..."` tags - one to get a field value, one to
set a field value and one to assign a source struct field value to the
corresponding field of a target.

Signed-off-by: Pavel Mores <pmores@redhat.com>

src/runtime/pkg/katautils/config.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"reflect"
 	goruntime "runtime"
 	"strings"
 
@@ -1308,6 +1309,54 @@ func decodeConfig(configPath string) (tomlConfig, string, error) {
 	return tomlConf, resolved, nil
 }
 
+
+// Copies a TOML value of the source field identified by its TOML key to the
+// corresponding field of the target.  Basically
+// 'target[tomlKeyName] = source[tomlKeyNmae]'.
+func copyFieldValue(source reflect.Value, tomlKeyName string, target reflect.Value) error {
+	val, err := getValue(source, tomlKeyName)
+	if err != nil {
+		return fmt.Errorf("error getting key %q from a decoded drop-in conf file: %s", tomlKeyName, err)
+	}
+	err = setValue(target, tomlKeyName, val)
+	if err != nil {
+		return fmt.Errorf("error setting key %q to a new value '%v': %s", tomlKeyName, val.Interface(), err)
+	}
+	return nil
+}
+
+// The first argument is expected to be a reflect.Value of a tomlConfig
+// substructure (hypervisor, agent), the second argument is a TOML key
+// corresponding to the substructure field whose TOML value is queried.
+// Return value corresponds to 'tomlConfStruct[tomlKey]'.
+func getValue(tomlConfStruct reflect.Value, tomlKey string) (reflect.Value, error) {
+	tomlConfStructType := tomlConfStruct.Type()
+	for j := 0; j < tomlConfStruct.NumField(); j++ {
+		fieldTomlTag := tomlConfStructType.Field(j).Tag.Get("toml")
+		if fieldTomlTag == tomlKey {
+			return tomlConfStruct.Field(j), nil
+		}
+	}
+	return reflect.Value{}, fmt.Errorf("key %q not found", tomlKey)
+}
+
+// The first argument is expected to be a reflect.Value of a tomlConfig
+// substructure (hypervisor, agent), the second argument is a TOML key
+// corresponding to the substructure field whose TOML value is to be changed,
+// the third argument is a reflect.Value representing the new TOML value.
+// An equivalent of 'tomlConfStruct[tomlKey] = newVal'.
+func setValue(tomlConfStruct reflect.Value, tomlKey string, newVal reflect.Value) error {
+	tomlConfStructType := tomlConfStruct.Type()
+	for j := 0; j < tomlConfStruct.NumField(); j++ {
+		fieldTomlTag := tomlConfStructType.Field(j).Tag.Get("toml")
+		if fieldTomlTag == tomlKey {
+			tomlConfStruct.Field(j).Set(newVal)
+			return nil
+		}
+	}
+	return fmt.Errorf("key %q not found", tomlKey)
+}
+
 // checkConfig checks the validity of the specified config.
 func checkConfig(config oci.RuntimeConfig) error {
 	if err := checkNetNsConfig(config); err != nil {