optimizer: workaround for data race in TPE optimization

pkg/optimizer/hpoptimizer.go

@@ -12,6 +12,7 @@ import (
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sync/errgroup"
 	"math"
+	"sync"
 )
 
 const (
@@ -76,6 +77,10 @@ func buildHyperparameterOptimizeTrialResults(study *goptuna.Study) []*Hyperparam
 type HyperparameterOptimizer struct {
 	StudyName string
 	Config    *Config
+
+	// Workaround for goptuna/tpe parameter suggestion. Remove this after fixed.
+	// ref: https://github.com/c-bata/goptuna/issues/236
+	paramSuggestionLock sync.Mutex
 }
 
 func (o *HyperparameterOptimizer) buildStudy(trialFinishChan chan goptuna.FrozenTrial) (*goptuna.Study, error) {
@@ -172,15 +177,23 @@ func (o *HyperparameterOptimizer) buildObjective(executor Executor, configJson [
 	}
 
 	return func(trial goptuna.Trial) (float64, error) {
-		var trialConfig = configJson
-		for _, domain := range paramDomains {
-			if patch, err := domain.buildPatch(&trial); err != nil {
-				return 0.0, err
-			} else if patchedConfig, err := patch.ApplyIndent(trialConfig, "  "); err != nil {
-				return 0.0, err
-			} else {
-				trialConfig = patchedConfig
+		trialConfig, err := func(trialConfig []byte) ([]byte, error) {
+			o.paramSuggestionLock.Lock()
+			defer o.paramSuggestionLock.Unlock()
+
+			for _, domain := range paramDomains {
+				if patch, err := domain.buildPatch(&trial); err != nil {
+					return nil, err
+				} else if patchedConfig, err := patch.ApplyIndent(trialConfig, "  "); err != nil {
+					return nil, err
+				} else {
+					trialConfig = patchedConfig
+				}
 			}
+			return trialConfig, nil
+		}(configJson)
+		if err != nil {
+			return 0.0, err
 		}
 
 		summary, err := executor.Execute(trialConfig)