*: Refactor session#domainMap use RWMutex to replace Mutex (#33739)

close #33737

session/tidb.go

@@ -44,12 +44,11 @@ import (
 
 type domainMap struct {
 	domains map[string]*domain.Domain
-	mu      sync.Mutex
+	mu      sync.RWMutex
 }
 
 func (dm *domainMap) Get(store kv.Storage) (d *domain.Domain, err error) {
-	dm.mu.Lock()
-	defer dm.mu.Unlock()
+	dm.mu.RLock()
 
 	// If this is the only domain instance, and the caller doesn't provide store.
 	if len(dm.domains) == 1 && store == nil {
@@ -60,6 +59,7 @@ func (dm *domainMap) Get(store kv.Storage) (d *domain.Domain, err error) {
 
 	key := store.UUID()
 	d = dm.domains[key]
+	dm.mu.RUnlock()
 	if d != nil {
 		return
 	}
@@ -92,7 +92,7 @@ func (dm *domainMap) Get(store kv.Storage) (d *domain.Domain, err error) {
 	if err != nil {
 		return nil, err
 	}
-	dm.domains[key] = d
+	dm.Set(store, d)
 
 	return
 }
@@ -103,6 +103,12 @@ func (dm *domainMap) Delete(store kv.Storage) {
 	dm.mu.Unlock()
 }
 
+func (dm *domainMap) Set(store kv.Storage, domain *domain.Domain) {
+	dm.mu.Lock()
+	dm.domains[store.UUID()] = domain
+	dm.mu.Unlock()
+}
+
 var (
 	domap = &domainMap{
 		domains: map[string]*domain.Domain{},