Defer mutex unlocks to prevent accidental deadlocks in connection pool

One such case happened when panicing because of nil HostInfo
(which in turn was caused by bad discovery mechanism).
Without the defer, it would stay silent and get deadlocked.

connectionpool.go

@@ -121,6 +121,7 @@ func newPolicyConnPool(session *Session, hostPolicy HostSelectionPolicy,
 
 func (p *policyConnPool) SetHosts(hosts []HostInfo) {
 	p.mu.Lock()
+	defer p.mu.Unlock()
 
 	toRemove := make(map[string]struct{})
 	for addr := range p.hostConnPools {
@@ -159,7 +160,6 @@ func (p *policyConnPool) SetHosts(hosts []HostInfo) {
 	// update the policy
 	p.hostPolicy.SetHosts(hosts)
 
-	p.mu.Unlock()
 }
 
 func (p *policyConnPool) SetPartitioner(partitioner string) {
@@ -186,6 +186,7 @@ func (p *policyConnPool) Pick(qry *Query) (SelectedHost, *Conn) {
 	)
 
 	p.mu.RLock()
+	defer p.mu.RUnlock()
 	for conn == nil {
 		host = nextHost()
 		if host == nil {
@@ -201,13 +202,12 @@ func (p *policyConnPool) Pick(qry *Query) (SelectedHost, *Conn) {
 
 		conn = pool.Pick(qry)
 	}
-
-	p.mu.RUnlock()
 	return host, conn
 }
 
 func (p *policyConnPool) Close() {
 	p.mu.Lock()
+	defer p.mu.Unlock()
 
 	// remove the hosts from the policy
 	p.hostPolicy.SetHosts([]HostInfo{})
@@ -217,7 +217,6 @@ func (p *policyConnPool) Close() {
 		delete(p.hostConnPools, addr)
 		pool.Close()
 	}
-	p.mu.Unlock()
 }
 
 // hostConnPool is a connection pool for a single host.