resolve various race condition

cpool.go

@@ -15,10 +15,7 @@ type CPool struct {
 }
 
 func (c *CPool) nextInPool() int {
-	c.lock.RLock()
-	defer c.lock.RUnlock()
-
-	if len(c.pool) == 0 {
+	if c.poolSize() == 0 {
 		return -1
 	}
 

database.go

@@ -184,21 +184,23 @@ func (db *DB) PrepareContext(ctx context.Context, query string) (Stmt, error) {
 		return nil, err
 	}
 
-	stmt.stmts = append([]*sql.Stmt{write}, stmt.stmts...)
+	stmt.stmts = []*sql.Stmt{write}
 
 	if db.cpool.poolSize() > 1 {
-		go func() {
-			reader, err := db.cpool.Reader()
+		reader, err := db.cpool.Reader()
+		if err == nil {
+			// we have writer statement prepared
+			// this error can be ignored
+			go func(reader *sql.DB) {
 
-			if err != nil {
-				// we have writer statement prepared
-				// this error can be ignored
-				return
-			}
+				read, _ := reader.PrepareContext(ctx, query)
+
+				stmt.lock.Lock()
+				defer stmt.lock.Unlock()
 
-			read, _ := reader.PrepareContext(ctx, query)
-			stmt.stmts = append(stmt.stmts, read)
-		}()
+				stmt.stmts = append(stmt.stmts, read)
+			}(reader)
+		}
 	}
 
 	return stmt, nil

stmt.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"sync"
 )
 
 // Stmt allows DB
@@ -20,6 +21,7 @@ type Stmt interface {
 // stmt holds at most 2 sql.Stmt
 type stmt struct {
 	stmts []*sql.Stmt
+	lock  sync.RWMutex
 }
 
 // Close will close the statments connections
@@ -39,6 +41,9 @@ func (s *stmt) Exec(args ...interface{}) (sql.Result, error) {
 // Exec execute statement with context
 // The statement is executed on the writer database
 func (s *stmt) ExecContext(ctx context.Context, args ...interface{}) (sql.Result, error) {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
 	if len(s.stmts) == 0 {
 		return nil, errors.New("zero statement executable")
 	}
@@ -54,6 +59,9 @@ func (s *stmt) Query(args ...interface{}) (*sql.Rows, error) {
 // Query execute statement with context
 // The statement is executed on reader database
 func (s *stmt) QueryContext(ctx context.Context, args ...interface{}) (*sql.Rows, error) {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
 	if len(s.stmts) == 0 {
 		return nil, errors.New("zero statement executable")
 	}
@@ -74,6 +82,9 @@ func (s *stmt) QueryRow(args ...interface{}) Row {
 
 // QueryRowContext is executed on reader database
 func (s *stmt) QueryRowContext(ctx context.Context, args ...interface{}) Row {
+	s.lock.RLock()
+	defer s.lock.RUnlock()
+
 	if len(s.stmts) == 0 {
 		return &row{err: errors.New("zero statement executable")}
 	}