From f472becd892d3cfca593ecca62aa92389e6f3ebb Mon Sep 17 00:00:00 2001
From: Yosyp Buchma <josephbuchma@gmail.com>
Date: Tue, 19 Sep 2023 20:56:54 +0300
Subject: [PATCH] simplified concurrent querying

---
 generator/mysql/query_set.go | 49 ++++++++++--------------------------
 go.mod                       |  1 +
 go.sum                       |  2 ++
 3 files changed, 16 insertions(+), 36 deletions(-)

diff --git a/generator/mysql/query_set.go b/generator/mysql/query_set.go
index f886357..b58ff82 100644
--- a/generator/mysql/query_set.go
+++ b/generator/mysql/query_set.go
@@ -4,12 +4,11 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
-	"runtime"
 	"strings"
-	"sync"
 
 	"github.com/go-jet/jet/v2/generator/metadata"
 	"github.com/go-jet/jet/v2/qrm"
+	"golang.org/x/sync/errgroup"
 )
 
 // mySqlQuerySet is dialect query set for MySQL
@@ -29,43 +28,21 @@ ORDER BY table_name;
 		return nil, fmt.Errorf("failed to query %s metadata result: %w", tableType, err)
 	}
 
-	tblChan := make(chan int, len(tables))
-	errChan := make(chan error, 1)
+	const maxConns = 32
+	db.SetMaxOpenConns(maxConns)
+	db.SetMaxIdleConns(maxConns)
 
-	wg := sync.WaitGroup{}
-	for i := 0; i < runtime.NumCPU(); i++ {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-			var err1 error
-			for tblIdx := range tblChan {
-				tables[tblIdx].Columns, err1 = m.GetTableColumnsMetaData(db, schemaName, tables[tblIdx].Name)
-				if err1 != nil {
-					select {
-					case errChan <- fmt.Errorf("failed to get '%s' table columns metadata: %w", tables[tblIdx].Name, err1):
-						return
-					default:
-					}
-					return
-				}
-			}
-		}()
+	wg := errgroup.Group{}
+	for i := 0; i < len(tables); i++ {
+		i := i
+		wg.Go(func() (err1 error) {
+			tables[i].Columns, err1 = m.GetTableColumnsMetaData(db, schemaName, tables[i].Name)
+			return err1
+		})
 	}
 
-	for i := range tables {
-		tblChan <- i
-	}
-
-	close(tblChan)
-	wg.Wait()
-
-	select {
-	case err = <-errChan:
-		return nil, err
-	default:
-	}
-
-	return tables, nil
+	err = wg.Wait()
+	return tables, err
 }
 
 func (m mySqlQuerySet) GetTableColumnsMetaData(db *sql.DB, schemaName string, tableName string) ([]metadata.Column, error) {
diff --git a/go.mod b/go.mod
index 685bebb..d0a810a 100644
--- a/go.mod
+++ b/go.mod
@@ -18,5 +18,6 @@ require (
 	github.com/shopspring/decimal v1.3.1
 	github.com/stretchr/testify v1.8.2
 	github.com/volatiletech/null/v8 v8.1.2
+	golang.org/x/sync v0.3.0
 	gopkg.in/guregu/null.v4 v4.0.0
 )
diff --git a/go.sum b/go.sum
index 2c11a02..4b06a0a 100644
--- a/go.sum
+++ b/go.sum
@@ -182,6 +182,8 @@ golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug
 golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.3.0 h1:ftCYgMx6zT/asHUrPw8BLLscYtGznsLAnjq5RH9P66E=
+golang.org/x/sync v0.3.0/go.mod h1:FU7BRWz2tNW+3quACPkgCx/L+uEAv1htQ0V83Z9Rj+Y=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190222072716-a9d3bda3a223/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=