Add maxAllowedPacket DSN Parameter

AUTHORS

@@ -43,6 +43,7 @@ Soroush Pour <me at soroushjp.com>
 Stan Putrya <root.vagner at gmail.com>
 Stanley Gunawan <gunawan.stanley at gmail.com>
 Xiaobing Jiang <s7v7nislands at gmail.com>
+Zhenye Xie <xiezhenye at gmail.com>
 Xiuming Chen <cc at cxm.cc>
 
 # Organizations

README.md

@@ -299,6 +299,15 @@ Default:        0
 I/O write timeout. The value must be a decimal number with an unit suffix ( *"ms"*, *"s"*, *"m"*, *"h"* ), such as *"30s"*, *"0.5m"* or *"1m30s"*.
 
 
+##### `maxPacketAllowed`
+```
+Type:          decimal number
+Default:       0
+```
+
+Max MySQL packet size allowed in bytes. Use `maxPacketAllowed` == 0 to fetch the `max_allowed_packet` variable from server.
+
+
 ##### System Variables
 
 All other parameters are interpreted as system variables:

driver.go

@@ -109,13 +109,17 @@ func (d MySQLDriver) Open(dsn string) (driver.Conn, error) {
 		return nil, err
 	}
 
-	// Get max allowed packet size
-	maxap, err := mc.getSystemVar("max_allowed_packet")
-	if err != nil {
-		mc.Close()
-		return nil, err
+	if mc.cfg.MaxPacketAllowed > 0 {
+		mc.maxPacketAllowed = mc.cfg.MaxPacketAllowed
+	} else {
+		// Get max allowed packet size
+		maxap, err := mc.getSystemVar("max_allowed_packet")
+		if err != nil {
+			mc.Close()
+			return nil, err
+		}
+		mc.maxPacketAllowed = stringToInt(maxap) - 1
 	}
-	mc.maxPacketAllowed = stringToInt(maxap) - 1
 	if mc.maxPacketAllowed < maxPacketSize {
 		mc.maxWriteSize = mc.maxPacketAllowed
 	}

dsn.go

@@ -17,6 +17,7 @@ import (
 	"net/url"
 	"strings"
 	"time"
+	"strconv"
 )
 
 var (
@@ -51,6 +52,7 @@ type Config struct {
 	MultiStatements         bool // Allow multiple statements in one query
 	ParseTime               bool // Parse time values to time.Time
 	Strict                  bool // Return warnings as errors
+	MaxPacketAllowed	int  // Max packet size allowed
 }
 
 // FormatDSN formats the given Config into a DSN string which can be passed to
@@ -222,6 +224,17 @@ func (cfg *Config) FormatDSN() string {
 		buf.WriteString(cfg.WriteTimeout.String())
 	}
 
+	if cfg.MaxPacketAllowed > 0 {
+		if hasParam {
+			buf.WriteString("&maxPacketAllowed=")
+		} else {
+			hasParam = true
+			buf.WriteString("?maxPacketAllowed=")
+		}
+		buf.WriteString(strconv.Itoa(cfg.MaxPacketAllowed))
+
+	}
+
 	// other params
 	if cfg.Params != nil {
 		for param, value := range cfg.Params {
@@ -496,7 +509,11 @@ func parseDSNParams(cfg *Config, params string) (err error) {
 			if err != nil {
 				return
 			}
-
+		case "maxPacketAllowed":
+			cfg.MaxPacketAllowed, err = strconv.Atoi(value)
+			if err != nil {
+				return
+			}
 		default:
 			// lazy init
 			if cfg.Params == nil {

dsn_test.go

@@ -39,8 +39,8 @@ var testDSNs = []struct {
 	"user:password@tcp(localhost:5555)/dbname?charset=utf8mb4,utf8&tls=skip-verify",
 	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8mb4,utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, TLSConfig: "skip-verify"},
 }, {
-	"user:password@/dbname?loc=UTC&timeout=30s&readTimeout=1s&writeTimeout=1s&allowAllFiles=1&clientFoundRows=true&allowOldPasswords=TRUE&collation=utf8mb4_unicode_ci",
-	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8mb4_unicode_ci", Loc: time.UTC, Timeout: 30 * time.Second, ReadTimeout: time.Second, WriteTimeout: time.Second, AllowAllFiles: true, AllowOldPasswords: true, ClientFoundRows: true},
+	"user:password@/dbname?loc=UTC&timeout=30s&readTimeout=1s&writeTimeout=1s&allowAllFiles=1&clientFoundRows=true&allowOldPasswords=TRUE&collation=utf8mb4_unicode_ci&maxPacketAllowed=16777216",
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8mb4_unicode_ci", Loc: time.UTC, Timeout: 30 * time.Second, ReadTimeout: time.Second, WriteTimeout: time.Second, AllowAllFiles: true, AllowOldPasswords: true, ClientFoundRows: true, MaxPacketAllowed: 16777216},
 }, {
 	"user:p@ss(word)@tcp([de:ad:be:ef::ca:fe]:80)/dbname?loc=Local",
 	&Config{User: "user", Passwd: "p@ss(word)", Net: "tcp", Addr: "[de:ad:be:ef::ca:fe]:80", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.Local},