深入了解Go语言中database/sql是如何设计的

type Driver interface {
	Open(name string) (Conn, error)
}

type Conn interface {
	Prepare(query string) (Stmt, error)
	Close() error
	Begin() (Tx, error)
}

type Stmt interface {
	Close() error
	NumInput() int
	Exec(args []Value) (Result, error)
	Query(args []Value) (Rows, error)
}

type Value any

type Result interface {
	LastInsertId() (int64, error)
	RowsAffected() (int64, error)
}

type Rows interface {
	Columns() []string
	Close() error
	Next(dest []Value) error
}

type Tx interface {
	Commit() error
	Rollback() error
}

type Connector interface {
	Connect(context.Context) (Conn, error)
	Driver() Driver
}
type Pinger interface {
	Ping(ctx context.Context) error
}
type Execer interface {
	Exec(query string, args []Value) (Result, error)
}
type ExecerContext interface {
	ExecContext(ctx context.Context, query string, args []NamedValue) (Result, error)
}
type Queryer interface {
	Query(query string, args []Value) (Rows, error)
}
type QueryerContext interface {
	QueryContext(ctx context.Context, query string, args []NamedValue) (Rows, error)
}

package main
import (
	"database/sql"
	"fmt"
	_ "github.com/go-sql-driver/mysql"
)
func main() {
	db, err := sql.Open("mysql", "user:password@tcp(127.0.0.1:3306)/demo?charset=utf8mb4&parseTime=true&loc=Local")
	if err != nil {
		panic(err.Error())
	}
	defer db.Close()
	rows, err := db.Query("SELECT id, name FROM user")
	if err != nil {
		panic(err.Error())
	}
	defer rows.Close()
	for rows.Next() {
		var (
			id   int
			name string
		)
		if err := rows.Scan(id, name); err != nil {
			panic(err.Error())
		}
		fmt.Printf("id: %d, name: %s\n", id, name)
	}
}

import _ "github.com/go-sql-driver/mysql"

import "database/sql"
func init() {
	sql.Register("mysql", &MySQLDriver{})
}

var (
	driversMu sync.RWMutex
	drivers   = make(map[string]driver.Driver)
)
func Register(name string, driver driver.Driver) {
	driversMu.Lock()
	defer driversMu.Unlock()
	if driver == nil {
		panic("sql: Register driver is nil")
	}
	if _, dup := drivers[name]; dup {
		panic("sql: Register called twice for driver " + name)
	}
	drivers[name] = driver
}

func Open(driverName, dataSourceName string) (*DB, error) {
	driversMu.RLock()
	driveri, ok := drivers[driverName]
	driversMu.RUnlock()
	if !ok {
		return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
	}
	if driverCtx, ok := driveri.(driver.DriverContext); ok {
		connector, err := driverCtx.OpenConnector(dataSourceName)
		if err != nil {
			return nil, err
		}
		return OpenDB(connector), nil
	}
	return OpenDB(dsnConnector{dsn: dataSourceName, driver: driveri}), nil
}

type DriverContext interface {
	OpenConnector(name string) (Connector, error)
}

type Connector interface {
	Connect(context.Context) (Conn, error)
	Driver() Driver
}

type dsnConnector struct {
	dsn    string
	driver driver.Driver
}

func (t dsnConnector) Connect(_ context.Context) (driver.Conn, error) {
	return t.driver.Open(t.dsn)
}
func (t dsnConnector) Driver() driver.Driver {
	return t.driver
}

func OpenDB(c driver.Connector) *DB {
	ctx, cancel := context.WithCancel(context.Background())
	db := &DB{
		connector:    c,
		openerCh:     make(chan struct{}, connectionRequestQueueSize),
		lastPut:      make(map[*driverConn]string),
		connRequests: make(map[uint64]chan connRequest),
		stop:         cancel,
	}
	go db.connectionOpener(ctx)
	return db
}

func (db *DB) connectionOpener(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case <-db.openerCh:
			db.openNewConnection(ctx)
		}
	}
}

func (db *DB) openNewConnection(ctx context.Context) {
	ci, err := db.connector.Connect(ctx)
	...
	dc := &driverConn{
		db:         db,
		createdAt:  nowFunc(),
		returnedAt: nowFunc(),
		ci:         ci,
	}
	if db.putConnDBLocked(dc, err) {
		db.addDepLocked(dc, dc)
	}
	...
}

func (db *DB) Query(query string, args ...any) (*Rows, error) {
	return db.QueryContext(context.Background(), query, args...)
}

func (db *DB) QueryContext(ctx context.Context, query string, args ...any) (*Rows, error) {
	var rows *Rows
	var err error
	err = db.retry(func(strategy connReuseStrategy) error {
		rows, err = db.query(ctx, query, args, strategy)
		return err
	})
	return rows, err
}

func (db *DB) query(ctx context.Context, query string, args []any, strategy connReuseStrategy) (*Rows, error) {
	dc, err := db.conn(ctx, strategy)
	if err != nil {
		return nil, err
	}
	return db.queryDC(ctx, nil, dc, dc.releaseConn, query, args)
}

func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn, error) {
	last := len(db.freeConn) - 1
	if strategy == cachedOrNewConn && last >= 0 {
		conn := db.freeConn[last]
		...
		return conn, nil
	}
	...
	ci, err := db.connector.Connect(ctx)
	if err != nil {
		db.mu.Lock()
		db.numOpen-- // correct for earlier optimism
		db.maybeOpenNewConnections()
		db.mu.Unlock()
		return nil, err
	}
	db.mu.Lock()
	dc := &driverConn{
		db:         db,
		createdAt:  nowFunc(),
		returnedAt: nowFunc(),
		ci:         ci,
		inUse:      true,
	}
	db.addDepLocked(dc, dc)
	db.mu.Unlock()
	return dc, nil
}

func (db *DB) maybeOpenNewConnections() {
	numRequests := len(db.connRequests)
	if db.maxOpen > 0 {
		numCanOpen := db.maxOpen - db.numOpen
		if numRequests > numCanOpen {
			numRequests = numCanOpen
		}
	}
	for numRequests > 0 {
		db.numOpen++ // optimistically
		numRequests--
		if db.closed {
			return
		}
		db.openerCh <- struct{}{}
	}
}

func (db *DB) QueryContext(ctx context.Context, query string, args ...any) (*Rows, error) {
	var rows *Rows
	var err error
	err = db.retry(func(strategy connReuseStrategy) error {
		rows, err = db.query(ctx, query, args, strategy)
		return err
	})
	return rows, err
}

func (db *DB) queryDC(ctx, txctx context.Context, dc *driverConn, releaseConn func(error), query string, args []any) (*Rows, error) {
	queryerCtx, ok := dc.ci.(driver.QueryerContext)
	var queryer driver.Queryer
	if !ok {
		queryer, ok = dc.ci.(driver.Queryer)
	}
	if ok {
		var nvdargs []driver.NamedValue
		var rowsi driver.Rows
		var err error
		withLock(dc, func() {
			nvdargs, err = driverArgsConnLocked(dc.ci, nil, args)
			if err != nil {
				return
			}
			rowsi, err = ctxDriverQuery(ctx, queryerCtx, queryer, query, nvdargs)
		})
		...
	}
	...
}

func ctxDriverQuery(ctx context.Context, queryerCtx driver.QueryerContext, queryer driver.Queryer, query string, nvdargs []driver.NamedValue) (driver.Rows, error) {
	if queryerCtx != nil {
		return queryerCtx.QueryContext(ctx, query, nvdargs)
	}
	dargs, err := namedValueToValue(nvdargs)
	if err != nil {
		return nil, err
	}
	select {
	default:
	case <-ctx.Done():
		return nil, ctx.Err()
	}
	return queryer.Query(query, dargs)
}