sql.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package sql provides a generic interface around SQL (or SQL-like)
// databases.
//
// The sql package must be used in conjunction with a database driver.
// See https://golang.org/s/sqldrivers for a list of drivers.
//
// For more usage examples, see the wiki page at
// https://golang.org/s/sqlwiki.
package sql

import (
	"context"
	"database/sql/driver"
	"database/sql/internal"
	"errors"
	"fmt"
	"io"
	"reflect"
	"runtime"
	"sort"
	"sync"
	"sync/atomic"
	"time"
)

var (
	driversMu sync.RWMutex
	drivers   = make(map[string]driver.Driver)
)

// nowFunc returns the current time; it's overridden in tests.
var nowFunc = time.Now

// Register makes a database driver available by the provided name.
// If Register is called twice with the same name or if driver is nil,
// it panics.
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 unregisterAllDrivers() {
	driversMu.Lock()
	defer driversMu.Unlock()
	// For tests.
	drivers = make(map[string]driver.Driver)
}

// Drivers returns a sorted list of the names of the registered drivers.
func Drivers() []string {
	driversMu.RLock()
	defer driversMu.RUnlock()
	var list []string
	for name := range drivers {
		list = append(list, name)
	}
	sort.Strings(list)
	return list
}

// NamedParam may be passed into query parameter arguments to associate
// a named placeholder with a value.
type NamedParam struct {
	// Name of the parameter placeholder. If empty the ordinal position in the
	// argument list will be used.
	Name string

	// Value of the parameter. It may be assigned the same value types as
	// the query arguments.
	Value interface{}
}

// Param provides a more concise way to create NamedParam values.
func Param(name string, value interface{}) NamedParam {
	// This method exists because the go1compat promise
	// doesn't guarantee that structs don't grow more fields,
	// so unkeyed struct literals are a vet error. Thus, we don't
	// want to encourage sql.NamedParam{name, value}.
	return NamedParam{Name: name, Value: value}
}

// IsolationLevel is the transaction isolation level stored in Context.
// The IsolationLevel is set with IsolationContext and the context
// should be passed to BeginContext.
type IsolationLevel int

// Various isolation levels that drivers may support in BeginContext.
// If a driver does not support a given isolation level an error may be returned.
//
// See https://en.wikipedia.org/wiki/Isolation_(database_systems)#Isolation_levels.
const (
	LevelDefault IsolationLevel = iota
	LevelReadUncommitted
	LevelReadCommitted
	LevelWriteCommitted
	LevelRepeatableRead
	LevelSnapshot
	LevelSerializable
	LevelLinearizable
)

// IsolationContext returns a new Context that carries the provided isolation level.
// The context must contain the isolation level before beginning the transaction
// with BeginContext.
func IsolationContext(ctx context.Context, level IsolationLevel) context.Context {
	return context.WithValue(ctx, internal.IsolationLevelKey{}, driver.IsolationLevel(level))
}

// ReadOnlyWithContext returns a new Context that carries the provided
// read-only transaction property. The context must contain the read-only property
// before beginning the transaction with BeginContext.
func ReadOnlyContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, internal.ReadOnlyKey{}, true)
}

// RawBytes is a byte slice that holds a reference to memory owned by
// the database itself. After a Scan into a RawBytes, the slice is only
// valid until the next call to Next, Scan, or Close.
type RawBytes []byte

// NullString represents a string that may be null.
// NullString implements the Scanner interface so
// it can be used as a scan destination:
//
//  var s NullString
//  err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
//  ...
//  if s.Valid {
//     // use s.String
//  } else {
//     // NULL value
//  }
//
type NullString struct {
	String string
	Valid  bool // Valid is true if String is not NULL
}

// Scan implements the Scanner interface.
func (ns *NullString) Scan(value interface{}) error {
	if value == nil {
		ns.String, ns.Valid = "", false
		return nil
	}
	ns.Valid = true
	return convertAssign(&ns.String, value)
}

// Value implements the driver Valuer interface.
func (ns NullString) Value() (driver.Value, error) {
	if !ns.Valid {
		return nil, nil
	}
	return ns.String, nil
}

// NullInt64 represents an int64 that may be null.
// NullInt64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullInt64 struct {
	Int64 int64
	Valid bool // Valid is true if Int64 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullInt64) Scan(value interface{}) error {
	if value == nil {
		n.Int64, n.Valid = 0, false
		return nil
	}
	n.Valid = true
	return convertAssign(&n.Int64, value)
}

// Value implements the driver Valuer interface.
func (n NullInt64) Value() (driver.Value, error) {
	if !n.Valid {
		return nil, nil
	}
	return n.Int64, nil
}

// NullFloat64 represents a float64 that may be null.
// NullFloat64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullFloat64 struct {
	Float64 float64
	Valid   bool // Valid is true if Float64 is not NULL
}