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Filename : driver.go
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// 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 driver defines interfaces to be implemented by database
// drivers as used by package sql.
//
// Most code should use the [database/sql] package.
//
// The driver interface has evolved over time. Drivers should implement
// [Connector] and [DriverContext] interfaces.
// The Connector.Connect and Driver.Open methods should never return [ErrBadConn].
// [ErrBadConn] should only be returned from [Validator], [SessionResetter], or
// a query method if the connection is already in an invalid (e.g. closed) state.
//
// All [Conn] implementations should implement the following interfaces:
// [Pinger], [SessionResetter], and [Validator].
//
// If named parameters or context are supported, the driver's [Conn] should implement:
// [ExecerContext], [QueryerContext], [ConnPrepareContext], and [ConnBeginTx].
//
// To support custom data types, implement [NamedValueChecker]. [NamedValueChecker]
// also allows queries to accept per-query options as a parameter by returning
// [ErrRemoveArgument] from CheckNamedValue.
//
// If multiple result sets are supported, [Rows] should implement [RowsNextResultSet].
// If the driver knows how to describe the types present in the returned result
// it should implement the following interfaces: [RowsColumnTypeScanType],
// [RowsColumnTypeDatabaseTypeName], [RowsColumnTypeLength], [RowsColumnTypeNullable],
// and [RowsColumnTypePrecisionScale]. A given row value may also return a [Rows]
// type, which may represent a database cursor value.
//
// If a [Conn] implements [Validator], then the IsValid method is called
// before returning the connection to the connection pool. If an entry in the
// connection pool implements [SessionResetter], then ResetSession
// is called before reusing the connection for another query. If a connection is
// never returned to the connection pool but is immediately reused, then
// ResetSession is called prior to reuse but IsValid is not called.
package driver

import (
	"context"
	"errors"
	"reflect"
)

// Value is a value that drivers must be able to handle.
// It is either nil, a type handled by a database driver's [NamedValueChecker]
// interface, or an instance of one of these types:
//
//	int64
//	float64
//	bool
//	[]byte
//	string
//	time.Time
//
// If the driver supports cursors, a returned Value may also implement the [Rows] interface
// in this package. This is used, for example, when a user selects a cursor
// such as "select cursor(select * from my_table) from dual". If the [Rows]
// from the select is closed, the cursor [Rows] will also be closed.
type Value any

// NamedValue holds both the value name and value.
type NamedValue struct {
	// If the Name is not empty it should be used for the parameter identifier and
	// not the ordinal position.
	//
	// Name will not have a symbol prefix.
	Name string

	// Ordinal position of the parameter starting from one and is always set.
	Ordinal int

	// Value is the parameter value.
	Value Value
}

// Driver is the interface that must be implemented by a database
// driver.
//
// Database drivers may implement [DriverContext] for access
// to contexts and to parse the name only once for a pool of connections,
// instead of once per connection.
type Driver interface {
	// Open returns a new connection to the database.
	// The name is a string in a driver-specific format.
	//
	// Open may return a cached connection (one previously
	// closed), but doing so is unnecessary; the sql package
	// maintains a pool of idle connections for efficient re-use.
	//
	// The returned connection is only used by one goroutine at a
	// time.
	Open(name string) (Conn, error)
}

// If a [Driver] implements DriverContext, then [database/sql.DB] will call
// OpenConnector to obtain a [Connector] and then invoke
// that [Connector]'s Connect method to obtain each needed connection,
// instead of invoking the [Driver]'s Open method for each connection.
// The two-step sequence allows drivers to parse the name just once
// and also provides access to per-[Conn] contexts.
type DriverContext interface {
	// OpenConnector must parse the name in the same format that Driver.Open
	// parses the name parameter.
	OpenConnector(name string) (Connector, error)
}

// A Connector represents a driver in a fixed configuration
// and can create any number of equivalent Conns for use
// by multiple goroutines.
//
// A Connector can be passed to [database/sql.OpenDB], to allow drivers
// to implement their own [database/sql.DB] constructors, or returned by
// [DriverContext]'s OpenConnector method, to allow drivers
// access to context and to avoid repeated parsing of driver
// configuration.
//
// If a Connector implements [io.Closer], the [database/sql.DB.Close]
// method will call the Close method and return error (if any).
type Connector interface {
	// Connect returns a connection to the database.
	// Connect may return a cached connection (one previously
	// closed), but doing so is unnecessary; the sql package
	// maintains a pool of idle connections for efficient re-use.
	//
	// The provided context.Context is for dialing purposes only
	// (see net.DialContext) and should not be stored or used for
	// other purposes. A default timeout should still be used
	// when dialing as a connection pool may call Connect
	// asynchronously to any query.
	//
	// The returned connection is only used by one goroutine at a
	// time.
	Connect(context.Context) (Conn, error)

	// Driver returns the underlying Driver of the Connector,
	// mainly to maintain compatibility with the Driver method
	// on sql.DB.
	Driver() Driver
}

// ErrSkip may be returned by some optional interfaces' methods to
// indicate at runtime that the fast path is unavailable and the sql
// package should continue as if the optional interface was not
// implemented. ErrSkip is only supported where explicitly
// documented.
var ErrSkip = errors.New("driver: skip fast-path; continue as if unimplemented")

// ErrBadConn should be returned by a driver to signal to the [database/sql]
// package that a driver.[Conn] is in a bad state (such as the server
// having earlier closed the connection) and the [database/sql] package should
// retry on a new connection.
//
// To prevent duplicate operations, ErrBadConn should NOT be returned
// if there's a possibility that the database server might have
// performed the operation. Even if the server sends back an error,
// you shouldn't return ErrBadConn.
//
// Errors will be checked using [errors.Is]. An error may
// wrap ErrBadConn or implement the Is(error) bool method.
var ErrBadConn = errors.New("driver: bad connection")

// Pinger is an optional interface that may be implemented by a [Conn].
//
// If a [Conn] does not implement Pinger, the [database/sql.DB.Ping] and
// [database/sql.DB.PingContext] will check if there is at least one [Conn] available.
//
// If Conn.Ping returns [ErrBadConn], [database/sql.DB.Ping] and [database/sql.DB.PingContext] will remove
// the [Conn] from pool.
type Pinger interface {
	Ping(ctx context.Context) error
}

// Execer is an optional interface that may be implemented by a [Conn].
//
// If a [Conn] implements neither [ExecerContext] nor [Execer],
// the [database/sql.DB.Exec] will first prepare a query, execute the statement,
// and then close the statement.
//
// Exec may return [ErrSkip].
//
// Deprecated: Drivers should implement [ExecerContext] instead.
type Execer interface {
	Exec(query string, args []Value) (Result, error)
}

// ExecerContext is an optional interface that may be implemented by a [Conn].
//
// If a [Conn] does not implement [ExecerContext], the [database/sql.DB.Exec]
// will fall back to [Execer]; if the Conn does not implement Execer either,
// [database/sql.DB.Exec] will first prepare a query, execute the statement, and then
// close the statement.
//
// ExecContext may return [ErrSkip].
//
// ExecContext must honor the context timeout and return when the context is canceled.
type ExecerContext interface {
	ExecContext(ctx context.Context, query string, args []NamedValue) (Result, error)
}

// Queryer is an optional interface that may be implemented by a [Conn].
//
// If a [Conn] implements neither [QueryerContext] nor [Queryer],
// the [database/sql.DB.Query] will first prepare a query, execute the statement,
// and then close the statement.
//
// Query may return [ErrSkip].
//
// Deprecated: Drivers should implement [QueryerContext] instead.
type Queryer interface {
	Query(query string, args []Value) (Rows, error)
}

// QueryerContext is an optional interface that may be implemented by a [Conn].
//
// If a [Conn] does not implement QueryerContext, the [database/sql.DB.Query]
// will fall back to [Queryer]; if the [Conn] does not implement [Queryer] either,
// [database/sql.DB.Query] will first prepare a query, execute the statement, and then
// close the statement.
//
// QueryContext may return [ErrSkip].
//
// QueryContext must honor the context timeout and return when the context is canceled.
type QueryerContext interface {
	QueryContext(ctx context.Context, query string, args []NamedValue) (Rows, error)
}

// Conn is a connection to a database. It is not used concurrently
// by multiple goroutines.
//
// Conn is assumed to be stateful.
type Conn interface {
	// Prepare returns a prepared statement, bound to this connection.
	Prepare(query string) (Stmt, error)

	// Close invalidates and potentially stops any current
	// prepared statements and transactions, marking this
	// connection as no longer in use.
	//
	// Because the sql package maintains a free pool of
	// connections and only calls Close when there's a surplus of
	// idle connections, it shouldn't be necessary for drivers to
	// do their own connection caching.
	//
	// Drivers must ensure all network calls made by Close
	// do not block indefinitely (e.g. apply a timeout).
	Close() error

	// Begin starts and returns a new transaction.
	//
	// Deprecated: Drivers should implement ConnBeginTx instead (or additionally).
	Begin() (Tx, error)
}

// ConnPrepareContext enhances the [Conn] interface with context.
type ConnPrepareContext interface {
	// PrepareContext returns a prepared statement, bound to this connection.
	// context is for the preparation of the statement,
	// it must not store the context within the statement itself.
	PrepareContext(ctx context.Context, query string) (Stmt, error)
}

// IsolationLevel is the transaction isolation level stored in [TxOptions].
//
// This type should be considered identical to [database/sql.IsolationLevel] along
// with any values defined on it.
type IsolationLevel int

// TxOptions holds the transaction options.
//
// This type should be considered identical to [database/sql.TxOptions].
type TxOptions struct {
	Isolation IsolationLevel
	ReadOnly  bool
}

// ConnBeginTx enhances the [Conn] interface with context and [TxOptions].
type ConnBeginTx interface {
	// BeginTx starts and returns a new transaction.
	// If the context is canceled by the user the sql package will
	// call Tx.Rollback before discarding and closing the connection.
	//
	// This must check opts.Isolation to determine if there is a set
	// isolation level. If the driver does not support a non-default
	// level and one is set or if there is a non-default isolation level
	// that is not supported, an error must be returned.
	//
	// This must also check opts.ReadOnly to determine if the read-only
	// value is true to either set the read-only transaction property if supported
	// or return an error if it is not supported.
	BeginTx(ctx context.Context, opts TxOptions) (Tx, error)
}

// SessionResetter may be implemented by [Conn] to allow drivers to reset the
// session state associated with the connection and to signal a bad connection.
type SessionResetter interface {
	// ResetSession is called prior to executing a query on the connection
	// if the connection has been used before. If the driver returns ErrBadConn
	// the connection is discarded.
	ResetSession(ctx context.Context) error
}

// Validator may be implemented by [Conn] to allow drivers to
// signal if a connection is valid or if it should be discarded.
//
// If implemented, drivers may return the underlying error from queries,
// even if the connection should be discarded by the connection pool.
type Validator interface {
	// IsValid is called prior to placing the connection into the
	// connection pool. The connection will be discarded if false is returned.
	IsValid() bool
}

// Result is the result of a query execution.
type Result interface {
	// LastInsertId returns the database's auto-generated ID
	// after, for example, an INSERT into a table with primary
	// key.
	LastInsertId() (int64, error)

	// RowsAffected returns the number of rows affected by the
	// query.
	RowsAffected() (int64, error)
}

// Stmt is a prepared statement. It is bound to a [Conn] and not
// used by multiple goroutines concurrently.
type Stmt interface {
	// Close closes the statement.
	//
	// As of Go 1.1, a Stmt will not be closed if it's in use
	// by any queries.
	//
	// Drivers must ensure all network calls made by Close
	// do not block indefinitely (e.g. apply a timeout).
	Close() error

	// NumInput returns the number of placeholder parameters.
	//
	// If NumInput returns >= 0, the sql package will sanity check
	// argument counts from callers and return errors to the caller
	// before the statement's Exec or Query methods are called.
	//
	// NumInput may also return -1, if the driver doesn't know
	// its number of placeholders. In that case, the sql package
	// will not sanity check Exec or Query argument counts.
	NumInput() int

	// Exec executes a query that doesn't return rows, such
	// as an INSERT or UPDATE.
	//
	// Deprecated: Drivers should implement StmtExecContext instead (or additionally).
	Exec(args []Value) (Result, error)

	// Query executes a query that may return rows, such as a
	// SELECT.
	//
	// Deprecated: Drivers should implement StmtQueryContext instead (or additionally).
	Query(args []Value) (Rows, error)
}

// StmtExecContext enhances the [Stmt] interface by providing Exec with context.
type StmtExecContext interface {
	// ExecContext executes a query that doesn't return rows, such
	// as an INSERT or UPDATE.
	//
	// ExecContext must honor the context timeout and return when it is canceled.
	ExecContext(ctx context.Context, args []NamedValue) (Result, error)
}

// StmtQueryContext enhances the [Stmt] interface by providing Query with context.
type StmtQueryContext interface {
	// QueryContext executes a query that may return rows, such as a
	// SELECT.
	//
	// QueryContext must honor the context timeout and return when it is canceled.
	QueryContext(ctx context.Context, args []NamedValue) (Rows, error)
}

// ErrRemoveArgument may be returned from [NamedValueChecker] to instruct the
// [database/sql] package to not pass the argument to the driver query interface.
// Return when accepting query specific options or structures that aren't
// SQL query arguments.
var ErrRemoveArgument = errors.New("driver: remove argument from query")

// NamedValueChecker may be optionally implemented by [Conn] or [Stmt]. It provides
// the driver more control to handle Go and database types beyond the default
// [Value] types allowed.
//
// The [database/sql] package checks for value checkers in the following order,
// stopping at the first found match: Stmt.NamedValueChecker, Conn.NamedValueChecker,
// Stmt.ColumnConverter, [DefaultParameterConverter].
//
// If CheckNamedValue returns [ErrRemoveArgument], the [NamedValue] will not be included in
// the final query arguments. This may be used to pass special options to
// the query itself.
//
// If [ErrSkip] is returned the column converter error checking
// path is used for the argument. Drivers may wish to return [ErrSkip] after
// they have exhausted their own special cases.
type NamedValueChecker interface {
	// CheckNamedValue is called before passing arguments to the driver
	// and is called in place of any ColumnConverter. CheckNamedValue must do type
	// validation and conversion as appropriate for the driver.
	CheckNamedValue(*NamedValue) error
}

// ColumnConverter may be optionally implemented by [Stmt] if the
// statement is aware of its own columns' types and can convert from
// any type to a driver [Value].
//
// Deprecated: Drivers should implement [NamedValueChecker].
type ColumnConverter interface {
	// ColumnConverter returns a ValueConverter for the provided
	// column index. If the type of a specific column isn't known
	// or shouldn't be handled specially, DefaultValueConverter
	// can be returned.
	ColumnConverter(idx int) ValueConverter
}

// Rows is an iterator over an executed query's results.
type Rows interface {
	// Columns returns the names of the columns. The number of
	// columns of the result is inferred from the length of the
	// slice. If a particular column name isn't known, an empty
	// string should be returned for that entry.
	Columns() []string

	// Close closes the rows iterator.
	Close() error

	// Next is called to populate the next row of data into
	// the provided slice. The provided slice will be the same
	// size as the Columns() are wide.
	//
	// Next should return io.EOF when there are no more rows.
	//
	// The dest should not be written to outside of Next. Care
	// should be taken when closing Rows not to modify
	// a buffer held in dest.
	Next(dest []Value) error
}

// RowsNextResultSet extends the [Rows] interface by providing a way to signal
// the driver to advance to the next result set.
type RowsNextResultSet interface {
	Rows

	// HasNextResultSet is called at the end of the current result set and
	// reports whether there is another result set after the current one.
	HasNextResultSet() bool

	// NextResultSet advances the driver to the next result set even
	// if there are remaining rows in the current result set.
	//
	// NextResultSet should return io.EOF when there are no more result sets.
	NextResultSet() error
}

// RowsColumnTypeScanType may be implemented by [Rows]. It should return
// the value type that can be used to scan types into. For example, the database
// column type "bigint" this should return "[reflect.TypeOf](int64(0))".
type RowsColumnTypeScanType interface {
	Rows
	ColumnTypeScanType(index int) reflect.Type
}

// RowsColumnTypeDatabaseTypeName may be implemented by [Rows]. It should return the
// database system type name without the length. Type names should be uppercase.
// Examples of returned types: "VARCHAR", "NVARCHAR", "VARCHAR2", "CHAR", "TEXT",
// "DECIMAL", "SMALLINT", "INT", "BIGINT", "BOOL", "[]BIGINT", "JSONB", "XML",
// "TIMESTAMP".
type RowsColumnTypeDatabaseTypeName interface {
	Rows
	ColumnTypeDatabaseTypeName(index int) string
}

// RowsColumnTypeLength may be implemented by [Rows]. It should return the length
// of the column type if the column is a variable length type. If the column is
// not a variable length type ok should return false.
// If length is not limited other than system limits, it should return [math.MaxInt64].
// The following are examples of returned values for various types:
//
//	TEXT          (math.MaxInt64, true)
//	varchar(10)   (10, true)
//	nvarchar(10)  (10, true)
//	decimal       (0, false)
//	int           (0, false)
//	bytea(30)     (30, true)
type RowsColumnTypeLength interface {
	Rows
	ColumnTypeLength(index int) (length int64, ok bool)
}

// RowsColumnTypeNullable may be implemented by [Rows]. The nullable value should
// be true if it is known the column may be null, or false if the column is known
// to be not nullable.
// If the column nullability is unknown, ok should be false.
type RowsColumnTypeNullable interface {
	Rows
	ColumnTypeNullable(index int) (nullable, ok bool)
}

// RowsColumnTypePrecisionScale may be implemented by [Rows]. It should return
// the precision and scale for decimal types. If not applicable, ok should be false.
// The following are examples of returned values for various types:
//
//	decimal(38, 4)    (38, 4, true)
//	int               (0, 0, false)
//	decimal           (math.MaxInt64, math.MaxInt64, true)
type RowsColumnTypePrecisionScale interface {
	Rows
	ColumnTypePrecisionScale(index int) (precision, scale int64, ok bool)
}

// Tx is a transaction.
type Tx interface {
	Commit() error
	Rollback() error
}

// RowsAffected implements [Result] for an INSERT or UPDATE operation
// which mutates a number of rows.
type RowsAffected int64

var _ Result = RowsAffected(0)

func (RowsAffected) LastInsertId() (int64, error) {
	return 0, errors.New("LastInsertId is not supported by this driver")
}

func (v RowsAffected) RowsAffected() (int64, error) {
	return int64(v), nil
}

// ResultNoRows is a pre-defined [Result] for drivers to return when a DDL
// command (such as a CREATE TABLE) succeeds. It returns an error for both
// LastInsertId and [RowsAffected].
var ResultNoRows noRows

type noRows struct{}

var _ Result = noRows{}

func (noRows) LastInsertId() (int64, error) {
	return 0, errors.New("no LastInsertId available after DDL statement")
}

func (noRows) RowsAffected() (int64, error) {
	return 0, errors.New("no RowsAffected available after DDL statement")
}