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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 package sql.
//
// Drivers only need to be aware of a subset of Go's types.  The sql package
// will convert all types into one of the following:
//
//   int64
//   float64
//   bool
//   nil
//   []byte
//   string   [*] everywhere except from Rows.Next.
//   time.Time
//
package driver

import "errors"

// Driver is the interface that must be implemented by a database
// driver.
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)
}

// 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")

// Execer is an optional interface that may be implemented by a Conn.
//
// If a Conn does not implement Execer, the db package's DB.Exec will
// first prepare a query, execute the statement, and then close the
// statement.
//
// All arguments are of a subset type as defined in the package docs.
//
// Exec may return ErrSkip.
type Execer interface {
        Exec(query string, args []interface{}) (Result, 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.
        Close() error

        // Begin starts and returns a new transaction.
        Begin() (Tx, error)
}

// 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.
        //
        // Closing a statement should not interrupt any outstanding
        // query created from that statement. That is, the following
        // order of operations is valid:
        //
        //  * create a driver statement
        //  * call Query on statement, returning Rows
        //  * close the statement
        //  * read from Rows
        //
        // If closing a statement invalidates currently-running
        // queries, the final step above will incorrectly fail.
        //
        // TODO(bradfitz): possibly remove the restriction above, if
        // enough driver authors object and find it complicates their
        // code too much. The sql package could be smarter about
        // refcounting the statement and closing it at the appropriate
        // time.
        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.  The args are all of a subset
        // type as defined above.
        Exec(args []interface{}) (Result, error)

        // Exec executes a query that may return rows, such as a
        // SELECT.  The args of all of a subset type as defined above.
        Query(args []interface{}) (Rows, error)
}

// ColumnConverter may be optionally implemented by Stmt if the
// the statement is aware of its own columns' types and can
// convert from any type to a driver subset type.
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.
        //
        // The dest slice may be populated with only with values
        // of subset types defined above, but excluding string.
        // All string values must be converted to []byte.
        //
        // Next should return io.EOF when there are no more rows.
        Next(dest []interface{}) error
}

// 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("no LastInsertId available")
}

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

// DDLSuccess is a pre-defined Result for drivers to return when a DDL
// command succeeds.
var DDLSuccess ddlSuccess

type ddlSuccess struct{}

var _ Result = ddlSuccess{}

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

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