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iterator.go
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/
iterator.go
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
package kivik
import (
"context"
"io"
"net/http"
"sync"
"github.com/go-kivik/kivik/v4/driver"
"github.com/go-kivik/kivik/v4/internal"
)
type iterator interface {
Next(interface{}) error
Close() error
}
// possible states of the result set iterator
const (
// stateReady is the initial state before [ResultSet.Next] or
// [ResultSet.NextResultSet] is called.
stateReady = iota
// stateResultSetReady is the state after calling [ResultSet.NextResultSet]
stateResultSetReady
// stateResultSetRowReady is the state after calling [ResultSet.Next] within
// a result set.
stateResultSetRowReady
// stateEOQ is the state after having reached the final row in a result set.
// [ResultSet.ResultSetNext] should be called next.
stateEOQ
// stateRowReady is the state when not iterating resultsets, after
// [ResultSet.Next] has been called.
stateRowReady
// stateClosed means the last row has been retrieved. The iterator is no
// longer usable.
stateClosed
)
type iter struct {
feed iterator
onClose func()
mu sync.Mutex
state int // Set to true once Next() has been called
err error // non-nil only if state == stateClosed
wg sync.WaitGroup
cancel func() // cancel function to exit context goroutine when iterator is closed
curVal interface{}
}
// isReady returns an error if the iterator is not ready, because it has been
// closed, or has not been made ready yet.
func (i *iter) isReady() error {
if i.state == stateClosed {
return &internal.Error{Status: http.StatusBadRequest, Message: "kivik: Iterator is closed"}
}
if !stateIsReady(i.state) {
return &internal.Error{Status: http.StatusBadRequest, Message: "kivik: Iterator access before calling Next"}
}
return nil
}
func stateIsReady(state int) bool {
switch state {
case stateRowReady, stateResultSetReady, stateResultSetRowReady, stateClosed:
return true
}
return false
}
// newIterator instantiates a new iterator.
//
// ctx is a possibly-cancellable context. zeroValue is an empty instance of
// the data type this iterator iterates over feed is the iterator interface,
// which typically wraps a driver.X iterator
func newIterator(ctx context.Context, onClose func(), feed iterator, zeroValue interface{}) *iter {
i := &iter{
onClose: onClose,
feed: feed,
curVal: zeroValue,
}
ctx, i.cancel = context.WithCancel(ctx)
go i.awaitDone(ctx)
return i
}
// errIterator instantiates a new iteratore that is already closed, and only
// returns an error.
func errIterator(err error) *iter {
return &iter{
state: stateClosed,
err: err,
}
}
// awaitDone blocks until the rows are closed or the context is cancelled, then
// closes the iterator if it's still open.
func (i *iter) awaitDone(ctx context.Context) {
<-ctx.Done()
i.mu.Lock()
_ = i.closeErr(ctx.Err())
i.mu.Unlock()
}
// Next prepares the next iterator result value for reading. It returns true on
// success, or false if there is no next result or an error occurs while
// preparing it. [Err] should be consulted to distinguish between the two.
func (i *iter) Next() bool {
i.mu.Lock()
defer i.mu.Unlock()
return i.next()
}
// next is the same as Next but doesn't do its own locking.
func (i *iter) next() bool {
if i.state == stateClosed {
return false
}
for {
err := i.feed.Next(i.curVal)
if err == driver.EOQ {
if i.state == stateResultSetReady || i.state == stateResultSetRowReady {
i.state = stateEOQ
i.err = nil
return false
}
continue
}
switch i.state {
case stateResultSetReady, stateResultSetRowReady:
i.state = stateResultSetRowReady
default:
i.state = stateRowReady
}
i.err = err
if i.err != nil {
_ = i.closeErr(nil)
return false
}
return true
}
}
// Close closes the iterator, preventing further enumeration, and freeing any
// resources (such as the http request body) of the underlying feed. If [Next]
// is called and there are no further results, the iterator is closed
// automatically and it will suffice to check the result of [Err]. Close is
// idempotent and does not affect the result of [Err].
func (i *iter) Close() error {
i.mu.Lock()
defer i.mu.Unlock()
i.wg.Wait()
return i.closeErr(nil)
}
func (i *iter) closeErr(err error) error {
if i.state == stateClosed {
return nil
}
i.state = stateClosed
if i.err == nil {
i.err = err
}
err = i.feed.Close()
if i.cancel != nil {
i.cancel()
}
if i.onClose != nil {
i.onClose()
}
return err
}
// Err returns the error, if any, that was encountered during iteration. Err may
// be called after an explicit or implicit [Close].
func (i *iter) Err() error {
i.mu.Lock()
defer i.mu.Unlock()
if i.err == io.EOF {
return nil
}
return i.err
}