cf67fe2cbc
The field cache is expected to be the authority on which field names are identified by which identifier. This code was optimized for the most common case in which fields already exist. However, if we deterimine the field is missing with the read lock (shared), we incorrectly immediately proceed to create a new row with the write lock (exclusive). The problem is that multiple goroutines might have come to the same conclusion, and they all proceed to add rows. The two choices were to do the whole operation with the write lock, or recheck the value again with the write lock. We have chosen to repeat the check inside the write-lock, as this optimizes for what we believe to be the most common case, in which most fields will already exist.
76 lines
2.1 KiB
Go
76 lines
2.1 KiB
Go
// Copyright (c) 2015 Couchbase, Inc.
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
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// except in compliance with the License. You may obtain a copy of the License at
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// http://www.apache.org/licenses/LICENSE-2.0
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// Unless required by applicable law or agreed to in writing, software distributed under the
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// License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
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// either express or implied. See the License for the specific language governing permissions
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// and limitations under the License.
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package index
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import (
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"sync"
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)
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type FieldCache struct {
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fieldIndexes map[string]uint16
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lastFieldIndex int
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mutex sync.RWMutex
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}
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func NewFieldCache() *FieldCache {
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return &FieldCache{
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fieldIndexes: make(map[string]uint16),
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lastFieldIndex: -1,
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}
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}
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func (f *FieldCache) AddExisting(field string, index uint16) {
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f.mutex.Lock()
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defer f.mutex.Unlock()
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f.fieldIndexes[field] = index
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if int(index) > f.lastFieldIndex {
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f.lastFieldIndex = int(index)
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}
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}
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// FieldNamed returns the index of the field, and whether or not it existed
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// before this call. if createIfMissing is true, and new field index is assigned
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// but the second return value will still be false
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func (f *FieldCache) FieldNamed(field string, createIfMissing bool) (uint16, bool) {
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f.mutex.RLock()
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if index, ok := f.fieldIndexes[field]; ok {
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f.mutex.RUnlock()
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return index, true
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} else if !createIfMissing {
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f.mutex.RUnlock()
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return 0, false
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}
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// trade read lock for write lock
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f.mutex.RUnlock()
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f.mutex.Lock()
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// need to check again with write lock
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if index, ok := f.fieldIndexes[field]; ok {
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f.mutex.Unlock()
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return index, true
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}
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// assign next field id
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index := uint16(f.lastFieldIndex + 1)
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f.fieldIndexes[field] = index
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f.lastFieldIndex = int(index)
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f.mutex.Unlock()
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return index, false
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}
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func (f *FieldCache) FieldIndexed(index uint16) string {
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f.mutex.RLock()
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defer f.mutex.RUnlock()
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for fieldName, fieldIndex := range f.fieldIndexes {
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if index == fieldIndex {
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return fieldName
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}
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}
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return ""
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}
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