Line data Source code
1 : // Copyright 2022 The LevelDB-Go and Pebble Authors. All rights reserved. Use
2 : // of this source code is governed by a BSD-style license that can be found in
3 : // the LICENSE file.
4 :
5 : package keyspan
6 :
7 : import "github.com/cockroachdb/pebble/internal/base"
8 :
9 : // TODO(jackson): Consider removing this type and adding bounds enforcement
10 : // directly to the MergingIter. This type is probably too lightweight to warrant
11 : // its own type, but for now we implement it separately for expediency.
12 :
13 : // boundedIterPos records the position of the BoundedIter relative to the
14 : // underlying iterator's position. It's used to avoid Next/Prev-ing the iterator
15 : // if there can't possibly be another span within bounds, because the current
16 : // span overlaps the bound.
17 : //
18 : // Imagine bounds [a,c) and an iterator that seeks to a span [b,d). The span
19 : // [b,d) overlaps some portion of the iterator bounds, so the iterator must
20 : // return it. If the iterator is subsequently Nexted, Next can tell that the
21 : // iterator is exhausted without advancing the underlying iterator because the
22 : // current span's end bound of d is ≥ the upper bound of c. In this case, the
23 : // bounded iterator returns nil and records i.pos as posAtUpperLimit to remember
24 : // that the underlying iterator position does not match the current BoundedIter
25 : // position.
26 : type boundedIterPos int8
27 :
28 : const (
29 : posAtLowerLimit boundedIterPos = -1
30 : posAtIterSpan boundedIterPos = 0
31 : posAtUpperLimit boundedIterPos = +1
32 : )
33 :
34 : // BoundedIter implements FragmentIterator and enforces bounds.
35 : //
36 : // Like the point InternalIterator interface, the bounded iterator's forward
37 : // positioning routines (SeekGE, First, and Next) only check the upper bound.
38 : // The reverse positioning routines (SeekLT, Last, and Prev) only check the
39 : // lower bound. It is up to the caller to ensure that the forward positioning
40 : // routines respect the lower bound and the reverse positioning routines respect
41 : // the upper bound (i.e. calling SeekGE instead of First if there is a lower
42 : // bound, and SeekLT instead of Last if there is an upper bound).
43 : //
44 : // When the hasPrefix parameter indicates that the iterator is in prefix
45 : // iteration mode, BoundedIter elides any spans that do not overlap with the
46 : // prefix's keyspace. In prefix iteration mode, reverse iteration is disallowed,
47 : // except for an initial SeekLT with a seek key greater than or equal to the
48 : // prefix. In prefix iteration mode, the first seek must position the iterator
49 : // at or immediately before the first fragment covering a key greater than or
50 : // equal to the prefix.
51 : type BoundedIter struct {
52 : iter FragmentIterator
53 : iterSpan *Span
54 : cmp base.Compare
55 : split base.Split
56 : lower []byte
57 : upper []byte
58 : hasPrefix *bool
59 : prefix *[]byte
60 : pos boundedIterPos
61 : }
62 :
63 : // Init initializes the bounded iterator.
64 : //
65 : // In addition to the iterator bounds, Init takes pointers to a boolean
66 : // indicating whether the iterator is in prefix iteration mode and the prefix
67 : // key if it is. This is used to exclude spans that are outside the iteration
68 : // prefix.
69 : //
70 : // hasPrefix and prefix are allowed to be nil, however if hasPrefix != nil,
71 : // prefix must also not be nil.
72 : func (i *BoundedIter) Init(
73 : cmp base.Compare,
74 : split base.Split,
75 : iter FragmentIterator,
76 : lower, upper []byte,
77 : hasPrefix *bool,
78 : prefix *[]byte,
79 1 : ) {
80 1 : *i = BoundedIter{
81 1 : iter: iter,
82 1 : cmp: cmp,
83 1 : split: split,
84 1 : lower: lower,
85 1 : upper: upper,
86 1 : hasPrefix: hasPrefix,
87 1 : prefix: prefix,
88 1 : }
89 1 : }
90 :
91 : var _ FragmentIterator = (*BoundedIter)(nil)
92 :
93 : // Seek calls.
94 : //
95 : // Seek calls check iterator bounds in the direction of the seek. Additionally,
96 : // if the iterator is in prefix iteration mode, seek calls check both start and
97 : // end bounds against the prefix's bounds. We check both bounds for defense in
98 : // depth. This optimization has been a source of various bugs due to various
99 : // other prefix iteration optimizations that can result in seek keys that don't
100 : // respect the prefix bounds.
101 :
102 : // SeekGE implements FragmentIterator.
103 1 : func (i *BoundedIter) SeekGE(key []byte) *Span {
104 1 : s := i.iter.SeekGE(key)
105 1 : s = i.checkPrefixSpanStart(s)
106 1 : s = i.checkPrefixSpanEnd(s)
107 1 : return i.checkForwardBound(s)
108 1 : }
109 :
110 : // SeekLT implements FragmentIterator.
111 1 : func (i *BoundedIter) SeekLT(key []byte) *Span {
112 1 : s := i.iter.SeekLT(key)
113 1 : s = i.checkPrefixSpanStart(s)
114 1 : s = i.checkPrefixSpanEnd(s)
115 1 : return i.checkBackwardBound(s)
116 1 : }
117 :
118 : // First implements FragmentIterator.
119 1 : func (i *BoundedIter) First() *Span {
120 1 : s := i.iter.First()
121 1 : s = i.checkPrefixSpanStart(s)
122 1 : return i.checkForwardBound(s)
123 1 : }
124 :
125 : // Last implements FragmentIterator.
126 1 : func (i *BoundedIter) Last() *Span {
127 1 : s := i.iter.Last()
128 1 : s = i.checkPrefixSpanEnd(s)
129 1 : return i.checkBackwardBound(s)
130 1 : }
131 :
132 : // Next implements FragmentIterator.
133 1 : func (i *BoundedIter) Next() *Span {
134 1 : switch i.pos {
135 1 : case posAtLowerLimit:
136 1 : // The BoundedIter had previously returned nil, because it knew from
137 1 : // i.iterSpan's bounds that there was no previous span. To Next, we only
138 1 : // need to return the current iter span and reset i.pos to reflect that
139 1 : // we're no longer positioned at the limit.
140 1 : i.pos = posAtIterSpan
141 1 : return i.iterSpan
142 1 : case posAtIterSpan:
143 1 : // If the span at the underlying iterator position extends to or beyond the
144 1 : // upper bound, we can avoid advancing because the next span is necessarily
145 1 : // out of bounds.
146 1 : if i.iterSpan != nil && i.upper != nil && i.cmp(i.iterSpan.End, i.upper) >= 0 {
147 1 : i.pos = posAtUpperLimit
148 1 : return nil
149 1 : }
150 : // Similarly, if the span extends to the next prefix and we're in prefix
151 : // iteration mode, we can avoid advancing.
152 1 : if i.iterSpan != nil && i.hasPrefix != nil && *i.hasPrefix {
153 1 : ei := i.split(i.iterSpan.End)
154 1 : if i.cmp(i.iterSpan.End[:ei], *i.prefix) > 0 {
155 1 : i.pos = posAtUpperLimit
156 1 : return nil
157 1 : }
158 : }
159 1 : return i.checkForwardBound(i.checkPrefixSpanStart(i.iter.Next()))
160 1 : case posAtUpperLimit:
161 1 : // Already exhausted.
162 1 : return nil
163 0 : default:
164 0 : panic("unreachable")
165 : }
166 : }
167 :
168 : // Prev implements FragmentIterator.
169 1 : func (i *BoundedIter) Prev() *Span {
170 1 : switch i.pos {
171 1 : case posAtLowerLimit:
172 1 : // Already exhausted.
173 1 : return nil
174 1 : case posAtIterSpan:
175 1 : // If the span at the underlying iterator position extends to or beyond
176 1 : // the lower bound, we can avoid advancing because the previous span is
177 1 : // necessarily out of bounds.
178 1 : if i.iterSpan != nil && i.lower != nil && i.cmp(i.iterSpan.Start, i.lower) <= 0 {
179 1 : i.pos = posAtLowerLimit
180 1 : return nil
181 1 : }
182 : // Similarly, if the span extends to or beyond the current prefix and
183 : // we're in prefix iteration mode, we can avoid advancing.
184 1 : if i.iterSpan != nil && i.hasPrefix != nil && *i.hasPrefix {
185 1 : si := i.split(i.iterSpan.Start)
186 1 : if i.cmp(i.iterSpan.Start[:si], *i.prefix) < 0 {
187 1 : i.pos = posAtLowerLimit
188 1 : return nil
189 1 : }
190 : }
191 1 : return i.checkBackwardBound(i.checkPrefixSpanEnd(i.iter.Prev()))
192 1 : case posAtUpperLimit:
193 1 : // The BoundedIter had previously returned nil, because it knew from
194 1 : // i.iterSpan's bounds that there was no next span. To Prev, we only
195 1 : // need to return the current iter span and reset i.pos to reflect that
196 1 : // we're no longer positioned at the limit.
197 1 : i.pos = posAtIterSpan
198 1 : return i.iterSpan
199 0 : default:
200 0 : panic("unreachable")
201 : }
202 : }
203 :
204 : // Error implements FragmentIterator.
205 1 : func (i *BoundedIter) Error() error {
206 1 : return i.iter.Error()
207 1 : }
208 :
209 : // Close implements FragmentIterator.
210 1 : func (i *BoundedIter) Close() error {
211 1 : return i.iter.Close()
212 1 : }
213 :
214 : // SetBounds modifies the FragmentIterator's bounds.
215 1 : func (i *BoundedIter) SetBounds(lower, upper []byte) {
216 1 : i.lower, i.upper = lower, upper
217 1 : }
218 :
219 1 : func (i *BoundedIter) checkPrefixSpanStart(span *Span) *Span {
220 1 : // Compare to the prefix's bounds, if in prefix iteration mode.
221 1 : if span != nil && i.hasPrefix != nil && *i.hasPrefix {
222 1 : si := i.split(span.Start)
223 1 : if i.cmp(span.Start[:si], *i.prefix) > 0 {
224 1 : // This span starts at a prefix that sorts after our current prefix.
225 1 : span = nil
226 1 : }
227 : }
228 1 : return span
229 : }
230 :
231 : // checkForwardBound enforces the upper bound, returning nil if the provided
232 : // span is wholly outside the upper bound. It also updates i.pos and i.iterSpan
233 : // to reflect the new iterator position.
234 1 : func (i *BoundedIter) checkForwardBound(span *Span) *Span {
235 1 : // Compare to the upper bound.
236 1 : if span != nil && i.upper != nil && i.cmp(span.Start, i.upper) >= 0 {
237 1 : span = nil
238 1 : }
239 1 : i.iterSpan = span
240 1 : if i.pos != posAtIterSpan {
241 1 : i.pos = posAtIterSpan
242 1 : }
243 1 : return span
244 : }
245 :
246 1 : func (i *BoundedIter) checkPrefixSpanEnd(span *Span) *Span {
247 1 : // Compare to the prefix's bounds, if in prefix iteration mode.
248 1 : if span != nil && i.hasPrefix != nil && *i.hasPrefix && i.cmp(span.End, *i.prefix) <= 0 {
249 1 : // This span ends before the current prefix.
250 1 : span = nil
251 1 : }
252 1 : return span
253 : }
254 :
255 : // checkBackward enforces the lower bound, returning nil if the provided span is
256 : // wholly outside the lower bound. It also updates i.pos and i.iterSpan to
257 : // reflect the new iterator position.
258 1 : func (i *BoundedIter) checkBackwardBound(span *Span) *Span {
259 1 : // Compare to the lower bound.
260 1 : if span != nil && i.lower != nil && i.cmp(span.End, i.lower) <= 0 {
261 1 : span = nil
262 1 : }
263 1 : i.iterSpan = span
264 1 : if i.pos != posAtIterSpan {
265 1 : i.pos = posAtIterSpan
266 1 : }
267 1 : return span
268 : }
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