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 (
8 : "bytes"
9 :
10 : "github.com/cockroachdb/pebble/internal/base"
11 : "github.com/cockroachdb/pebble/internal/bytealloc"
12 : "github.com/cockroachdb/pebble/internal/invariants"
13 : )
14 :
15 : // bufferReuseMaxCapacity is the maximum capacity of a DefragmentingIter buffer
16 : // that DefragmentingIter will reuse. Buffers larger than this will be
17 : // discarded and reallocated as necessary.
18 : const bufferReuseMaxCapacity = 10 << 10 // 10 KB
19 :
20 : // keysReuseMaxCapacity is the maximum capacity of a []keyspan.Key buffer that
21 : // DefragmentingIter will reuse. Buffers larger than this will be discarded and
22 : // reallocated as necessary.
23 : const keysReuseMaxCapacity = 100
24 :
25 : // DefragmentMethod configures the defragmentation performed by the
26 : // DefragmentingIter.
27 : type DefragmentMethod interface {
28 : // ShouldDefragment takes two abutting spans and returns whether the two
29 : // spans should be combined into a single, defragmented Span.
30 : ShouldDefragment(equal base.Equal, left, right *Span) bool
31 : }
32 :
33 : // The DefragmentMethodFunc type is an adapter to allow the use of ordinary
34 : // functions as DefragmentMethods. If f is a function with the appropriate
35 : // signature, DefragmentMethodFunc(f) is a DefragmentMethod that calls f.
36 : type DefragmentMethodFunc func(equal base.Equal, left, right *Span) bool
37 :
38 : // ShouldDefragment calls f(equal, left, right).
39 2 : func (f DefragmentMethodFunc) ShouldDefragment(equal base.Equal, left, right *Span) bool {
40 2 : return f(equal, left, right)
41 2 : }
42 :
43 : // DefragmentInternal configures a DefragmentingIter to defragment spans
44 : // only if they have identical keys. It requires spans' keys to be sorted in
45 : // trailer descending order.
46 : //
47 : // This defragmenting method is intended for use in compactions that may see
48 : // internal range keys fragments that may now be joined, because the state that
49 : // required their fragmentation has been dropped.
50 2 : var DefragmentInternal DefragmentMethod = DefragmentMethodFunc(func(equal base.Equal, a, b *Span) bool {
51 2 : if a.KeysOrder != ByTrailerDesc || b.KeysOrder != ByTrailerDesc {
52 0 : panic("pebble: span keys unexpectedly not in trailer descending order")
53 : }
54 2 : if len(a.Keys) != len(b.Keys) {
55 2 : return false
56 2 : }
57 2 : for i := range a.Keys {
58 2 : if a.Keys[i].Trailer != b.Keys[i].Trailer {
59 2 : return false
60 2 : }
61 2 : if !equal(a.Keys[i].Suffix, b.Keys[i].Suffix) {
62 0 : return false
63 0 : }
64 2 : if !bytes.Equal(a.Keys[i].Value, b.Keys[i].Value) {
65 0 : return false
66 0 : }
67 : }
68 2 : return true
69 : })
70 :
71 : // DefragmentReducer merges the current and next Key slices, returning a new Key
72 : // slice.
73 : //
74 : // Implementations should modify and return `cur` to save on allocations, or
75 : // consider allocating a new slice, as the `cur` slice may be retained by the
76 : // DefragmentingIter and mutated. The `next` slice must not be mutated.
77 : //
78 : // The incoming slices are sorted by (SeqNum, Kind) descending. The output slice
79 : // must also have this sort order.
80 : type DefragmentReducer func(cur, next []Key) []Key
81 :
82 : // StaticDefragmentReducer is a no-op DefragmentReducer that simply returns the
83 : // current key slice, effectively retaining the first set of keys encountered
84 : // for a defragmented span.
85 : //
86 : // This reducer can be used, for example, when the set of Keys for each Span
87 : // being reduced is not expected to change, and therefore the keys from the
88 : // first span encountered can be used without considering keys in subsequent
89 : // spans.
90 2 : var StaticDefragmentReducer DefragmentReducer = func(cur, _ []Key) []Key {
91 2 : return cur
92 2 : }
93 :
94 : // iterPos is an enum indicating the position of the defragmenting iter's
95 : // wrapped iter. The defragmenting iter must look ahead or behind when
96 : // defragmenting forward or backwards respectively, and this enum records that
97 : // current position.
98 : type iterPos int8
99 :
100 : const (
101 : iterPosPrev iterPos = -1
102 : iterPosCurr iterPos = 0
103 : iterPosNext iterPos = +1
104 : )
105 :
106 : // DefragmentingIter wraps a key span iterator, defragmenting physical
107 : // fragmentation during iteration.
108 : //
109 : // During flushes and compactions, keys applied over a span may be split at
110 : // sstable boundaries. This fragmentation can produce internal key bounds that
111 : // do not match any of the bounds ever supplied to a user operation. This
112 : // physical fragmentation is necessary to avoid excessively wide sstables.
113 : //
114 : // The defragmenting iterator undoes this physical fragmentation, joining spans
115 : // with abutting bounds and equal state. The defragmenting iterator takes a
116 : // DefragmentMethod to determine what is "equal state" for a span. The
117 : // DefragmentMethod is a function type, allowing arbitrary comparisons between
118 : // Span keys.
119 : //
120 : // Seeking (SeekGE, SeekLT) poses an obstacle to defragmentation. A seek may
121 : // land on a physical fragment in the middle of several fragments that must be
122 : // defragmented. A seek that lands in a fragment straddling the seek key must
123 : // first degfragment in the opposite direction of iteration to find the
124 : // beginning of the defragmented span, and then defragments in the iteration
125 : // direction, ensuring it's found a whole defragmented span.
126 : type DefragmentingIter struct {
127 : // DefragmentingBuffers holds buffers used for copying iterator state.
128 : *DefragmentingBuffers
129 : comparer *base.Comparer
130 : equal base.Equal
131 : iter FragmentIterator
132 : iterSpan *Span
133 : iterPos iterPos
134 :
135 : // curr holds the span at the current iterator position.
136 : curr Span
137 :
138 : // method is a comparison function for two spans. method is called when two
139 : // spans are abutting to determine whether they may be defragmented.
140 : // method does not itself check for adjacency for the two spans.
141 : method DefragmentMethod
142 :
143 : // reduce is the reducer function used to collect Keys across all spans that
144 : // constitute a defragmented span.
145 : reduce DefragmentReducer
146 : }
147 :
148 : // DefragmentingBuffers holds buffers used for copying iterator state.
149 : type DefragmentingBuffers struct {
150 : // currBuf is a buffer for use when copying user keys for curr. currBuf is
151 : // cleared between positioning methods.
152 : currBuf bytealloc.A
153 : // keysBuf is a buffer for use when copying Keys for DefragmentingIter.curr.
154 : keysBuf []Key
155 : // keyBuf is a buffer specifically for the defragmented start key when
156 : // defragmenting backwards or the defragmented end key when defragmenting
157 : // forwards. These bounds are overwritten repeatedly during defragmentation,
158 : // and the defragmentation routines overwrite keyBuf repeatedly to store
159 : // these extended bounds.
160 : keyBuf []byte
161 : }
162 :
163 : // PrepareForReuse discards any excessively large buffers.
164 2 : func (bufs *DefragmentingBuffers) PrepareForReuse() {
165 2 : if cap(bufs.currBuf) > bufferReuseMaxCapacity {
166 2 : bufs.currBuf = nil
167 2 : }
168 2 : if cap(bufs.keyBuf) > bufferReuseMaxCapacity {
169 0 : bufs.keyBuf = nil
170 0 : }
171 2 : if cap(bufs.keysBuf) > keysReuseMaxCapacity {
172 0 : bufs.keysBuf = nil
173 0 : }
174 : }
175 :
176 : // Assert that *DefragmentingIter implements the FragmentIterator interface.
177 : var _ FragmentIterator = (*DefragmentingIter)(nil)
178 :
179 : // Init initializes the defragmenting iter using the provided defragment
180 : // method.
181 : func (i *DefragmentingIter) Init(
182 : comparer *base.Comparer,
183 : iter FragmentIterator,
184 : equal DefragmentMethod,
185 : reducer DefragmentReducer,
186 : bufs *DefragmentingBuffers,
187 2 : ) {
188 2 : *i = DefragmentingIter{
189 2 : DefragmentingBuffers: bufs,
190 2 : comparer: comparer,
191 2 : equal: comparer.Equal,
192 2 : iter: iter,
193 2 : method: equal,
194 2 : reduce: reducer,
195 2 : }
196 2 : }
197 :
198 : // Error returns any accumulated error.
199 2 : func (i *DefragmentingIter) Error() error {
200 2 : return i.iter.Error()
201 2 : }
202 :
203 : // Close closes the underlying iterators.
204 2 : func (i *DefragmentingIter) Close() error {
205 2 : return i.iter.Close()
206 2 : }
207 :
208 : // SeekGE moves the iterator to the first span covering a key greater than or
209 : // equal to the given key. This is equivalent to seeking to the first span with
210 : // an end key greater than the given key.
211 2 : func (i *DefragmentingIter) SeekGE(key []byte) *Span {
212 2 : i.iterSpan = i.iter.SeekGE(key)
213 2 : if i.iterSpan == nil {
214 2 : i.iterPos = iterPosCurr
215 2 : return nil
216 2 : } else if i.iterSpan.Empty() {
217 2 : i.iterPos = iterPosCurr
218 2 : return i.iterSpan
219 2 : }
220 : // If the span starts strictly after key, we know there mustn't be an
221 : // earlier span that ends at i.iterSpan.Start, otherwise i.iter would've
222 : // returned that span instead.
223 2 : if i.comparer.Compare(i.iterSpan.Start, key) > 0 {
224 2 : return i.defragmentForward()
225 2 : }
226 :
227 : // The span we landed on has a Start bound ≤ key. There may be additional
228 : // fragments before this span. Defragment backward to find the start of the
229 : // defragmented span.
230 2 : i.defragmentBackward()
231 2 : if i.iterPos == iterPosPrev {
232 2 : // Next once back onto the span.
233 2 : i.iterSpan = i.iter.Next()
234 2 : }
235 : // Defragment the full span from its start.
236 2 : return i.defragmentForward()
237 : }
238 :
239 : // SeekLT moves the iterator to the last span covering a key less than the
240 : // given key. This is equivalent to seeking to the last span with a start
241 : // key less than the given key.
242 2 : func (i *DefragmentingIter) SeekLT(key []byte) *Span {
243 2 : i.iterSpan = i.iter.SeekLT(key)
244 2 : if i.iterSpan == nil {
245 2 : i.iterPos = iterPosCurr
246 2 : return nil
247 2 : } else if i.iterSpan.Empty() {
248 2 : i.iterPos = iterPosCurr
249 2 : return i.iterSpan
250 2 : }
251 : // If the span ends strictly before key, we know there mustn't be a later
252 : // span that starts at i.iterSpan.End, otherwise i.iter would've returned
253 : // that span instead.
254 2 : if i.comparer.Compare(i.iterSpan.End, key) < 0 {
255 2 : return i.defragmentBackward()
256 2 : }
257 :
258 : // The span we landed on has a End bound ≥ key. There may be additional
259 : // fragments after this span. Defragment forward to find the end of the
260 : // defragmented span.
261 2 : i.defragmentForward()
262 2 : if i.iterPos == iterPosNext {
263 2 : // Prev once back onto the span.
264 2 : i.iterSpan = i.iter.Prev()
265 2 : }
266 : // Defragment the full span from its end.
267 2 : return i.defragmentBackward()
268 : }
269 :
270 : // First seeks the iterator to the first span and returns it.
271 2 : func (i *DefragmentingIter) First() *Span {
272 2 : i.iterSpan = i.iter.First()
273 2 : if i.iterSpan == nil {
274 2 : i.iterPos = iterPosCurr
275 2 : return nil
276 2 : }
277 2 : return i.defragmentForward()
278 : }
279 :
280 : // Last seeks the iterator to the last span and returns it.
281 2 : func (i *DefragmentingIter) Last() *Span {
282 2 : i.iterSpan = i.iter.Last()
283 2 : if i.iterSpan == nil {
284 1 : i.iterPos = iterPosCurr
285 1 : return nil
286 1 : }
287 2 : return i.defragmentBackward()
288 : }
289 :
290 : // Next advances to the next span and returns it.
291 2 : func (i *DefragmentingIter) Next() *Span {
292 2 : switch i.iterPos {
293 2 : case iterPosPrev:
294 2 : // Switching directions; The iterator is currently positioned over the
295 2 : // last span of the previous set of fragments. In the below diagram,
296 2 : // the iterator is positioned over the last span that contributes to
297 2 : // the defragmented x position. We want to be positioned over the first
298 2 : // span that contributes to the z position.
299 2 : //
300 2 : // x x x y y y z z z
301 2 : // ^ ^
302 2 : // old new
303 2 : //
304 2 : // Next once to move onto y, defragment forward to land on the first z
305 2 : // position.
306 2 : i.iterSpan = i.iter.Next()
307 2 : if invariants.Enabled && i.iterSpan == nil {
308 0 : panic("pebble: invariant violation: no next span while switching directions")
309 : }
310 : // We're now positioned on the first span that was defragmented into the
311 : // current iterator position. Skip over the rest of the current iterator
312 : // position's constitutent fragments. In the above example, this would
313 : // land on the first 'z'.
314 2 : i.defragmentForward()
315 2 : if i.iterSpan == nil {
316 2 : i.iterPos = iterPosCurr
317 2 : return nil
318 2 : }
319 :
320 : // Now that we're positioned over the first of the next set of
321 : // fragments, defragment forward.
322 2 : return i.defragmentForward()
323 2 : case iterPosCurr:
324 2 : // iterPosCurr is only used when the iter is exhausted or when the iterator
325 2 : // is at an empty span.
326 2 : if invariants.Enabled && i.iterSpan != nil && !i.iterSpan.Empty() {
327 0 : panic("pebble: invariant violation: iterPosCurr with valid iterSpan")
328 : }
329 :
330 2 : i.iterSpan = i.iter.Next()
331 2 : if i.iterSpan == nil {
332 2 : return nil
333 2 : }
334 2 : return i.defragmentForward()
335 2 : case iterPosNext:
336 2 : // Already at the next span.
337 2 : if i.iterSpan == nil {
338 2 : i.iterPos = iterPosCurr
339 2 : return nil
340 2 : }
341 2 : return i.defragmentForward()
342 0 : default:
343 0 : panic("unreachable")
344 : }
345 : }
346 :
347 : // Prev steps back to the previous span and returns it.
348 2 : func (i *DefragmentingIter) Prev() *Span {
349 2 : switch i.iterPos {
350 2 : case iterPosPrev:
351 2 : // Already at the previous span.
352 2 : if i.iterSpan == nil {
353 2 : i.iterPos = iterPosCurr
354 2 : return nil
355 2 : }
356 2 : return i.defragmentBackward()
357 2 : case iterPosCurr:
358 2 : // iterPosCurr is only used when the iter is exhausted or when the iterator
359 2 : // is at an empty span.
360 2 : if invariants.Enabled && i.iterSpan != nil && !i.iterSpan.Empty() {
361 0 : panic("pebble: invariant violation: iterPosCurr with valid iterSpan")
362 : }
363 :
364 2 : i.iterSpan = i.iter.Prev()
365 2 : if i.iterSpan == nil {
366 2 : return nil
367 2 : }
368 2 : return i.defragmentBackward()
369 2 : case iterPosNext:
370 2 : // Switching directions; The iterator is currently positioned over the
371 2 : // first fragment of the next set of fragments. In the below diagram,
372 2 : // the iterator is positioned over the first span that contributes to
373 2 : // the defragmented z position. We want to be positioned over the last
374 2 : // span that contributes to the x position.
375 2 : //
376 2 : // x x x y y y z z z
377 2 : // ^ ^
378 2 : // new old
379 2 : //
380 2 : // Prev once to move onto y, defragment backward to land on the last x
381 2 : // position.
382 2 : i.iterSpan = i.iter.Prev()
383 2 : if invariants.Enabled && i.iterSpan == nil {
384 0 : panic("pebble: invariant violation: no previous span while switching directions")
385 : }
386 : // We're now positioned on the last span that was defragmented into the
387 : // current iterator position. Skip over the rest of the current iterator
388 : // position's constitutent fragments. In the above example, this would
389 : // land on the last 'x'.
390 2 : i.defragmentBackward()
391 2 :
392 2 : // Now that we're positioned over the last of the prev set of
393 2 : // fragments, defragment backward.
394 2 : if i.iterSpan == nil {
395 2 : i.iterPos = iterPosCurr
396 2 : return nil
397 2 : }
398 2 : return i.defragmentBackward()
399 0 : default:
400 0 : panic("unreachable")
401 : }
402 : }
403 :
404 : // checkEqual checks the two spans for logical equivalence. It uses the passed-in
405 : // DefragmentMethod and ensures both spans are NOT empty; not defragmenting empty
406 : // spans is an optimization that lets us load fewer sstable blocks.
407 2 : func (i *DefragmentingIter) checkEqual(left, right *Span) bool {
408 2 : return (!left.Empty() && !right.Empty()) && i.method.ShouldDefragment(i.equal, i.iterSpan, &i.curr)
409 2 : }
410 :
411 : // defragmentForward defragments spans in the forward direction, starting from
412 : // i.iter's current position. The span at the current position must be non-nil,
413 : // but may be Empty().
414 2 : func (i *DefragmentingIter) defragmentForward() *Span {
415 2 : if i.iterSpan.Empty() {
416 2 : // An empty span will never be equal to another span; see checkEqual for
417 2 : // why. To avoid loading non-empty range keys further ahead by calling Next,
418 2 : // return early.
419 2 : i.iterPos = iterPosCurr
420 2 : return i.iterSpan
421 2 : }
422 2 : i.saveCurrent()
423 2 :
424 2 : i.iterPos = iterPosNext
425 2 : i.iterSpan = i.iter.Next()
426 2 : for i.iterSpan != nil {
427 2 : if !i.equal(i.curr.End, i.iterSpan.Start) {
428 2 : // Not a continuation.
429 2 : break
430 : }
431 2 : if !i.checkEqual(i.iterSpan, &i.curr) {
432 2 : // Not a continuation.
433 2 : break
434 : }
435 2 : i.keyBuf = append(i.keyBuf[:0], i.iterSpan.End...)
436 2 : i.curr.End = i.keyBuf
437 2 : i.keysBuf = i.reduce(i.keysBuf, i.iterSpan.Keys)
438 2 : i.iterSpan = i.iter.Next()
439 : }
440 2 : i.curr.Keys = i.keysBuf
441 2 : return &i.curr
442 : }
443 :
444 : // defragmentBackward defragments spans in the backward direction, starting from
445 : // i.iter's current position. The span at the current position must be non-nil,
446 : // but may be Empty().
447 2 : func (i *DefragmentingIter) defragmentBackward() *Span {
448 2 : if i.iterSpan.Empty() {
449 2 : // An empty span will never be equal to another span; see checkEqual for
450 2 : // why. To avoid loading non-empty range keys further ahead by calling Next,
451 2 : // return early.
452 2 : i.iterPos = iterPosCurr
453 2 : return i.iterSpan
454 2 : }
455 2 : i.saveCurrent()
456 2 :
457 2 : i.iterPos = iterPosPrev
458 2 : i.iterSpan = i.iter.Prev()
459 2 : for i.iterSpan != nil {
460 2 : if !i.equal(i.curr.Start, i.iterSpan.End) {
461 2 : // Not a continuation.
462 2 : break
463 : }
464 2 : if !i.checkEqual(i.iterSpan, &i.curr) {
465 2 : // Not a continuation.
466 2 : break
467 : }
468 2 : i.keyBuf = append(i.keyBuf[:0], i.iterSpan.Start...)
469 2 : i.curr.Start = i.keyBuf
470 2 : i.keysBuf = i.reduce(i.keysBuf, i.iterSpan.Keys)
471 2 : i.iterSpan = i.iter.Prev()
472 : }
473 2 : i.curr.Keys = i.keysBuf
474 2 : return &i.curr
475 : }
476 :
477 2 : func (i *DefragmentingIter) saveCurrent() {
478 2 : i.currBuf.Reset()
479 2 : i.keysBuf = i.keysBuf[:0]
480 2 : i.keyBuf = i.keyBuf[:0]
481 2 : if i.iterSpan == nil {
482 0 : return
483 0 : }
484 2 : i.curr = Span{
485 2 : Start: i.saveBytes(i.iterSpan.Start),
486 2 : End: i.saveBytes(i.iterSpan.End),
487 2 : KeysOrder: i.iterSpan.KeysOrder,
488 2 : }
489 2 : for j := range i.iterSpan.Keys {
490 2 : i.keysBuf = append(i.keysBuf, Key{
491 2 : Trailer: i.iterSpan.Keys[j].Trailer,
492 2 : Suffix: i.saveBytes(i.iterSpan.Keys[j].Suffix),
493 2 : Value: i.saveBytes(i.iterSpan.Keys[j].Value),
494 2 : })
495 2 : }
496 2 : i.curr.Keys = i.keysBuf
497 : }
498 :
499 2 : func (i *DefragmentingIter) saveBytes(b []byte) []byte {
500 2 : if b == nil {
501 2 : return nil
502 2 : }
503 2 : i.currBuf, b = i.currBuf.Copy(b)
504 2 : return b
505 : }
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