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1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * generic_xlog.c
4 : : * Implementation of generic xlog records.
5 : : *
6 : : *
7 : : * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
8 : : * Portions Copyright (c) 1994, Regents of the University of California
9 : : *
10 : : * src/backend/access/transam/generic_xlog.c
11 : : *
12 : : *-------------------------------------------------------------------------
13 : : */
14 : : #include "postgres.h"
15 : :
16 : : #include "access/bufmask.h"
17 : : #include "access/generic_xlog.h"
18 : : #include "access/xlogutils.h"
19 : : #include "miscadmin.h"
20 : :
21 : : /*-------------------------------------------------------------------------
22 : : * Internally, a delta between pages consists of a set of fragments. Each
23 : : * fragment represents changes made in a given region of a page. A fragment
24 : : * is made up as follows:
25 : : *
26 : : * - offset of page region (OffsetNumber)
27 : : * - length of page region (OffsetNumber)
28 : : * - data - the data to place into the region ('length' number of bytes)
29 : : *
30 : : * Unchanged regions of a page are not represented in its delta. As a result,
31 : : * a delta can be more compact than the full page image. But having an
32 : : * unchanged region between two fragments that is smaller than the fragment
33 : : * header (offset+length) does not pay off in terms of the overall size of
34 : : * the delta. For this reason, we merge adjacent fragments if the unchanged
35 : : * region between them is <= MATCH_THRESHOLD bytes.
36 : : *
37 : : * We do not bother to merge fragments across the "lower" and "upper" parts
38 : : * of a page; it's very seldom the case that pd_lower and pd_upper are within
39 : : * MATCH_THRESHOLD bytes of each other, and handling that infrequent case
40 : : * would complicate and slow down the delta-computation code unduly.
41 : : * Therefore, the worst-case delta size includes two fragment headers plus
42 : : * a full page's worth of data.
43 : : *-------------------------------------------------------------------------
44 : : */
45 : : #define FRAGMENT_HEADER_SIZE (2 * sizeof(OffsetNumber))
46 : : #define MATCH_THRESHOLD FRAGMENT_HEADER_SIZE
47 : : #define MAX_DELTA_SIZE (BLCKSZ + 2 * FRAGMENT_HEADER_SIZE)
48 : :
49 : : /* Struct of generic xlog data for single page */
50 : : typedef struct
51 : : {
52 : : Buffer buffer; /* registered buffer */
53 : : int flags; /* flags for this buffer */
54 : : int deltaLen; /* space consumed in delta field */
55 : : char *image; /* copy of page image for modification, do not
56 : : * do it in-place to have aligned memory chunk */
57 : : char delta[MAX_DELTA_SIZE]; /* delta between page images */
58 : : } GenericXLogPageData;
59 : :
60 : : /*
61 : : * State of generic xlog record construction. Must be allocated at an I/O
62 : : * aligned address.
63 : : */
64 : : struct GenericXLogState
65 : : {
66 : : /* Page images (properly aligned, must be first) */
67 : : PGIOAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
68 : : /* Info about each page, see above */
69 : : GenericXLogPageData pages[MAX_GENERIC_XLOG_PAGES];
70 : : bool isLogged;
71 : : };
72 : :
73 : : static void writeFragment(GenericXLogPageData *pageData, OffsetNumber offset,
74 : : OffsetNumber length, const char *data);
75 : : static void computeRegionDelta(GenericXLogPageData *pageData,
76 : : const char *curpage, const char *targetpage,
77 : : int targetStart, int targetEnd,
78 : : int validStart, int validEnd);
79 : : static void computeDelta(GenericXLogPageData *pageData, Page curpage, Page targetpage);
80 : : static void applyPageRedo(Page page, const char *delta, Size deltaSize);
81 : :
82 : :
83 : : /*
84 : : * Write next fragment into pageData's delta.
85 : : *
86 : : * The fragment has the given offset and length, and data points to the
87 : : * actual data (of length length).
88 : : */
89 : : static void
90 : 0 : writeFragment(GenericXLogPageData *pageData, OffsetNumber offset, OffsetNumber length,
91 : : const char *data)
92 : : {
93 : 0 : char *ptr = pageData->delta + pageData->deltaLen;
94 : :
95 : : /* Verify we have enough space */
96 [ # # ]: 0 : Assert(pageData->deltaLen + sizeof(offset) +
97 : : sizeof(length) + length <= sizeof(pageData->delta));
98 : :
99 : : /* Write fragment data */
100 : 0 : memcpy(ptr, &offset, sizeof(offset));
101 : 0 : ptr += sizeof(offset);
102 : 0 : memcpy(ptr, &length, sizeof(length));
103 : 0 : ptr += sizeof(length);
104 : 0 : memcpy(ptr, data, length);
105 : 0 : ptr += length;
106 : :
107 : 0 : pageData->deltaLen = ptr - pageData->delta;
108 : 0 : }
109 : :
110 : : /*
111 : : * Compute the XLOG fragments needed to transform a region of curpage into the
112 : : * corresponding region of targetpage, and append them to pageData's delta
113 : : * field. The region to transform runs from targetStart to targetEnd-1.
114 : : * Bytes in curpage outside the range validStart to validEnd-1 should be
115 : : * considered invalid, and always overwritten with target data.
116 : : *
117 : : * This function is a hot spot, so it's worth being as tense as possible
118 : : * about the data-matching loops.
119 : : */
120 : : static void
121 : 0 : computeRegionDelta(GenericXLogPageData *pageData,
122 : : const char *curpage, const char *targetpage,
123 : : int targetStart, int targetEnd,
124 : : int validStart, int validEnd)
125 : : {
126 : 0 : int i,
127 : : loopEnd,
128 : 0 : fragmentBegin = -1,
129 : 0 : fragmentEnd = -1;
130 : :
131 : : /* Deal with any invalid start region by including it in first fragment */
132 [ # # ]: 0 : if (validStart > targetStart)
133 : : {
134 : 0 : fragmentBegin = targetStart;
135 : 0 : targetStart = validStart;
136 : 0 : }
137 : :
138 : : /* We'll deal with any invalid end region after the main loop */
139 [ # # ]: 0 : loopEnd = Min(targetEnd, validEnd);
140 : :
141 : : /* Examine all the potentially matchable bytes */
142 : 0 : i = targetStart;
143 [ # # ]: 0 : while (i < loopEnd)
144 : : {
145 [ # # ]: 0 : if (curpage[i] != targetpage[i])
146 : : {
147 : : /* On unmatched byte, start new fragment if not already in one */
148 [ # # ]: 0 : if (fragmentBegin < 0)
149 : 0 : fragmentBegin = i;
150 : : /* Mark unmatched-data endpoint as uncertain */
151 : 0 : fragmentEnd = -1;
152 : : /* Extend the fragment as far as possible in a tight loop */
153 : 0 : i++;
154 [ # # # # ]: 0 : while (i < loopEnd && curpage[i] != targetpage[i])
155 : 0 : i++;
156 [ # # ]: 0 : if (i >= loopEnd)
157 : 0 : break;
158 : 0 : }
159 : :
160 : : /* Found a matched byte, so remember end of unmatched fragment */
161 : 0 : fragmentEnd = i;
162 : :
163 : : /*
164 : : * Extend the match as far as possible in a tight loop. (On typical
165 : : * workloads, this inner loop is the bulk of this function's runtime.)
166 : : */
167 : 0 : i++;
168 [ # # # # ]: 0 : while (i < loopEnd && curpage[i] == targetpage[i])
169 : 0 : i++;
170 : :
171 : : /*
172 : : * There are several possible cases at this point:
173 : : *
174 : : * 1. We have no unwritten fragment (fragmentBegin < 0). There's
175 : : * nothing to write; and it doesn't matter what fragmentEnd is.
176 : : *
177 : : * 2. We found more than MATCH_THRESHOLD consecutive matching bytes.
178 : : * Dump out the unwritten fragment, stopping at fragmentEnd.
179 : : *
180 : : * 3. The match extends to loopEnd. We'll do nothing here, exit the
181 : : * loop, and then dump the unwritten fragment, after merging it with
182 : : * the invalid end region if any. If we don't so merge, fragmentEnd
183 : : * establishes how much the final writeFragment call needs to write.
184 : : *
185 : : * 4. We found an unmatched byte before loopEnd. The loop will repeat
186 : : * and will enter the unmatched-byte stanza above. So in this case
187 : : * also, it doesn't matter what fragmentEnd is. The matched bytes
188 : : * will get merged into the continuing unmatched fragment.
189 : : *
190 : : * Only in case 3 do we reach the bottom of the loop with a meaningful
191 : : * fragmentEnd value, which is why it's OK that we unconditionally
192 : : * assign "fragmentEnd = i" above.
193 : : */
194 [ # # # # ]: 0 : if (fragmentBegin >= 0 && i - fragmentEnd > MATCH_THRESHOLD)
195 : : {
196 : 0 : writeFragment(pageData, fragmentBegin,
197 : 0 : fragmentEnd - fragmentBegin,
198 : 0 : targetpage + fragmentBegin);
199 : 0 : fragmentBegin = -1;
200 : 0 : fragmentEnd = -1; /* not really necessary */
201 : 0 : }
202 : : }
203 : :
204 : : /* Deal with any invalid end region by including it in final fragment */
205 [ # # ]: 0 : if (loopEnd < targetEnd)
206 : : {
207 [ # # ]: 0 : if (fragmentBegin < 0)
208 : 0 : fragmentBegin = loopEnd;
209 : 0 : fragmentEnd = targetEnd;
210 : 0 : }
211 : :
212 : : /* Write final fragment if any */
213 [ # # ]: 0 : if (fragmentBegin >= 0)
214 : : {
215 [ # # ]: 0 : if (fragmentEnd < 0)
216 : 0 : fragmentEnd = targetEnd;
217 : 0 : writeFragment(pageData, fragmentBegin,
218 : 0 : fragmentEnd - fragmentBegin,
219 : 0 : targetpage + fragmentBegin);
220 : 0 : }
221 : 0 : }
222 : :
223 : : /*
224 : : * Compute the XLOG delta record needed to transform curpage into targetpage,
225 : : * and store it in pageData's delta field.
226 : : */
227 : : static void
228 : 0 : computeDelta(GenericXLogPageData *pageData, Page curpage, Page targetpage)
229 : : {
230 : 0 : int targetLower = ((PageHeader) targetpage)->pd_lower,
231 : 0 : targetUpper = ((PageHeader) targetpage)->pd_upper,
232 : 0 : curLower = ((PageHeader) curpage)->pd_lower,
233 : 0 : curUpper = ((PageHeader) curpage)->pd_upper;
234 : :
235 : 0 : pageData->deltaLen = 0;
236 : :
237 : : /* Compute delta records for lower part of page ... */
238 : 0 : computeRegionDelta(pageData, curpage, targetpage,
239 : 0 : 0, targetLower,
240 : 0 : 0, curLower);
241 : : /* ... and for upper part, ignoring what's between */
242 : 0 : computeRegionDelta(pageData, curpage, targetpage,
243 : 0 : targetUpper, BLCKSZ,
244 : 0 : curUpper, BLCKSZ);
245 : :
246 : : /*
247 : : * If xlog debug is enabled, then check produced delta. Result of delta
248 : : * application to curpage should be equivalent to targetpage.
249 : : */
250 : : #ifdef WAL_DEBUG
251 : : if (XLOG_DEBUG)
252 : : {
253 : : PGAlignedBlock tmp;
254 : :
255 : : memcpy(tmp.data, curpage, BLCKSZ);
256 : : applyPageRedo(tmp.data, pageData->delta, pageData->deltaLen);
257 : : if (memcmp(tmp.data, targetpage, targetLower) != 0 ||
258 : : memcmp(tmp.data + targetUpper, targetpage + targetUpper,
259 : : BLCKSZ - targetUpper) != 0)
260 : : elog(ERROR, "result of generic xlog apply does not match");
261 : : }
262 : : #endif
263 : 0 : }
264 : :
265 : : /*
266 : : * Start new generic xlog record for modifications to specified relation.
267 : : */
268 : : GenericXLogState *
269 : 0 : GenericXLogStart(Relation relation)
270 : : {
271 : 0 : GenericXLogState *state;
272 : 0 : int i;
273 : :
274 : 0 : state = (GenericXLogState *) palloc_aligned(sizeof(GenericXLogState),
275 : : PG_IO_ALIGN_SIZE,
276 : : 0);
277 [ # # # # : 0 : state->isLogged = RelationNeedsWAL(relation);
# # ]
278 : :
279 [ # # ]: 0 : for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
280 : : {
281 : 0 : state->pages[i].image = state->images[i].data;
282 : 0 : state->pages[i].buffer = InvalidBuffer;
283 : 0 : }
284 : :
285 : 0 : return state;
286 : 0 : }
287 : :
288 : : /*
289 : : * Register new buffer for generic xlog record.
290 : : *
291 : : * Returns pointer to the page's image in the GenericXLogState, which
292 : : * is what the caller should modify.
293 : : *
294 : : * If the buffer is already registered, just return its existing entry.
295 : : * (It's not very clear what to do with the flags in such a case, but
296 : : * for now we stay with the original flags.)
297 : : */
298 : : Page
299 : 0 : GenericXLogRegisterBuffer(GenericXLogState *state, Buffer buffer, int flags)
300 : : {
301 : 0 : int block_id;
302 : :
303 : : /* Search array for existing entry or first unused slot */
304 [ # # ]: 0 : for (block_id = 0; block_id < MAX_GENERIC_XLOG_PAGES; block_id++)
305 : : {
306 : 0 : GenericXLogPageData *page = &state->pages[block_id];
307 : :
308 [ # # ]: 0 : if (BufferIsInvalid(page->buffer))
309 : : {
310 : : /* Empty slot, so use it (there cannot be a match later) */
311 : 0 : page->buffer = buffer;
312 : 0 : page->flags = flags;
313 : 0 : memcpy(page->image, BufferGetPage(buffer), BLCKSZ);
314 : 0 : return (Page) page->image;
315 : : }
316 [ # # ]: 0 : else if (page->buffer == buffer)
317 : : {
318 : : /*
319 : : * Buffer is already registered. Just return the image, which is
320 : : * already prepared.
321 : : */
322 : 0 : return (Page) page->image;
323 : : }
324 [ # # ]: 0 : }
325 : :
326 [ # # # # ]: 0 : elog(ERROR, "maximum number %d of generic xlog buffers is exceeded",
327 : : MAX_GENERIC_XLOG_PAGES);
328 : : /* keep compiler quiet */
329 : 0 : return NULL;
330 : 0 : }
331 : :
332 : : /*
333 : : * Apply changes represented by GenericXLogState to the actual buffers,
334 : : * and emit a generic xlog record.
335 : : */
336 : : XLogRecPtr
337 : 0 : GenericXLogFinish(GenericXLogState *state)
338 : : {
339 : 0 : XLogRecPtr lsn;
340 : 0 : int i;
341 : :
342 [ # # ]: 0 : if (state->isLogged)
343 : : {
344 : : /* Logged relation: make xlog record in critical section. */
345 : 0 : XLogBeginInsert();
346 : :
347 : 0 : START_CRIT_SECTION();
348 : :
349 : : /*
350 : : * Compute deltas if necessary, write changes to buffers, mark buffers
351 : : * dirty, and register changes.
352 : : */
353 [ # # ]: 0 : for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
354 : : {
355 : 0 : GenericXLogPageData *pageData = &state->pages[i];
356 : 0 : Page page;
357 : 0 : PageHeader pageHeader;
358 : :
359 [ # # ]: 0 : if (BufferIsInvalid(pageData->buffer))
360 : 0 : continue;
361 : :
362 : 0 : page = BufferGetPage(pageData->buffer);
363 : 0 : pageHeader = (PageHeader) pageData->image;
364 : :
365 : : /*
366 : : * Compute delta while we still have both the unmodified page and
367 : : * the new image. Not needed if we are logging the full image.
368 : : */
369 [ # # ]: 0 : if (!(pageData->flags & GENERIC_XLOG_FULL_IMAGE))
370 : 0 : computeDelta(pageData, page, (Page) pageData->image);
371 : :
372 : : /*
373 : : * Apply the image, being careful to zero the "hole" between
374 : : * pd_lower and pd_upper in order to avoid divergence between
375 : : * actual page state and what replay would produce.
376 : : */
377 : 0 : memcpy(page, pageData->image, pageHeader->pd_lower);
378 : 0 : memset(page + pageHeader->pd_lower, 0,
379 : : pageHeader->pd_upper - pageHeader->pd_lower);
380 : 0 : memcpy(page + pageHeader->pd_upper,
381 : : pageData->image + pageHeader->pd_upper,
382 : : BLCKSZ - pageHeader->pd_upper);
383 : :
384 : 0 : MarkBufferDirty(pageData->buffer);
385 : :
386 [ # # ]: 0 : if (pageData->flags & GENERIC_XLOG_FULL_IMAGE)
387 : : {
388 : 0 : XLogRegisterBuffer(i, pageData->buffer,
389 : : REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
390 : 0 : }
391 : : else
392 : : {
393 : 0 : XLogRegisterBuffer(i, pageData->buffer, REGBUF_STANDARD);
394 : 0 : XLogRegisterBufData(i, pageData->delta, pageData->deltaLen);
395 : : }
396 [ # # ]: 0 : }
397 : :
398 : : /* Insert xlog record */
399 : 0 : lsn = XLogInsert(RM_GENERIC_ID, 0);
400 : :
401 : : /* Set LSN */
402 [ # # ]: 0 : for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
403 : : {
404 : 0 : GenericXLogPageData *pageData = &state->pages[i];
405 : :
406 [ # # ]: 0 : if (BufferIsInvalid(pageData->buffer))
407 : 0 : continue;
408 : 0 : PageSetLSN(BufferGetPage(pageData->buffer), lsn);
409 [ # # ]: 0 : }
410 [ # # ]: 0 : END_CRIT_SECTION();
411 : 0 : }
412 : : else
413 : : {
414 : : /* Unlogged relation: skip xlog-related stuff */
415 : 0 : START_CRIT_SECTION();
416 [ # # ]: 0 : for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
417 : : {
418 : 0 : GenericXLogPageData *pageData = &state->pages[i];
419 : :
420 [ # # ]: 0 : if (BufferIsInvalid(pageData->buffer))
421 : 0 : continue;
422 : 0 : memcpy(BufferGetPage(pageData->buffer),
423 : : pageData->image,
424 : : BLCKSZ);
425 : : /* We don't worry about zeroing the "hole" in this case */
426 : 0 : MarkBufferDirty(pageData->buffer);
427 [ # # ]: 0 : }
428 [ # # ]: 0 : END_CRIT_SECTION();
429 : : /* We don't have a LSN to return, in this case */
430 : 0 : lsn = InvalidXLogRecPtr;
431 : : }
432 : :
433 : 0 : pfree(state);
434 : :
435 : 0 : return lsn;
436 : 0 : }
437 : :
438 : : /*
439 : : * Abort generic xlog record construction. No changes are applied to buffers.
440 : : *
441 : : * Note: caller is responsible for releasing locks/pins on buffers, if needed.
442 : : */
443 : : void
444 : 0 : GenericXLogAbort(GenericXLogState *state)
445 : : {
446 : 0 : pfree(state);
447 : 0 : }
448 : :
449 : : /*
450 : : * Apply delta to given page image.
451 : : */
452 : : static void
453 : 0 : applyPageRedo(Page page, const char *delta, Size deltaSize)
454 : : {
455 : 0 : const char *ptr = delta;
456 : 0 : const char *end = delta + deltaSize;
457 : :
458 [ # # ]: 0 : while (ptr < end)
459 : : {
460 : 0 : OffsetNumber offset,
461 : : length;
462 : :
463 : 0 : memcpy(&offset, ptr, sizeof(offset));
464 : 0 : ptr += sizeof(offset);
465 : 0 : memcpy(&length, ptr, sizeof(length));
466 : 0 : ptr += sizeof(length);
467 : :
468 : 0 : memcpy(page + offset, ptr, length);
469 : :
470 : 0 : ptr += length;
471 : 0 : }
472 : 0 : }
473 : :
474 : : /*
475 : : * Redo function for generic xlog record.
476 : : */
477 : : void
478 : 0 : generic_redo(XLogReaderState *record)
479 : : {
480 : 0 : XLogRecPtr lsn = record->EndRecPtr;
481 : 0 : Buffer buffers[MAX_GENERIC_XLOG_PAGES];
482 : 0 : uint8 block_id;
483 : :
484 : : /* Protect limited size of buffers[] array */
485 [ # # ]: 0 : Assert(XLogRecMaxBlockId(record) < MAX_GENERIC_XLOG_PAGES);
486 : :
487 : : /* Iterate over blocks */
488 [ # # ]: 0 : for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
489 : : {
490 : 0 : XLogRedoAction action;
491 : :
492 [ # # # # ]: 0 : if (!XLogRecHasBlockRef(record, block_id))
493 : : {
494 : 0 : buffers[block_id] = InvalidBuffer;
495 : 0 : continue;
496 : : }
497 : :
498 : 0 : action = XLogReadBufferForRedo(record, block_id, &buffers[block_id]);
499 : :
500 : : /* Apply redo to given block if needed */
501 [ # # ]: 0 : if (action == BLK_NEEDS_REDO)
502 : : {
503 : 0 : Page page;
504 : 0 : PageHeader pageHeader;
505 : 0 : char *blockDelta;
506 : 0 : Size blockDeltaSize;
507 : :
508 : 0 : page = BufferGetPage(buffers[block_id]);
509 : 0 : blockDelta = XLogRecGetBlockData(record, block_id, &blockDeltaSize);
510 : 0 : applyPageRedo(page, blockDelta, blockDeltaSize);
511 : :
512 : : /*
513 : : * Since the delta contains no information about what's in the
514 : : * "hole" between pd_lower and pd_upper, set that to zero to
515 : : * ensure we produce the same page state that application of the
516 : : * logged action by GenericXLogFinish did.
517 : : */
518 : 0 : pageHeader = (PageHeader) page;
519 : 0 : memset(page + pageHeader->pd_lower, 0,
520 : : pageHeader->pd_upper - pageHeader->pd_lower);
521 : :
522 : 0 : PageSetLSN(page, lsn);
523 : 0 : MarkBufferDirty(buffers[block_id]);
524 : 0 : }
525 [ # # # ]: 0 : }
526 : :
527 : : /* Changes are done: unlock and release all buffers */
528 [ # # ]: 0 : for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
529 : : {
530 [ # # ]: 0 : if (BufferIsValid(buffers[block_id]))
531 : 0 : UnlockReleaseBuffer(buffers[block_id]);
532 : 0 : }
533 : 0 : }
534 : :
535 : : /*
536 : : * Mask a generic page before performing consistency checks on it.
537 : : */
538 : : void
539 : 0 : generic_mask(char *page, BlockNumber blkno)
540 : : {
541 : 0 : mask_page_lsn_and_checksum(page);
542 : :
543 : 0 : mask_unused_space(page);
544 : 0 : }
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