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Commitdb03cf3

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committed
Code review/prettification for generic_xlog.c.
Improve commentary, use more specific names for the delta fields,const-ify pointer arguments where possible, avoid assuming thatinitializing only the first element of a local array will guaranteethat the remaining elements end up as we need them. (I think thatcode in generic_redo actually worked, but only because InvalidBufferis zero; this is a particularly ugly way of depending on that ...)
1 parent2dd318d commitdb03cf3

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1 file changed

+92
-67
lines changed

1 file changed

+92
-67
lines changed

‎src/backend/access/transam/generic_xlog.c

Lines changed: 92 additions & 67 deletions
Original file line numberDiff line numberDiff line change
@@ -27,12 +27,12 @@
2727
* - length of page region (OffsetNumber)
2828
* - data - the data to place into the region ('length' number of bytes)
2929
*
30-
* Unchanged regions of a page are not represented in its delta. As a
31-
*result,a delta can be more compact than the full page image. But having
32-
*anunchanged regionin the middle oftwo fragments that is smaller than
33-
*the fragmentheader (offset andlength) does not pay off in terms of the
34-
*overall size ofthe delta. For this reason, webreakfragmentsonly if
35-
*the unchanged region isbigger thanMATCH_THRESHOLD.
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 regionbetweentwo 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, wemerge adjacentfragmentsif the unchanged
35+
*region between them is<=MATCH_THRESHOLD bytes.
3636
*
3737
* The worst case for delta sizes occurs when we did not find any unchanged
3838
* region in the page. The size of the delta will be the size of the page plus
@@ -41,16 +41,16 @@
4141
*/
4242
#defineFRAGMENT_HEADER_SIZE(2 * sizeof(OffsetNumber))
4343
#defineMATCH_THRESHOLDFRAGMENT_HEADER_SIZE
44-
#defineMAX_DELTA_SIZEBLCKSZ + FRAGMENT_HEADER_SIZE
44+
#defineMAX_DELTA_SIZE(BLCKSZ + FRAGMENT_HEADER_SIZE)
4545

4646
/* Struct of generic xlog data for single page */
4747
typedefstruct
4848
{
4949
Bufferbuffer;/* registered buffer */
50-
charimage[BLCKSZ];/* copy of page image for modification */
51-
chardata[MAX_DELTA_SIZE];/* delta between page images */
52-
intdataLen;/* space consumed in data field */
5350
boolfullImage;/* are we taking a full image of this page? */
51+
intdeltaLen;/* space consumed in delta field */
52+
charimage[BLCKSZ];/* copy of page image for modification */
53+
chardelta[MAX_DELTA_SIZE];/* delta between page images */
5454
}PageData;
5555

5656
/* State of generic xlog record construction */
@@ -61,22 +61,26 @@ struct GenericXLogState
6161
};
6262

6363
staticvoidwriteFragment(PageData*pageData,OffsetNumberoffset,
64-
OffsetNumberlen,Pointerdata);
65-
staticvoidwriteDelta(PageData*pageData);
66-
staticvoidapplyPageRedo(Pagepage,Pointerdata,SizedataSize);
64+
OffsetNumberlen,constchar*data);
65+
staticvoidcomputeDelta(PageData*pageData);
66+
staticvoidapplyPageRedo(Pagepage,constchar*delta,SizedeltaSize);
67+
6768

6869
/*
69-
* Write next fragment into delta.
70+
* Write next fragment into pageData's delta.
71+
*
72+
* The fragment has the given offset and length, and data points to the
73+
* actual data (of length length).
7074
*/
7175
staticvoid
7276
writeFragment(PageData*pageData,OffsetNumberoffset,OffsetNumberlength,
73-
Pointerdata)
77+
constchar*data)
7478
{
75-
Pointerptr=pageData->data+pageData->dataLen;
79+
char*ptr=pageData->delta+pageData->deltaLen;
7680

77-
/*Check if we have enough space */
78-
Assert(pageData->dataLen+sizeof(offset)+
79-
sizeof(length)+length <=sizeof(pageData->data));
81+
/*Verify we have enough space */
82+
Assert(pageData->deltaLen+sizeof(offset)+
83+
sizeof(length)+length <=sizeof(pageData->delta));
8084

8185
/* Write fragment data */
8286
memcpy(ptr,&offset,sizeof(offset));
@@ -86,14 +90,14 @@ writeFragment(PageData *pageData, OffsetNumber offset, OffsetNumber length,
8690
memcpy(ptr,data,length);
8791
ptr+=length;
8892

89-
pageData->dataLen=ptr-pageData->data;
93+
pageData->deltaLen=ptr-pageData->delta;
9094
}
9195

9296
/*
93-
*Makedelta for given page.
97+
*Compute thedelta record for given page.
9498
*/
9599
staticvoid
96-
writeDelta(PageData*pageData)
100+
computeDelta(PageData*pageData)
97101
{
98102
Pagepage=BufferGetPage(pageData->buffer,NULL,NULL,
99103
BGP_NO_SNAPSHOT_TEST),
@@ -106,6 +110,8 @@ writeDelta(PageData *pageData)
106110
imageLower= ((PageHeader)image)->pd_lower,
107111
imageUpper= ((PageHeader)image)->pd_upper;
108112

113+
pageData->deltaLen=0;
114+
109115
for (i=0;i<BLCKSZ;i++)
110116
{
111117
boolmatch;
@@ -181,22 +187,22 @@ writeDelta(PageData *pageData)
181187
chartmp[BLCKSZ];
182188

183189
memcpy(tmp,image,BLCKSZ);
184-
applyPageRedo(tmp,pageData->data,pageData->dataLen);
185-
if (memcmp(tmp,page,pageLower)
186-
||memcmp(tmp+pageUpper,page+pageUpper,BLCKSZ-pageUpper))
190+
applyPageRedo(tmp,pageData->delta,pageData->deltaLen);
191+
if (memcmp(tmp,page,pageLower)!=0||
192+
memcmp(tmp+pageUpper,page+pageUpper,BLCKSZ-pageUpper)!=0)
187193
elog(ERROR,"result of generic xlog apply does not match");
188194
}
189195
#endif
190196
}
191197

192198
/*
193-
* Start new generic xlog record.
199+
* Start new generic xlog record for modifications to specified relation.
194200
*/
195201
GenericXLogState*
196202
GenericXLogStart(Relationrelation)
197203
{
198-
inti;
199204
GenericXLogState*state;
205+
inti;
200206

201207
state= (GenericXLogState*)palloc(sizeof(GenericXLogState));
202208

@@ -209,24 +215,30 @@ GenericXLogStart(Relation relation)
209215

210216
/*
211217
* Register new buffer for generic xlog record.
218+
*
219+
* Returns pointer to the page's image in the GenericXLogState, which
220+
* is what the caller should modify.
221+
*
222+
* If the buffer is already registered, just return its existing entry.
212223
*/
213224
Page
214225
GenericXLogRegister(GenericXLogState*state,Bufferbuffer,boolisNew)
215226
{
216227
intblock_id;
217228

218-
/*Place new buffer to unused slot in array */
229+
/*Search array for existing entry or first unused slot */
219230
for (block_id=0;block_id<MAX_GENERIC_XLOG_PAGES;block_id++)
220231
{
221232
PageData*page=&state->pages[block_id];
222233

223234
if (BufferIsInvalid(page->buffer))
224235
{
236+
/* Empty slot, so use it (there cannot be a match later) */
225237
page->buffer=buffer;
226-
memcpy(page->image,BufferGetPage(buffer,NULL,NULL,
227-
BGP_NO_SNAPSHOT_TEST),BLCKSZ);
228-
page->dataLen=0;
229238
page->fullImage=isNew;
239+
memcpy(page->image,
240+
BufferGetPage(buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST),
241+
BLCKSZ);
230242
return (Page)page->image;
231243
}
232244
elseif (page->buffer==buffer)
@@ -239,15 +251,16 @@ GenericXLogRegister(GenericXLogState *state, Buffer buffer, bool isNew)
239251
}
240252
}
241253

242-
elog(ERROR,"maximum numberof %d generic xlog buffers is exceeded",
254+
elog(ERROR,"maximum number%d of generic xlog buffers is exceeded",
243255
MAX_GENERIC_XLOG_PAGES);
244-
245256
/* keep compiler quiet */
246257
returnNULL;
247258
}
248259

249260
/*
250261
* Unregister particular buffer for generic xlog record.
262+
*
263+
* XXX this is dangerous and should go away.
251264
*/
252265
void
253266
GenericXLogUnregister(GenericXLogState*state,Bufferbuffer)
@@ -274,7 +287,8 @@ GenericXLogUnregister(GenericXLogState *state, Buffer buffer)
274287
}
275288

276289
/*
277-
* Put all changes in registered buffers to generic xlog record.
290+
* Apply changes represented by GenericXLogState to the actual buffers,
291+
* and emit a generic xlog record.
278292
*/
279293
XLogRecPtr
280294
GenericXLogFinish(GenericXLogState*state)
@@ -291,33 +305,35 @@ GenericXLogFinish(GenericXLogState *state)
291305

292306
for (i=0;i<MAX_GENERIC_XLOG_PAGES;i++)
293307
{
308+
PageData*pageData=&state->pages[i];
309+
Pagepage;
294310
chartmp[BLCKSZ];
295-
PageData*page=&state->pages[i];
296311

297-
if (BufferIsInvalid(page->buffer))
312+
if (BufferIsInvalid(pageData->buffer))
298313
continue;
299314

315+
page=BufferGetPage(pageData->buffer,NULL,NULL,
316+
BGP_NO_SNAPSHOT_TEST);
317+
300318
/* Swap current and saved page image. */
301-
memcpy(tmp,page->image,BLCKSZ);
302-
memcpy(page->image,BufferGetPage(page->buffer,NULL,NULL,
303-
BGP_NO_SNAPSHOT_TEST),BLCKSZ);
304-
memcpy(BufferGetPage(page->buffer,NULL,NULL,
305-
BGP_NO_SNAPSHOT_TEST),tmp,BLCKSZ);
319+
memcpy(tmp,pageData->image,BLCKSZ);
320+
memcpy(pageData->image,page,BLCKSZ);
321+
memcpy(page,tmp,BLCKSZ);
306322

307-
if (page->fullImage)
323+
if (pageData->fullImage)
308324
{
309325
/* A full page image does not require anything special */
310-
XLogRegisterBuffer(i,page->buffer,REGBUF_FORCE_IMAGE);
326+
XLogRegisterBuffer(i,pageData->buffer,REGBUF_FORCE_IMAGE);
311327
}
312328
else
313329
{
314330
/*
315-
* In normal mode, calculate delta and write it as data
331+
* In normal mode, calculate delta and write it asxlogdata
316332
* associated with this page.
317333
*/
318-
XLogRegisterBuffer(i,page->buffer,REGBUF_STANDARD);
319-
writeDelta(page);
320-
XLogRegisterBufData(i,page->data,page->dataLen);
334+
XLogRegisterBuffer(i,pageData->buffer,REGBUF_STANDARD);
335+
computeDelta(pageData);
336+
XLogRegisterBufData(i,pageData->delta,pageData->deltaLen);
321337
}
322338
}
323339

@@ -327,13 +343,13 @@ GenericXLogFinish(GenericXLogState *state)
327343
/* Set LSN and mark buffers dirty */
328344
for (i=0;i<MAX_GENERIC_XLOG_PAGES;i++)
329345
{
330-
PageData*page=&state->pages[i];
346+
PageData*pageData=&state->pages[i];
331347

332-
if (BufferIsInvalid(page->buffer))
348+
if (BufferIsInvalid(pageData->buffer))
333349
continue;
334-
PageSetLSN(BufferGetPage(page->buffer,NULL,NULL,
350+
PageSetLSN(BufferGetPage(pageData->buffer,NULL,NULL,
335351
BGP_NO_SNAPSHOT_TEST),lsn);
336-
MarkBufferDirty(page->buffer);
352+
MarkBufferDirty(pageData->buffer);
337353
}
338354
END_CRIT_SECTION();
339355
}
@@ -343,13 +359,15 @@ GenericXLogFinish(GenericXLogState *state)
343359
START_CRIT_SECTION();
344360
for (i=0;i<MAX_GENERIC_XLOG_PAGES;i++)
345361
{
346-
PageData*page=&state->pages[i];
362+
PageData*pageData=&state->pages[i];
347363

348-
if (BufferIsInvalid(page->buffer))
364+
if (BufferIsInvalid(pageData->buffer))
349365
continue;
350-
memcpy(BufferGetPage(page->buffer,NULL,NULL,
351-
BGP_NO_SNAPSHOT_TEST),page->image,BLCKSZ);
352-
MarkBufferDirty(page->buffer);
366+
memcpy(BufferGetPage(pageData->buffer,NULL,NULL,
367+
BGP_NO_SNAPSHOT_TEST),
368+
pageData->image,
369+
BLCKSZ);
370+
MarkBufferDirty(pageData->buffer);
353371
}
354372
END_CRIT_SECTION();
355373
}
@@ -360,7 +378,9 @@ GenericXLogFinish(GenericXLogState *state)
360378
}
361379

362380
/*
363-
* Abort generic xlog record.
381+
* Abort generic xlog record construction. No changes are applied to buffers.
382+
*
383+
* Note: caller is responsible for releasing locks/pins on buffers, if needed.
364384
*/
365385
void
366386
GenericXLogAbort(GenericXLogState*state)
@@ -372,10 +392,10 @@ GenericXLogAbort(GenericXLogState *state)
372392
* Apply delta to given page image.
373393
*/
374394
staticvoid
375-
applyPageRedo(Pagepage,Pointerdata,SizedataSize)
395+
applyPageRedo(Pagepage,constchar*delta,SizedeltaSize)
376396
{
377-
Pointerptr=data,
378-
end=data+dataSize;
397+
constchar*ptr=delta;
398+
constchar*end=delta+deltaSize;
379399

380400
while (ptr<end)
381401
{
@@ -399,10 +419,11 @@ applyPageRedo(Page page, Pointer data, Size dataSize)
399419
void
400420
generic_redo(XLogReaderState*record)
401421
{
402-
uint8block_id;
403-
Bufferbuffers[MAX_GENERIC_XLOG_PAGES]= {InvalidBuffer};
404422
XLogRecPtrlsn=record->EndRecPtr;
423+
Bufferbuffers[MAX_GENERIC_XLOG_PAGES];
424+
uint8block_id;
405425

426+
/* Protect limited size of buffers[] array */
406427
Assert(record->max_block_id<MAX_GENERIC_XLOG_PAGES);
407428

408429
/* Iterate over blocks */
@@ -411,20 +432,24 @@ generic_redo(XLogReaderState *record)
411432
XLogRedoActionaction;
412433

413434
if (!XLogRecHasBlockRef(record,block_id))
435+
{
436+
buffers[block_id]=InvalidBuffer;
414437
continue;
438+
}
415439

416440
action=XLogReadBufferForRedo(record,block_id,&buffers[block_id]);
417441

418442
/* Apply redo to given block if needed */
419443
if (action==BLK_NEEDS_REDO)
420444
{
421-
PointerblockData;
422-
SizeblockDataSize;
423445
Pagepage;
446+
char*blockDelta;
447+
SizeblockDeltaSize;
424448

425-
page=BufferGetPage(buffers[block_id],NULL,NULL,BGP_NO_SNAPSHOT_TEST);
426-
blockData=XLogRecGetBlockData(record,block_id,&blockDataSize);
427-
applyPageRedo(page,blockData,blockDataSize);
449+
page=BufferGetPage(buffers[block_id],NULL,NULL,
450+
BGP_NO_SNAPSHOT_TEST);
451+
blockDelta=XLogRecGetBlockData(record,block_id,&blockDeltaSize);
452+
applyPageRedo(page,blockDelta,blockDeltaSize);
428453

429454
PageSetLSN(page,lsn);
430455
MarkBufferDirty(buffers[block_id]);

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