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subtrans.c
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1 /*-------------------------------------------------------------------------
2  *
3  * subtrans.c
4  * PostgreSQL subtransaction-log manager
5  *
6  * The pg_subtrans manager is a pg_xact-like manager that stores the parent
7  * transaction Id for each transaction. It is a fundamental part of the
8  * nested transactions implementation. A main transaction has a parent
9  * of InvalidTransactionId, and each subtransaction has its immediate parent.
10  * The tree can easily be walked from child to parent, but not in the
11  * opposite direction.
12  *
13  * This code is based on xact.c, but the robustness requirements
14  * are completely different from pg_xact, because we only need to remember
15  * pg_subtrans information for currently-open transactions. Thus, there is
16  * no need to preserve data over a crash and restart.
17  *
18  * There are no XLOG interactions since we do not care about preserving
19  * data across crashes. During database startup, we simply force the
20  * currently-active page of SUBTRANS to zeroes.
21  *
22  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
23  * Portions Copyright (c) 1994, Regents of the University of California
24  *
25  * src/backend/access/transam/subtrans.c
26  *
27  *-------------------------------------------------------------------------
28  */
29 #include "postgres.h"
30 
31 #include "access/slru.h"
32 #include "access/subtrans.h"
33 #include "access/transam.h"
34 #include "pg_trace.h"
35 #include "utils/snapmgr.h"
36 
37 
38 /*
39  * Defines for SubTrans page sizes. A page is the same BLCKSZ as is used
40  * everywhere else in Postgres.
41  *
42  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
43  * SubTrans page numbering also wraps around at
44  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
45  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
46  * explicit notice of that fact in this module, except when comparing segment
47  * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes) and zeroing
48  * them in StartupSUBTRANS.
49  */
50 
51 /* We need four bytes per xact */
52 #define SUBTRANS_XACTS_PER_PAGE (BLCKSZ / sizeof(TransactionId))
53 
54 #define TransactionIdToPage(xid) ((xid) / (TransactionId) SUBTRANS_XACTS_PER_PAGE)
55 #define TransactionIdToEntry(xid) ((xid) % (TransactionId) SUBTRANS_XACTS_PER_PAGE)
56 
57 
58 /*
59  * Link to shared-memory data structures for SUBTRANS control
60  */
62 
63 #define SubTransCtl (&SubTransCtlData)
64 
65 
66 static int ZeroSUBTRANSPage(int pageno);
67 static bool SubTransPagePrecedes(int page1, int page2);
68 
69 
70 /*
71  * Record the parent of a subtransaction in the subtrans log.
72  */
73 void
75 {
76  int pageno = TransactionIdToPage(xid);
77  int entryno = TransactionIdToEntry(xid);
78  int slotno;
79  TransactionId *ptr;
80 
82  Assert(TransactionIdFollows(xid, parent));
83 
84  LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
85 
86  slotno = SimpleLruReadPage(SubTransCtl, pageno, true, xid);
87  ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
88  ptr += entryno;
89 
90  /*
91  * It's possible we'll try to set the parent xid multiple times but we
92  * shouldn't ever be changing the xid from one valid xid to another valid
93  * xid, which would corrupt the data structure.
94  */
95  if (*ptr != parent)
96  {
98  *ptr = parent;
99  SubTransCtl->shared->page_dirty[slotno] = true;
100  }
101 
102  LWLockRelease(SubtransSLRULock);
103 }
104 
105 /*
106  * Interrogate the parent of a transaction in the subtrans log.
107  */
110 {
111  int pageno = TransactionIdToPage(xid);
112  int entryno = TransactionIdToEntry(xid);
113  int slotno;
114  TransactionId *ptr;
115  TransactionId parent;
116 
117  /* Can't ask about stuff that might not be around anymore */
119 
120  /* Bootstrap and frozen XIDs have no parent */
121  if (!TransactionIdIsNormal(xid))
122  return InvalidTransactionId;
123 
124  /* lock is acquired by SimpleLruReadPage_ReadOnly */
125 
126  slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid);
127  ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
128  ptr += entryno;
129 
130  parent = *ptr;
131 
132  LWLockRelease(SubtransSLRULock);
133 
134  return parent;
135 }
136 
137 /*
138  * SubTransGetTopmostTransaction
139  *
140  * Returns the topmost transaction of the given transaction id.
141  *
142  * Because we cannot look back further than TransactionXmin, it is possible
143  * that this function will lie and return an intermediate subtransaction ID
144  * instead of the true topmost parent ID. This is OK, because in practice
145  * we only care about detecting whether the topmost parent is still running
146  * or is part of a current snapshot's list of still-running transactions.
147  * Therefore, any XID before TransactionXmin is as good as any other.
148  */
151 {
152  TransactionId parentXid = xid,
153  previousXid = xid;
154 
155  /* Can't ask about stuff that might not be around anymore */
157 
158  while (TransactionIdIsValid(parentXid))
159  {
160  previousXid = parentXid;
161  if (TransactionIdPrecedes(parentXid, TransactionXmin))
162  break;
163  parentXid = SubTransGetParent(parentXid);
164 
165  /*
166  * By convention the parent xid gets allocated first, so should always
167  * precede the child xid. Anything else points to a corrupted data
168  * structure that could lead to an infinite loop, so exit.
169  */
170  if (!TransactionIdPrecedes(parentXid, previousXid))
171  elog(ERROR, "pg_subtrans contains invalid entry: xid %u points to parent xid %u",
172  previousXid, parentXid);
173  }
174 
175  Assert(TransactionIdIsValid(previousXid));
176 
177  return previousXid;
178 }
179 
180 
181 /*
182  * Initialization of shared memory for SUBTRANS
183  */
184 Size
186 {
188 }
189 
190 void
192 {
193  SubTransCtl->PagePrecedes = SubTransPagePrecedes;
195  SubtransSLRULock, "pg_subtrans",
198 }
199 
200 /*
201  * This func must be called ONCE on system install. It creates
202  * the initial SUBTRANS segment. (The SUBTRANS directory is assumed to
203  * have been created by the initdb shell script, and SUBTRANSShmemInit
204  * must have been called already.)
205  *
206  * Note: it's not really necessary to create the initial segment now,
207  * since slru.c would create it on first write anyway. But we may as well
208  * do it to be sure the directory is set up correctly.
209  */
210 void
212 {
213  int slotno;
214 
215  LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
216 
217  /* Create and zero the first page of the subtrans log */
218  slotno = ZeroSUBTRANSPage(0);
219 
220  /* Make sure it's written out */
222  Assert(!SubTransCtl->shared->page_dirty[slotno]);
223 
224  LWLockRelease(SubtransSLRULock);
225 }
226 
227 /*
228  * Initialize (or reinitialize) a page of SUBTRANS to zeroes.
229  *
230  * The page is not actually written, just set up in shared memory.
231  * The slot number of the new page is returned.
232  *
233  * Control lock must be held at entry, and will be held at exit.
234  */
235 static int
236 ZeroSUBTRANSPage(int pageno)
237 {
238  return SimpleLruZeroPage(SubTransCtl, pageno);
239 }
240 
241 /*
242  * This must be called ONCE during postmaster or standalone-backend startup,
243  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
244  *
245  * oldestActiveXID is the oldest XID of any prepared transaction, or nextXid
246  * if there are none.
247  */
248 void
250 {
251  FullTransactionId nextXid;
252  int startPage;
253  int endPage;
254 
255  /*
256  * Since we don't expect pg_subtrans to be valid across crashes, we
257  * initialize the currently-active page(s) to zeroes during startup.
258  * Whenever we advance into a new page, ExtendSUBTRANS will likewise zero
259  * the new page without regard to whatever was previously on disk.
260  */
261  LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
262 
263  startPage = TransactionIdToPage(oldestActiveXID);
264  nextXid = ShmemVariableCache->nextXid;
265  endPage = TransactionIdToPage(XidFromFullTransactionId(nextXid));
266 
267  while (startPage != endPage)
268  {
269  (void) ZeroSUBTRANSPage(startPage);
270  startPage++;
271  /* must account for wraparound */
272  if (startPage > TransactionIdToPage(MaxTransactionId))
273  startPage = 0;
274  }
275  (void) ZeroSUBTRANSPage(startPage);
276 
277  LWLockRelease(SubtransSLRULock);
278 }
279 
280 /*
281  * Perform a checkpoint --- either during shutdown, or on-the-fly
282  */
283 void
285 {
286  /*
287  * Write dirty SUBTRANS pages to disk
288  *
289  * This is not actually necessary from a correctness point of view. We do
290  * it merely to improve the odds that writing of dirty pages is done by
291  * the checkpoint process and not by backends.
292  */
293  TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(true);
295  TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(true);
296 }
297 
298 
299 /*
300  * Make sure that SUBTRANS has room for a newly-allocated XID.
301  *
302  * NB: this is called while holding XidGenLock. We want it to be very fast
303  * most of the time; even when it's not so fast, no actual I/O need happen
304  * unless we're forced to write out a dirty subtrans page to make room
305  * in shared memory.
306  */
307 void
309 {
310  int pageno;
311 
312  /*
313  * No work except at first XID of a page. But beware: just after
314  * wraparound, the first XID of page zero is FirstNormalTransactionId.
315  */
316  if (TransactionIdToEntry(newestXact) != 0 &&
318  return;
319 
320  pageno = TransactionIdToPage(newestXact);
321 
322  LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
323 
324  /* Zero the page */
325  ZeroSUBTRANSPage(pageno);
326 
327  LWLockRelease(SubtransSLRULock);
328 }
329 
330 
331 /*
332  * Remove all SUBTRANS segments before the one holding the passed transaction ID
333  *
334  * oldestXact is the oldest TransactionXmin of any running transaction. This
335  * is called only during checkpoint.
336  */
337 void
339 {
340  int cutoffPage;
341 
342  /*
343  * The cutoff point is the start of the segment containing oldestXact. We
344  * pass the *page* containing oldestXact to SimpleLruTruncate. We step
345  * back one transaction to avoid passing a cutoff page that hasn't been
346  * created yet in the rare case that oldestXact would be the first item on
347  * a page and oldestXact == next XID. In that case, if we didn't subtract
348  * one, we'd trigger SimpleLruTruncate's wraparound detection.
349  */
350  TransactionIdRetreat(oldestXact);
351  cutoffPage = TransactionIdToPage(oldestXact);
352 
353  SimpleLruTruncate(SubTransCtl, cutoffPage);
354 }
355 
356 
357 /*
358  * Decide whether a SUBTRANS page number is "older" for truncation purposes.
359  * Analogous to CLOGPagePrecedes().
360  */
361 static bool
362 SubTransPagePrecedes(int page1, int page2)
363 {
364  TransactionId xid1;
365  TransactionId xid2;
366 
367  xid1 = ((TransactionId) page1) * SUBTRANS_XACTS_PER_PAGE;
368  xid1 += FirstNormalTransactionId + 1;
369  xid2 = ((TransactionId) page2) * SUBTRANS_XACTS_PER_PAGE;
370  xid2 += FirstNormalTransactionId + 1;
371 
372  return (TransactionIdPrecedes(xid1, xid2) &&
374 }
static int ZeroSUBTRANSPage(int pageno)
Definition: subtrans.c:236
#define TransactionIdToEntry(xid)
Definition: subtrans.c:55
#define TransactionIdEquals(id1, id2)
Definition: transam.h:43
bool TransactionIdFollows(TransactionId id1, TransactionId id2)
Definition: transam.c:334
uint32 TransactionId
Definition: c.h:587
void SUBTRANSShmemInit(void)
Definition: subtrans.c:191
TransactionId SubTransGetTopmostTransaction(TransactionId xid)
Definition: subtrans.c:150
TransactionId SubTransGetParent(TransactionId xid)
Definition: subtrans.c:109
void SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
Definition: slru.c:1226
bool TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
Definition: transam.c:349
void TruncateSUBTRANS(TransactionId oldestXact)
Definition: subtrans.c:338
#define TransactionIdRetreat(dest)
Definition: transam.h:141
FullTransactionId nextXid
Definition: transam.h:220
Size SimpleLruShmemSize(int nslots, int nlsns)
Definition: slru.c:156
void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns, LWLock *ctllock, const char *subdir, int tranche_id, SyncRequestHandler sync_handler)
Definition: slru.c:187
#define XidFromFullTransactionId(x)
Definition: transam.h:48
TransactionId TransactionXmin
Definition: snapmgr.c:112
#define SubTransCtl
Definition: subtrans.c:63
void LWLockRelease(LWLock *lock)
Definition: lwlock.c:1803
void BootStrapSUBTRANS(void)
Definition: subtrans.c:211
#define FirstNormalTransactionId
Definition: transam.h:34
#define ERROR
Definition: elog.h:46
void ExtendSUBTRANS(TransactionId newestXact)
Definition: subtrans.c:308
int SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok, TransactionId xid)
Definition: slru.c:395
VariableCache ShmemVariableCache
Definition: varsup.c:34
void SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied)
Definition: slru.c:1156
#define InvalidTransactionId
Definition: transam.h:31
void SimpleLruWritePage(SlruCtl ctl, int slotno)
Definition: slru.c:614
bool TransactionIdPrecedes(TransactionId id1, TransactionId id2)
Definition: transam.c:300
#define MaxTransactionId
Definition: transam.h:35
static bool SubTransPagePrecedes(int page1, int page2)
Definition: subtrans.c:362
#define TransactionIdToPage(xid)
Definition: subtrans.c:54
int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
Definition: slru.c:495
#define NUM_SUBTRANS_BUFFERS
Definition: subtrans.h:15
void StartupSUBTRANS(TransactionId oldestActiveXID)
Definition: subtrans.c:249
#define Assert(condition)
Definition: c.h:804
size_t Size
Definition: c.h:540
void CheckPointSUBTRANS(void)
Definition: subtrans.c:284
bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1199
Size SUBTRANSShmemSize(void)
Definition: subtrans.c:185
#define SUBTRANS_XACTS_PER_PAGE
Definition: subtrans.c:52
#define elog(elevel,...)
Definition: elog.h:232
#define SlruPagePrecedesUnitTests(ctl, per_page)
Definition: slru.h:156
#define TransactionIdIsValid(xid)
Definition: transam.h:41
#define TransactionIdIsNormal(xid)
Definition: transam.h:42
void SubTransSetParent(TransactionId xid, TransactionId parent)
Definition: subtrans.c:74
int SimpleLruZeroPage(SlruCtl ctl, int pageno)
Definition: slru.c:280
static SlruCtlData SubTransCtlData
Definition: subtrans.c:61