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/*-------------------------------------------------------------------------
* epoch.c
*
* Detect current epoch.
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <limits.h>
#include "access/transam.h"
#include "executor/spi.h"
#include "miscadmin.h"
#include "catalog/pg_control.h"
#include "access/xlog.h"
#include "txid.h"
/*
* do a TransactionId -> txid conversion
*/
txid txid_convert_xid(TransactionId xid, TxidEpoch *state)
{
uint64 epoch;
/* avoid issues with the the special meaning of 0 */
if (xid == InvalidTransactionId)
return MAX_INT64;
/* return special xid's as-is */
if (xid < FirstNormalTransactionId)
return xid;
/* xid can on both sides on wrap-around */
epoch = state->epoch;
if (TransactionIdPrecedes(xid, state->last_value)) {
if (xid > state->last_value)
epoch--;
} else if (TransactionIdFollows(xid, state->last_value)) {
if (xid < state->last_value)
epoch++;
}
return (epoch << 32) | xid;
}
#if PG_CONTROL_VERSION >= 820
/*
* PostgreSQl 8.2 keeps track of epoch internally.
*/
void txid_load_epoch(TxidEpoch *state, int try_write)
{
TransactionId xid;
uint32 epoch;
GetNextXidAndEpoch(&xid, &epoch);
state->epoch = epoch;
state->last_value = xid;
}
#else
/*
* For older PostgreSQL keep epoch in table.
*/
/*
* this caches the txid_epoch table.
* The struct should be updated only together with the table.
*/
static TxidEpoch epoch_state = { 0, 0 };
/*
* load values from txid_epoch table.
*/
static int load_epoch(void)
{
HeapTuple row;
TupleDesc rdesc;
bool isnull = false;
Datum tmp;
int res;
uint64 db_epoch, db_value;
res = SPI_connect();
if (res < 0)
elog(ERROR, "cannot connect to SPI");
res = SPI_execute("select epoch, last_value from txid.epoch", true, 0);
if (res != SPI_OK_SELECT)
elog(ERROR, "load_epoch: select failed?");
if (SPI_processed != 1)
elog(ERROR, "load_epoch: there must be exactly 1 row");
row = SPI_tuptable->vals[0];
rdesc = SPI_tuptable->tupdesc;
tmp = SPI_getbinval(row, rdesc, 1, &isnull);
if (isnull)
elog(ERROR, "load_epoch: epoch is NULL");
db_epoch = DatumGetInt64(tmp);
tmp = SPI_getbinval(row, rdesc, 2, &isnull);
if (isnull)
elog(ERROR, "load_epoch: last_value is NULL");
db_value = DatumGetInt64(tmp);
SPI_finish();
/*
* If the db has lesser values, then some updates were lost.
*
* Should that be special-cased? ATM just use db values.
* Thus immidiate update.
*/
epoch_state.epoch = db_epoch;
epoch_state.last_value = db_value;
return 1;
}
/*
* updates last_value and epoch, if needed
*/
static void save_epoch(void)
{
int res;
char qbuf[200];
uint64 new_epoch, new_value;
TransactionId xid = GetTopTransactionId();
TransactionId old_value;
/* store old state */
MemoryContext oldcontext = CurrentMemoryContext;
ResourceOwner oldowner = CurrentResourceOwner;
/*
* avoid changing internal values.
*/
new_value = xid;
new_epoch = epoch_state.epoch;
old_value = (TransactionId)epoch_state.last_value;
if (xid < old_value) {
if (TransactionIdFollows(xid, old_value))
new_epoch++;
else
return;
}
sprintf(qbuf, "update txid.epoch set epoch = %llu, last_value = %llu",
(unsigned long long)new_epoch,
(unsigned long long)new_value);
/*
* The update may fail in case of SERIALIZABLE transaction.
* Try to catch the error and hide it.
*/
BeginInternalSubTransaction(NULL);
PG_TRY();
{
/* do the update */
res = SPI_connect();
if (res < 0)
elog(ERROR, "cannot connect to SPI");
res = SPI_execute(qbuf, false, 0);
SPI_finish();
ReleaseCurrentSubTransaction();
}
PG_CATCH();
{
/* we expect rollback to clean up inner SPI call */
RollbackAndReleaseCurrentSubTransaction();
FlushErrorState();
res = -1; /* remember failure */
}
PG_END_TRY();
/* restore old state */
MemoryContextSwitchTo(oldcontext);
CurrentResourceOwner = oldowner;
if (res < 0)
return;
/*
* Seems the update was successful, update internal state too.
*
* There is a chance that the TX will be rollbacked, but then
* another backend will do the update, or this one at next
* checkpoint.
*/
epoch_state.epoch = new_epoch;
epoch_state.last_value = new_value;
}
static void check_epoch(int update_prio)
{
TransactionId xid = GetTopTransactionId();
TransactionId recheck, tx_next;
int ok = 1;
/* should not happen, but just in case */
if (xid == InvalidTransactionId)
return;
/* new backend */
if (epoch_state.last_value == 0)
load_epoch();
/* try to avoid concurrent access */
if (update_prio)
recheck = 50000 + 100 * (MyProcPid & 0x1FF);
else
recheck = 300000 + 1000 * (MyProcPid & 0x1FF);
/* read table */
tx_next = (TransactionId)epoch_state.last_value + recheck;
if (TransactionIdFollows(xid, tx_next))
ok = load_epoch();
/*
* check if save is needed. last_value may be updated above.
*/
tx_next = (TransactionId)epoch_state.last_value + recheck;
if (!ok || TransactionIdFollows(xid, tx_next))
save_epoch();
}
void txid_load_epoch(TxidEpoch *state, int try_write)
{
check_epoch(try_write);
state->epoch = epoch_state.epoch;
state->last_value = epoch_state.last_value;
}
#endif
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