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When possible, avoid taking wal file read-lock 0 in sqlite3_snapshot_get().

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FossilOrigin-Name: 34b6ac3d76dbc6819778ec2a0f81cbcdcc0cd1a6303381d97f1c479e4ecdd132
This commit is contained in:
dan
2024-09-26 18:02:17 +00:00
parent 8cd30e3f5b
commit d0720eee5e
5 changed files with 72 additions and 13 deletions
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18
src/wal.c
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@@ -541,6 +541,7 @@ struct Wal {
#endif
#ifdef SQLITE_ENABLE_SNAPSHOT
WalIndexHdr *pSnapshot; /* Start transaction here if not NULL */
int bGetSnapshot; /* Transaction opened for sqlite3_get_snapshot() */
#endif
#ifdef SQLITE_ENABLE_SETLK_TIMEOUT
sqlite3 *db;
@@ -3097,7 +3098,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int *pCnt){
SEH_INJECT_FAULT;
if( !useWal && AtomicLoad(&pInfo->nBackfill)==pWal->hdr.mxFrame
#ifdef SQLITE_ENABLE_SNAPSHOT
&& (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)
&& ((pWal->bGetSnapshot==0 && pWal->pSnapshot==0) || pWal->hdr.mxFrame==0)
#endif
){
/* The WAL has been completely backfilled (or it is empty).
@@ -4497,7 +4498,20 @@ void sqlite3WalSnapshotOpen(
Wal *pWal,
sqlite3_snapshot *pSnapshot
){
pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
if( pSnapshot && ((WalIndexHdr*)pSnapshot)->iVersion==0 ){
/* iVersion==0 means that this is a call to sqlite3_snapshot_get(). In
** this case set the bGetSnapshot flag so that if the call to
** sqlite3_snapshot_get() is about to read transaction on this wal
** file, it does not take read-lock 0 if the wal file has been completely
** checkpointed. Taking read-lock 0 would work, but then it would be
** possible for a subsequent writer to destroy the snapshot even while
** this connection is holding its read-transaction open. This is contrary
** to user expectations, so we avoid it by not taking read-lock 0. */
pWal->bGetSnapshot = 1;
}else{
pWal->pSnapshot = (WalIndexHdr*)pSnapshot;
pWal->bGetSnapshot = 0;
}
}
/*