database/sqlite
1
0
Fork 0
mirror of https://github.com/sqlite/sqlite.git synced 2025-08-05 15:55:57 +03:00
Code Activity

Update sqlite3_snapshot_open() to reduce the chances of reading a corrupt snapshot created by a checkpointer process exiting unexpectedly.

Browse Source 
FossilOrigin-Name: 7315f7cbf4179aadda0f1a0baa1526a9b9f9729f
This commit is contained in:
dan
2015-12-09 20:05:27 +00:00
parent ee3b7a2767
commit 65127cd57d
5 changed files with 130 additions and 32 deletions
Download Patch File Download Diff File Expand all files Collapse all files

66
src/wal.c
View File

@@ -2275,6 +2275,9 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
{
if( (pWal->readOnly & WAL_SHM_RDONLY)==0
&& (mxReadMark<mxFrame || mxI==0)
#ifdef SQLITE_ENABLE_SNAPSHOT
&& pWal->pSnapshot==0
#endif
){
for(i=1; i<WAL_NREADER; i++){
rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
@@ -2289,6 +2292,9 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
}
}
if( mxI==0 ){
#ifdef SQLITE_ENABLE_SNAPSHOT
if( pWal->pSnapshot ) return SQLITE_BUSY_SNAPSHOT;
#endif
assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTLOCK;
}
@@ -2383,22 +2389,54 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
#ifdef SQLITE_ENABLE_SNAPSHOT
if( rc==SQLITE_OK ){
if( pSnapshot && memcmp(pSnapshot, &pWal->hdr, sizeof(WalIndexHdr)) ){
/* At this point the client has a lock on an aReadMark[] slot holding
** a value equal to or smaller than pSnapshot->mxFrame. This client
** did not populate the aReadMark[] slot. pWal->hdr is populated with
** the wal-index header for the snapshot currently at the head of the
** wal file, which is different from pSnapshot.
**
** The presence of the aReadMark[] slot entry makes it very likely
** that either there is currently another read-transaction open on
** pSnapshot, or that there has been one more recently than the last
** checkpoint of any frames greater than pSnapshot->mxFrame was
** started. There is an exception though: client 1 may have called
** walTryBeginRead and started to open snapshot pSnapshot, setting
** the aReadMark[] slot to do so. At the same time, client 2 may
** have committed a new snapshot to disk and started a checkpoint.
** In this circumstance client 1 does not end up reading pSnapshot,
** but may leave the aReadMark[] slot populated.
**
** The race condition above is difficult to detect. One approach would
** be to check the aReadMark[] slot for another client. But this is
** prone to false-positives from other snapshot clients. And there
** is no equivalent to xCheckReservedLock() for wal locks. Another
** approach would be to take the checkpointer lock and check that
** fewer than pSnapshot->mxFrame frames have been checkpointed. But
** that does not account for checkpointer processes that failed after
** checkpointing frames but before updating WalCkptInfo.nBackfill.
** And it would mean that this function would block on checkpointers
** and vice versa.
**
** TODO: For now, this race condition is ignored.
*/
volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
rc = walLockShared(pWal, WAL_READ_LOCK(0));
if( rc==SQLITE_OK ){
if( pInfo->nBackfill<=pSnapshot->mxFrame
&& pSnapshot->aSalt[0]==pWal->hdr.aSalt[0]
&& pSnapshot->aSalt[1]==pWal->hdr.aSalt[1]
){
assert( pWal->readLock>0 );
assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );
memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
*pChanged = bChanged;
}else{
rc = SQLITE_BUSY_SNAPSHOT;
}
walUnlockShared(pWal, WAL_READ_LOCK(0));
assert( pWal->readLock>0 );
assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );
/* Check that the wal file has not been wrapped. Assuming it has not,
** overwrite pWal->hdr with *pSnapshot and set *pChanged as appropriate
** for opening the snapshot. Or, if the wal file has been wrapped
** since pSnapshot was written, return SQLITE_BUSY_SNAPSHOT. */
if( pSnapshot->aSalt[0]==pWal->hdr.aSalt[0]
&& pSnapshot->aSalt[1]==pWal->hdr.aSalt[1]
){
memcpy(&pWal->hdr, pSnapshot, sizeof(WalIndexHdr));
*pChanged = bChanged;
}else{
rc = SQLITE_BUSY_SNAPSHOT;
}
if( rc!=SQLITE_OK ){
sqlite3WalEndReadTransaction(pWal);
}