Many hyperlinks are disabled.
Use anonymous login
to enable hyperlinks.
Overview
| Comment: | Clarify the purpose of a test for a race-condition in sqlite3WalBeginReadTransaction(). |
|---|---|
| Downloads: | Tarball | ZIP archive |
| Timelines: | family | ancestors | descendants | both | trunk |
| Files: | files | file ages | folders |
| SHA1: |
c041c6a9786bc9ebb82527f7a2c96d25 |
| User & Date: | dan 2010-06-05 18:34:26.000 |
| Original Comment: | Clarify the purpose of a test for a race-condition in walIndexReadHdr(). |
Context
|
2010-06-05
| ||
| 19:18 | Add further test cases for the logic in sqlite3WalBeginReadTransaction(). (check-in: a49713db39 user: dan tags: trunk) | |
| 18:34 | Clarify the purpose of a test for a race-condition in sqlite3WalBeginReadTransaction(). (check-in: c041c6a978 user: dan tags: trunk) | |
| 18:12 | Remove a condition from sqlite3WalRead() that is unreachable as of the changes to clear entries out of the wal-index hash tables on transaction or savepoint rollback. (check-in: 394204735a user: dan tags: trunk) | |
Changes
Changes to src/wal.c.
| ︙ | ︙ | |||
1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 |
pInfo = (volatile WalCkptInfo*)&pHdr[2];
assert( pInfo==walCkptInfo(pWal) );
if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame ){
/* The WAL has been completely backfilled (or it is empty).
** and can be safely ignored.
*/
rc = walLockShared(pWal, WAL_READ_LOCK(0));
if( rc==SQLITE_OK ){
if( memcmp(pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
/* It is not safe to allow the reader to continue here if frames
** may have been appended to the log before READ_LOCK(0) was obtained.
** When holding READ_LOCK(0), the reader ignores the entire log file,
** which implies that the database file contains a trustworthy
** snapshoT. Since holding READ_LOCK(0) prevents a checkpoint from
| > | 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 |
pInfo = (volatile WalCkptInfo*)&pHdr[2];
assert( pInfo==walCkptInfo(pWal) );
if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame ){
/* The WAL has been completely backfilled (or it is empty).
** and can be safely ignored.
*/
rc = walLockShared(pWal, WAL_READ_LOCK(0));
sqlite3OsShmBarrier(pWal->pDbFd);
if( rc==SQLITE_OK ){
if( memcmp(pHdr, &pWal->hdr, sizeof(WalIndexHdr)) ){
/* It is not safe to allow the reader to continue here if frames
** may have been appended to the log before READ_LOCK(0) was obtained.
** When holding READ_LOCK(0), the reader ignores the entire log file,
** which implies that the database file contains a trustworthy
** snapshoT. Since holding READ_LOCK(0) prevents a checkpoint from
|
| ︙ | ︙ | |||
1813 1814 1815 1816 1817 1818 1819 1820 |
}
}
rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
if( rc ){
return rc==SQLITE_BUSY ? WAL_RETRY : rc;
}
if( pInfo->aReadMark[mxI]!=mxReadMark
| > > > > > > > > > > > > > > > > > > > > > | < | 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 |
}
}
rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
if( rc ){
return rc==SQLITE_BUSY ? WAL_RETRY : rc;
}
/* Now that the read-lock has been obtained, check that neither the
** value in the aReadMark[] array or the contents of the wal-index
** header have changed.
**
** It is necessary to check that the wal-index header did not change
** between the time it was read and when the shared-lock was obtained
** on WAL_READ_LOCK(mxI) was obtained to account for the possibility
** that the log file may have been wrapped by a writer, or that frames
** that occur later in the log than pWal->hdr.mxFrame may have been
** copied into the database by a checkpointer. If either of these things
** happened, then reading the database with the current value of
** pWal->hdr.mxFrame risks reading a corrupted snapshot. So, retry
** instead.
**
** This does not guarantee that the copy wal-index header is up to
** date before proceeding. This would not be possible without somehow
** blocking writers. It only guarantees that a damaging checkpoint or
** log-wrap (either of which would require an exclusive lock on
** WAL_READ_LOCK(mxI)) has not occurred since the snapshot was valid.
*/
sqlite3OsShmBarrier(pWal->pDbFd);
if( pInfo->aReadMark[mxI]!=mxReadMark
|| memcmp(pHdr, &pWal->hdr, sizeof(WalIndexHdr))
){
walUnlockShared(pWal, WAL_READ_LOCK(mxI));
return WAL_RETRY;
}else{
pWal->readLock = mxI;
}
}
|
| ︙ | ︙ |