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Overview
Comment: | Make sure the wal-index reader detects an incorrect version number even if it had to hold a lock in order to read the wal-index. Also, expand and enhance various comments in wal.c. |
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Timelines: | family | ancestors | descendants | both | trunk |
Files: | files | file ages | folders |
SHA1: |
2e6a462cebc05bfd4648d26dd5ae70b6 |
User & Date: | drh 2010-06-24 02:46:49.000 |
Context
2010-06-24
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10:50 | Add coverage tests. Remove a NEVER macro from pager.c, as the condition can now be true in wal mode. (check-in: 7aac9ad6dd user: dan tags: trunk) | |
02:46 | Make sure the wal-index reader detects an incorrect version number even if it had to hold a lock in order to read the wal-index. Also, expand and enhance various comments in wal.c. (check-in: 2e6a462ceb user: drh tags: trunk) | |
2010-06-23
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22:00 | Fix and/or improve comments in wal.c. No code changes. (check-in: ee9991be08 user: drh tags: trunk) | |
Changes
Changes to src/wal.c.
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1659 1660 1661 1662 1663 1664 1665 | /* The header was successfully read. Return zero. */ return 0; } /* ** Read the wal-index header from the wal-index and into pWal->hdr. | | | < < < < < < | | < < < | 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 | /* The header was successfully read. Return zero. */ return 0; } /* ** Read the wal-index header from the wal-index and into pWal->hdr. ** If the wal-header appears to be corrupt, try to reconstruct the ** wal-index from the WAL before returning. ** ** Set *pChanged to 1 if the wal-index header value in pWal->hdr is ** changed by this opertion. If pWal->hdr is unchanged, set *pChanged ** to 0. ** ** If the wal-index header is successfully read, return SQLITE_OK. ** Otherwise an SQLite error code. */ static int walIndexReadHdr(Wal *pWal, int *pChanged){ int rc; /* Return code */ int badHdr; /* True if a header read failed */ volatile u32 *page0; /* Chunk of wal-index containing header */ /* Ensure that page 0 of the wal-index (the page that contains the ** wal-index header) is mapped. Return early if an error occurs here. */ assert( pChanged ); rc = walIndexPage(pWal, 0, &page0); if( rc!=SQLITE_OK ){ return rc; }; assert( page0 || pWal->writeLock==0 ); /* If the first page of the wal-index has been mapped, try to read the ** wal-index header immediately, without holding any lock. This usually ** works, but may fail if the wal-index header is corrupt or currently ** being modified by another thread or process. */ badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1); /* If the first attempt failed, it might have been due to a race ** with a writer. So get a WRITE lock and try again. */ assert( badHdr==0 || pWal->writeLock==0 ); if( badHdr && SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) ){ pWal->writeLock = 1; |
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1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 | rc = walIndexRecover(pWal); *pChanged = 1; } } pWal->writeLock = 0; walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1); } return rc; } /* ** This is the value that walTryBeginRead returns when it needs to ** be retried. */ #define WAL_RETRY (-1) /* ** Attempt to start a read transaction. This might fail due to a race or ** other transient condition. When that happens, it returns WAL_RETRY to ** indicate to the caller that it is safe to retry immediately. ** | > > > > > > > > | > > > > > > > > > > > > > > > > > > > > > > | 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 | rc = walIndexRecover(pWal); *pChanged = 1; } } pWal->writeLock = 0; walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1); } /* If the header is read successfully, check the version number to make ** sure the wal-index was not constructed with some future format that ** this version of SQLite cannot understand. */ if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){ rc = SQLITE_CANTOPEN_BKPT; } return rc; } /* ** This is the value that walTryBeginRead returns when it needs to ** be retried. */ #define WAL_RETRY (-1) /* ** Attempt to start a read transaction. This might fail due to a race or ** other transient condition. When that happens, it returns WAL_RETRY to ** indicate to the caller that it is safe to retry immediately. ** ** On success return SQLITE_OK. On a permanent failure (such an ** I/O error or an SQLITE_BUSY because another process is running ** recovery) return a positive error code. ** ** The useWal parameter is true to force the use of the WAL and disable ** the case where the WAL is bypassed because it has been completely ** checkpointed. If useWal==0 then this routine calls walIndexReadHdr() ** to make a copy of the wal-index header into pWal->hdr. If the ** wal-index header has changed, *pChanged is set to 1 (as an indication ** to the caller that the local paget cache is obsolete and needs to be ** flushed.) When useWal==1, the wal-index header is assumed to already ** be loaded and the pChanged parameter is unused. ** ** The caller must set the cnt parameter to the number of prior calls to ** this routine during the current read attempt that returned WAL_RETRY. ** This routine will start taking more aggressive measures to clear the ** race conditions after multiple WAL_RETRY returns, and after an excessive ** number of errors will ultimately return SQLITE_PROTOCOL. The ** SQLITE_PROTOCOL return indicates that some other process has gone rogue ** and is not honoring the locking protocol. There is a vanishingly small ** chance that SQLITE_PROTOCOL could be returned because of a run of really ** bad luck when there is lots of contention for the wal-index, but that ** possibility is so small that it can be safely neglected, we believe. ** ** On success, this routine obtains a read lock on ** WAL_READ_LOCK(pWal->readLock). The pWal->readLock integer is ** in the range 0 <= pWal->readLock < WAL_NREADER. If pWal->readLock==(-1) ** that means the Wal does not hold any read lock. The reader must not ** access any database page that is modified by a WAL frame up to and ** including frame number aReadMark[pWal->readLock]. The reader will ** use WAL frames up to and including pWal->hdr.mxFrame if pWal->readLock>0 ** Or if pWal->readLock==0, then the reader will ignore the WAL ** completely and get all content directly from the database file. ** If the useWal parameter is 1 then the WAL will never be ignored and ** this routine will always set pWal->readLock>0 on success. ** When the read transaction is completed, the caller must release the ** lock on WAL_READ_LOCK(pWal->readLock) and set pWal->readLock to -1. ** ** This routine uses the nBackfill and aReadMark[] fields of the header ** to select a particular WAL_READ_LOCK() that strives to let the ** checkpoint process do as much work as possible. This routine might ** update values of the aReadMark[] array in the header, but if it does |
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1792 1793 1794 1795 1796 1797 1798 | if( rc==SQLITE_OK ){ walUnlockShared(pWal, WAL_RECOVER_LOCK); rc = WAL_RETRY; }else if( rc==SQLITE_BUSY ){ rc = SQLITE_BUSY_RECOVERY; } } | < | | > | 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 | if( rc==SQLITE_OK ){ walUnlockShared(pWal, WAL_RECOVER_LOCK); rc = WAL_RETRY; }else if( rc==SQLITE_BUSY ){ rc = SQLITE_BUSY_RECOVERY; } } if( rc!=SQLITE_OK ){ return rc; } } 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. */ |
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1970 1971 1972 1973 1974 1975 1976 | int iHash; /* Used to loop through N hash tables */ /* This routine is only be called from within a read transaction. */ assert( pWal->readLock>=0 || pWal->lockError ); /* If the "last page" field of the wal-index header snapshot is 0, then ** no data will be read from the wal under any circumstances. Return early | | | | | | 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 | int iHash; /* Used to loop through N hash tables */ /* This routine is only be called from within a read transaction. */ assert( pWal->readLock>=0 || pWal->lockError ); /* If the "last page" field of the wal-index header snapshot is 0, then ** no data will be read from the wal under any circumstances. Return early ** in this case as an optimization. Likewise, if pWal->readLock==0, ** then the WAL is ignored by the reader so return early, as if the ** WAL were empty. */ if( iLast==0 || pWal->readLock==0 ){ *pInWal = 0; return SQLITE_OK; } /* Search the hash table or tables for an entry matching page number ** pgno. Each iteration of the following for() loop searches one ** hash table (each hash table indexes up to HASHTABLE_NPAGE frames). ** ** This code might run concurrently to the code in walIndexAppend() ** that adds entries to the wal-index (and possibly to this hash ** table). This means the value just read from the hash ** slot (aHash[iKey]) may have been added before or after the ** current read transaction was opened. Values added after the ** read transaction was opened may have been written incorrectly - ** i.e. these slots may contain garbage data. However, we assume ** that any slots written before the current read transaction was |
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