/* ** 2010 February 1 ** ** The author disclaims copyright to this source code. In place of ** a legal notice, here is a blessing: ** ** May you do good and not evil. ** May you find forgiveness for yourself and forgive others. ** May you share freely, never taking more than you give. ** ************************************************************************* ** ** This file contains the implementation of a write-ahead log (WAL) used in ** "journal_mode=WAL" mode. ** ** WRITE-AHEAD LOG (WAL) FILE FORMAT ** ** A WAL file consists of a header followed by zero or more "frames". ** Each frame records the revised content of a single page from the ** database file. All changes to the database are recorded by writing ** frames into the WAL. Transactions commit when a frame is written that ** contains a commit marker. A single WAL can and usually does record ** multiple transactions. Periodically, the content of the WAL is ** transferred back into the database file in an operation called a ** "checkpoint". ** ** A single WAL file can be used multiple times. In other words, the ** WAL can fill up with frames and then be checkpointed and then new ** frames can overwrite the old ones. A WAL always grows from beginning ** toward the end. Checksums and counters attached to each frame are ** used to determine which frames within the WAL are valid and which ** are leftovers from prior checkpoints. ** ** The WAL header is 24 bytes in size and consists of the following six ** big-endian 32-bit unsigned integer values: ** ** 0: Magic number. 0x377f0682 (big endian) ** 4: File format version. Currently 3007000 ** 8: Database page size. Example: 1024 ** 12: Checkpoint sequence number ** 16: Salt-1, random integer incremented with each checkpoint ** 20: Salt-2, a different random integer changing with each ckpt ** ** Immediately following the wal-header are zero or more frames. Each ** frame consists of a 24-byte frame-header followed by a bytes ** of page data. The frame-header is broken into 6 big-endian 32-bit unsigned ** integer values, as follows: ** ** 0: Page number. ** 4: For commit records, the size of the database image in pages ** after the commit. For all other records, zero. ** 8: Salt-1 (copied from the header) ** 12: Salt-2 (copied from the header) ** 16: Checksum-1. ** 20: Checksum-2. ** ** A frame is considered valid if and only if the following conditions are ** true: ** ** (1) The salt-1 and salt-2 values in the frame-header match ** salt values in the wal-header ** ** (2) The checksum values in the final 8 bytes of the frame-header ** exactly match the checksum computed consecutively on ** (a) the first 16 bytes of the frame-header, and ** (b) the frame data. ** ** On a checkpoint, the WAL is first VFS.xSync-ed, then valid content of the ** WAL is transferred into the database, then the database is VFS.xSync-ed. ** The VFS.xSync operations server as write barriers - all writes launched ** before the xSync must complete before any write that launches after the ** xSync begins. ** ** After each checkpoint, the salt-1 value is incremented and the salt-2 ** value is randomized. This prevents old and new frames in the WAL from ** being considered valid at the same time and being checkpointing together ** following a crash. ** ** READER ALGORITHM ** ** To read a page from the database (call it page number P), a reader ** first checks the WAL to see if it contains page P. If so, then the ** last valid instance of page P that is or is followed by a commit frame ** become the value read. If the WAL contains no copies of page P that ** are valid and which are or are followed by a commit frame, then page ** P is read from the database file. ** ** The reader algorithm in the previous paragraph works correctly, but ** because frames for page P can appear anywhere within the WAL, the ** reader has to scan the entire WAL looking for page P frames. If the ** WAL is large (multiple megabytes is typical) that scan can be slow, ** and read performance suffers. To overcome this problem, a separate ** data structure called the wal-index is maintained to expedite the ** search for frames of a particular page. ** ** WAL-INDEX FORMAT ** ** Conceptually, the wal-index is shared memory, though VFS implementations ** might choose to implement the wal-index using a mmapped file. Because ** the wal-index is shared memory, SQLite does not support journal_mode=WAL ** on a network filesystem. All users of the database must be able to ** share memory. ** ** The wal-index is transient. After a crash, the wal-index can (and should ** be) reconstructed from the original WAL file. In fact, the VFS is required ** to either truncate or zero the header of the wal-index when the last ** connection to it closes. Because the wal-index is transient, it can ** use an architecture-specific format; it does not have to be cross-platform. ** Hence, unlike the database and WAL file formats which store all values ** as big endian, the wal-index can store multi-byte values in the native ** byte order of the host computer. ** ** The purpose of the wal-index is to answer this question quickly: Given ** a page number P, return the index of the last frame for page P in the WAL, ** or return NULL if there are no frames for page P in the WAL. ** ** The wal-index consists of a header region, followed by an one or ** more index blocks. ** ** The wal-index header contains the total number of frames within the WAL ** in the the mxFrame field. Each index block contains information on ** HASHTABLE_NPAGE frames. Each index block contains two sections, a ** mapping which is a database page number for each frame, and a hash ** table used to look up frames by page number. The mapping section is ** an array of HASHTABLE_NPAGE 32-bit page numbers. The first entry on the ** array is the page number for the first frame; the second entry is the ** page number for the second frame; and so forth. The last index block ** holds a total of (mxFrame%HASHTABLE_NPAGE) page numbers. All index ** blocks other than the last are completely full with HASHTABLE_NPAGE ** page numbers. All index blocks are the same size; the mapping section ** of the last index block merely contains unused entries if mxFrame is ** not an even multiple of HASHTABLE_NPAGE. ** ** Even without using the hash table, the last frame for page P ** can be found by scanning the mapping sections of each index block ** starting with the last index block and moving toward the first, and ** within each index block, starting at the end and moving toward the ** beginning. The first entry that equals P corresponds to the frame ** holding the content for that page. ** ** The hash table consists of HASHTABLE_NSLOT 16-bit unsigned integers. ** HASHTABLE_NSLOT = 2*HASHTABLE_NPAGE, and there is one entry in the ** hash table for each page number in the mapping section, so the hash ** table is never more than half full. The expected number of collisions ** prior to finding a match is 1. Each entry of the hash table is an ** 1-based index of an entry in the mapping section of the same ** index block. Let K be the 1-based index of the largest entry in ** the mapping section. (For index blocks other than the last, K will ** always be exactly HASHTABLE_NPAGE (4096) and for the last index block ** K will be (mxFrame%HASHTABLE_NPAGE).) Unused slots of the hash table ** contain a value greater than K. Note that no hash table slot ever ** contains a zero value. ** ** To look for page P in the hash table, first compute a hash iKey on ** P as follows: ** ** iKey = (P * 383) % HASHTABLE_NSLOT ** ** Then start scanning entries of the hash table, starting with iKey ** (wrapping around to the beginning when the end of the hash table is ** reached) until an unused hash slot is found. Let the first unused slot ** be at index iUnused. (iUnused might be less than iKey if there was ** wrap-around.) Because the hash table is never more than half full, ** the search is guaranteed to eventually hit an unused entry. Let ** iMax be the value between iKey and iUnused, closest to iUnused, ** where aHash[iMax]==P. If there is no iMax entry (if there exists ** no hash slot such that aHash[i]==p) then page P is not in the ** current index block. Otherwise the iMax-th mapping entry of the ** current index block corresponds to the last entry that references ** page P. ** ** A hash search begins with the last index block and moves toward the ** first index block, looking for entries corresponding to page P. On ** average, only two or three slots in each index block need to be ** examined in order to either find the last entry for page P, or to ** establish that no such entry exists in the block. Each index block ** holds over 4000 entries. So two or three index blocks are sufficient ** to cover a typical 10 megabyte WAL file, assuming 1K pages. 8 or 10 ** comparisons (on average) suffice to either locate a frame in the ** WAL or to establish that the frame does not exist in the WAL. This ** is much faster than scanning the entire 10MB WAL. ** ** Note that entries are added in order of increasing K. Hence, one ** reader might be using some value K0 and a second reader that started ** at a later time (after additional transactions were added to the WAL ** and to the wal-index) might be using a different value K1, where K1>K0. ** Both readers can use the same hash table and mapping section to get ** the correct result. There may be entries in the hash table with ** K>K0 but to the first reader, those entries will appear to be unused ** slots in the hash table and so the first reader will get an answer as ** if no values greater than K0 had ever been inserted into the hash table ** in the first place - which is what reader one wants. Meanwhile, the ** second reader using K1 will see additional values that were inserted ** later, which is exactly what reader two wants. ** ** When a rollback occurs, the value of K is decreased. Hash table entries ** that correspond to frames greater than the new K value are removed ** from the hash table at this point. */ #ifndef SQLITE_OMIT_WAL #include "wal.h" /* Object declarations */ typedef struct WalIndexHdr WalIndexHdr; typedef struct WalIterator WalIterator; /* ** The following object holds a copy of the wal-index header content. ** ** The actual header in the wal-index consists of two copies of this ** object. */ struct WalIndexHdr { u32 iChange; /* Counter incremented each transaction */ u32 szPage; /* Database page size in bytes */ u32 mxFrame; /* Index of last valid frame in the WAL */ u32 nPage; /* Size of database in pages */ u32 aSalt[2]; /* Salt-1 and salt-2 values copied from WAL header */ u32 aCksum[2]; /* Checksum over all prior fields */ }; /* A block of WALINDEX_LOCK_RESERVED bytes beginning at ** WALINDEX_LOCK_OFFSET is reserved for locks. Since some systems ** only support mandatory file-locks, we do not read or write data ** from the region of the file on which locks are applied. */ #define WALINDEX_LOCK_OFFSET (sizeof(WalIndexHdr)*2) #define WALINDEX_LOCK_RESERVED 8 /* Size of header before each frame in wal */ #define WAL_FRAME_HDRSIZE 24 /* Size of write ahead log header */ #define WAL_HDRSIZE 24 /* ** Return the offset of frame iFrame in the write-ahead log file, ** assuming a database page size of szPage bytes. The offset returned ** is to the start of the write-ahead log frame-header. */ #define walFrameOffset(iFrame, szPage) ( \ WAL_HDRSIZE + ((iFrame)-1)*((szPage)+WAL_FRAME_HDRSIZE) \ ) /* ** An open write-ahead log file is represented by an instance of the ** following object. */ struct Wal { sqlite3_vfs *pVfs; /* The VFS used to create pFd */ sqlite3_file *pDbFd; /* File handle for the database file */ sqlite3_file *pWalFd; /* File handle for WAL file */ u32 iCallback; /* Value to pass to log callback (or 0) */ int szWIndex; /* Size of the wal-index that is mapped in mem */ volatile u32 *pWiData; /* Pointer to wal-index content in memory */ u8 lockState; /* SQLITE_SHM_xxxx constant showing lock state */ u8 readerType; /* SQLITE_SHM_READ or SQLITE_SHM_READ_FULL */ u8 exclusiveMode; /* Non-zero if connection is in exclusive mode */ u8 isWindexOpen; /* True if ShmOpen() called on pDbFd */ WalIndexHdr hdr; /* Wal-index for current snapshot */ char *zWalName; /* Name of WAL file */ int szPage; /* Database page size */ u32 nCkpt; /* Checkpoint sequence counter in the wal-header */ }; /* ** This structure is used to implement an iterator that loops through ** all frames in the WAL in database page order. Where two or more frames ** correspond to the same database page, the iterator visits only the ** frame most recently written to the WAL (in other words, the frame with ** the largest index). ** ** The internals of this structure are only accessed by: ** ** walIteratorInit() - Create a new iterator, ** walIteratorNext() - Step an iterator, ** walIteratorFree() - Free an iterator. ** ** This functionality is used by the checkpoint code (see walCheckpoint()). */ struct WalIterator { int iPrior; /* Last result returned from the iterator */ int nSegment; /* Size of the aSegment[] array */ int nFinal; /* Elements in aSegment[nSegment-1] */ struct WalSegment { int iNext; /* Next slot in aIndex[] not previously returned */ u8 *aIndex; /* i0, i1, i2... such that aPgno[iN] ascending */ u32 *aPgno; /* 256 page numbers. Pointer to Wal.pWiData */ } aSegment[1]; /* One for every 256 entries in the WAL */ }; /* ** Generate or extend an 8 byte checksum based on the data in ** array aByte[] and the initial values of aIn[0] and aIn[1] (or ** initial values of 0 and 0 if aIn==NULL). ** ** The checksum is written back into aOut[] before returning. ** ** nByte must be a positive multiple of 8. */ static void walChecksumBytes( u8 *a, /* Content to be checksummed */ int nByte, /* Bytes of content in a[]. Must be a multiple of 8. */ const u32 *aIn, /* Initial checksum value input */ u32 *aOut /* OUT: Final checksum value output */ ){ u32 s1, s2; u8 *aEnd = (u8*)&a[nByte]; if( aIn ){ s1 = aIn[0]; s2 = aIn[1]; }else{ s1 = s2 = 0; } assert( nByte>=8 ); assert( (nByte&0x00000003)==0 ); do { s1 += (a[0]<<24) + (a[1]<<16) + (a[2]<<8) + a[3] + s2; s2 += (a[4]<<24) + (a[5]<<16) + (a[6]<<8) + a[7] + s1; a += 8; }while( areaderType. */ static int walSetLock(Wal *pWal, int desiredStatus){ int rc = SQLITE_OK; /* Return code */ if( pWal->exclusiveMode || pWal->lockState==desiredStatus ){ pWal->lockState = desiredStatus; }else{ int got = pWal->lockState; rc = sqlite3OsShmLock(pWal->pDbFd, desiredStatus, &got); pWal->lockState = got; if( got==SQLITE_SHM_READ_FULL || got==SQLITE_SHM_READ ){ pWal->readerType = got; pWal->lockState = SQLITE_SHM_READ; } } return rc; } /* ** Write the header information in pWal->hdr into the wal-index. ** ** The checksum on pWal->hdr is updated before it is written. */ static void walIndexWriteHdr(Wal *pWal){ WalIndexHdr *aHdr; walChecksumBytes((u8*)&pWal->hdr, sizeof(pWal->hdr) - sizeof(pWal->hdr.aCksum), 0, pWal->hdr.aCksum); aHdr = (WalIndexHdr*)pWal->pWiData; memcpy(&aHdr[1], &pWal->hdr, sizeof(pWal->hdr)); sqlite3OsShmBarrier(pWal->pDbFd); memcpy(&aHdr[0], &pWal->hdr, sizeof(pWal->hdr)); } /* ** This function encodes a single frame header and writes it to a buffer ** supplied by the caller. A frame-header is made up of a series of ** 4-byte big-endian integers, as follows: ** ** 0: Page number. ** 4: For commit records, the size of the database image in pages ** after the commit. For all other records, zero. ** 8: Salt-1 (copied from the wal-header) ** 12: Salt-2 (copied from the wal-header) ** 16: Checksum-1. ** 20: Checksum-2. */ static void walEncodeFrame( Wal *pWal, /* The write-ahead log */ u32 iPage, /* Database page number for frame */ u32 nTruncate, /* New db size (or 0 for non-commit frames) */ u8 *aData, /* Pointer to page data */ u8 *aFrame /* OUT: Write encoded frame here */ ){ u32 aCksum[2]; assert( WAL_FRAME_HDRSIZE==24 ); sqlite3Put4byte(&aFrame[0], iPage); sqlite3Put4byte(&aFrame[4], nTruncate); memcpy(&aFrame[8], pWal->hdr.aSalt, 8); walChecksumBytes(aFrame, 16, 0, aCksum); walChecksumBytes(aData, pWal->szPage, aCksum, aCksum); sqlite3Put4byte(&aFrame[16], aCksum[0]); sqlite3Put4byte(&aFrame[20], aCksum[1]); } /* ** Check to see if the frame with header in aFrame[] and content ** in aData[] is valid. If it is a valid frame, fill *piPage and ** *pnTruncate and return true. Return if the frame is not valid. */ static int walDecodeFrame( Wal *pWal, /* The write-ahead log */ u32 *piPage, /* OUT: Database page number for frame */ u32 *pnTruncate, /* OUT: New db size (or 0 if not commit) */ u8 *aData, /* Pointer to page data (for checksum) */ u8 *aFrame /* Frame data */ ){ u32 aCksum[2]; assert( WAL_FRAME_HDRSIZE==24 ); /* A frame is only valid if the salt values in the frame-header ** match the salt values in the wal-header. */ if( memcmp(&pWal->hdr.aSalt, &aFrame[8], 8)!=0 ){ return 0; } /* A frame is only valid if a checksum of the first 16 bytes ** of the frame-header, and the frame-data matches ** the checksum in the last 8 bytes of the frame-header. */ walChecksumBytes(aFrame, 16, 0, aCksum); walChecksumBytes(aData, pWal->szPage, aCksum, aCksum); if( aCksum[0]!=sqlite3Get4byte(&aFrame[16]) || aCksum[1]!=sqlite3Get4byte(&aFrame[20]) ){ /* Checksum failed. */ return 0; } /* If we reach this point, the frame is valid. Return the page number ** and the new database size. */ *piPage = sqlite3Get4byte(&aFrame[0]); *pnTruncate = sqlite3Get4byte(&aFrame[4]); return 1; } /* ** Define the parameters of the hash tables in the wal-index file. There ** is a hash-table following every HASHTABLE_NPAGE page numbers in the ** wal-index. ** ** Changing any of these constants will alter the wal-index format and ** create incompatibilities. */ #define HASHTABLE_NPAGE 4096 /* Must be power of 2 and multiple of 256 */ #define HASHTABLE_DATATYPE u16 #define HASHTABLE_HASH_1 383 /* Should be prime */ #define HASHTABLE_NSLOT (HASHTABLE_NPAGE*2) /* Must be a power of 2 */ #define HASHTABLE_NBYTE (sizeof(HASHTABLE_DATATYPE)*HASHTABLE_NSLOT) /* ** Return the index in the Wal.pWiData array that corresponds to ** frame iFrame. ** ** Wal.pWiData is an array of u32 elements that is the wal-index. ** The array begins with a header and is then followed by alternating ** "map" and "hash-table" blocks. Each "map" block consists of ** HASHTABLE_NPAGE u32 elements which are page numbers corresponding ** to frames in the WAL file. ** ** This routine returns an index X such that Wal.pWiData[X] is part ** of a "map" block that contains the page number of the iFrame-th ** frame in the WAL file. */ static int walIndexEntry(u32 iFrame){ return ( (WALINDEX_LOCK_OFFSET+WALINDEX_LOCK_RESERVED)/sizeof(u32) + (((iFrame-1)/HASHTABLE_NPAGE) * HASHTABLE_NBYTE)/sizeof(u32) + (iFrame-1) ); } /* ** Return the minimum mapping size in bytes that can be used to read the ** wal-index up to and including frame iFrame. If iFrame is the last frame ** in a block of 256 frames, the returned byte-count includes the space ** required by the 256-byte index block. */ static int walMappingSize(u32 iFrame){ const int nByte = (sizeof(u32)*HASHTABLE_NPAGE + HASHTABLE_NBYTE) ; return ( WALINDEX_LOCK_OFFSET + WALINDEX_LOCK_RESERVED + nByte * ((iFrame + HASHTABLE_NPAGE - 1)/HASHTABLE_NPAGE) ); } /* ** Release our reference to the wal-index memory map, if we are holding ** it. */ static void walIndexUnmap(Wal *pWal){ if( pWal->pWiData ){ sqlite3OsShmRelease(pWal->pDbFd); pWal->pWiData = 0; } } /* ** Map the wal-index file into memory if it isn't already. ** ** The reqSize parameter is the minimum required size of the mapping. ** A value of -1 means "don't care". */ static int walIndexMap(Wal *pWal, int reqSize){ int rc = SQLITE_OK; if( pWal->pWiData==0 || reqSize>pWal->szWIndex ){ walIndexUnmap(pWal); rc = sqlite3OsShmGet(pWal->pDbFd, reqSize, &pWal->szWIndex, (void volatile**)(char volatile*)&pWal->pWiData); if( rc==SQLITE_OK && pWal->pWiData==0 ){ /* Make sure pWal->pWiData is not NULL while we are holding the ** lock on the mapping. */ assert( pWal->szWIndex==0 ); pWal->pWiData = &pWal->iCallback; } if( rc!=SQLITE_OK ){ walIndexUnmap(pWal); } } return rc; } /* ** Remap the wal-index so that the mapping covers the full size ** of the underlying file. ** ** If enlargeTo is non-negative, then increase the size of the underlying ** storage to be at least as big as enlargeTo before remapping. */ static int walIndexRemap(Wal *pWal, int enlargeTo){ int rc; int sz; rc = sqlite3OsShmSize(pWal->pDbFd, enlargeTo, &sz); if( rc==SQLITE_OK && sz>pWal->szWIndex ){ walIndexUnmap(pWal); rc = walIndexMap(pWal, sz); } return rc; } /* ** Increment by which to increase the wal-index file size. */ #define WALINDEX_MMAP_INCREMENT (64*1024) /* ** Compute a hash on a page number. The resulting hash value must land ** between 0 and (HASHTABLE_NSLOT-1). */ static int walHash(u32 iPage){ assert( iPage>0 ); assert( (HASHTABLE_NSLOT & (HASHTABLE_NSLOT-1))==0 ); return (iPage*HASHTABLE_HASH_1) & (HASHTABLE_NSLOT-1); } static int walNextHash(int iPriorHash){ return (iPriorHash+1)&(HASHTABLE_NSLOT-1); } /* ** Find the hash table and (section of the) page number array used to ** store data for WAL frame iFrame. ** ** Set output variable *paHash to point to the start of the hash table ** in the wal-index file. Set *piZero to one less than the frame ** number of the first frame indexed by this hash table. If a ** slot in the hash table is set to N, it refers to frame number ** (*piZero+N) in the log. ** ** Finally, set *paPgno such that for all frames F between (*piZero+1) and ** (*piZero+HASHTABLE_NPAGE), (*paPgno)[F] is the database page number ** associated with frame F. */ static void walHashFind( Wal *pWal, /* WAL handle */ u32 iFrame, /* Find the hash table indexing this frame */ volatile HASHTABLE_DATATYPE **paHash, /* OUT: Pointer to hash index */ volatile u32 **paPgno, /* OUT: Pointer to page number array */ u32 *piZero /* OUT: Frame associated with *paPgno[0] */ ){ u32 iZero; volatile u32 *aPgno; volatile HASHTABLE_DATATYPE *aHash; iZero = ((iFrame-1)/HASHTABLE_NPAGE) * HASHTABLE_NPAGE; aPgno = &pWal->pWiData[walIndexEntry(iZero+1)-iZero-1]; aHash = (HASHTABLE_DATATYPE *)&aPgno[iZero+HASHTABLE_NPAGE+1]; /* Assert that: ** ** + the mapping is large enough for this hash-table, and ** ** + that aPgno[iZero+1] really is the database page number associated ** with the first frame indexed by this hash table. */ assert( (u32*)(&aHash[HASHTABLE_NSLOT])<=&pWal->pWiData[pWal->szWIndex/4] ); assert( walIndexEntry(iZero+1)==(&aPgno[iZero+1] - pWal->pWiData) ); *paHash = aHash; *paPgno = aPgno; *piZero = iZero; } /* ** Set an entry in the wal-index that will map database page number ** pPage into WAL frame iFrame. */ static int walIndexAppend(Wal *pWal, u32 iFrame, u32 iPage){ int rc; /* Return code */ int nMapping; /* Required mapping size in bytes */ /* Make sure the wal-index is mapped. Enlarge the mapping if required. */ nMapping = walMappingSize(iFrame); rc = walIndexMap(pWal, -1); while( rc==SQLITE_OK && nMapping>pWal->szWIndex ){ int nByte = pWal->szWIndex + WALINDEX_MMAP_INCREMENT; rc = walIndexRemap(pWal, nByte); } /* Assuming the wal-index file was successfully mapped, find the hash ** table and section of of the page number array that pertain to frame ** iFrame of the WAL. Then populate the page number array and the hash ** table entry. */ if( rc==SQLITE_OK ){ int iKey; /* Hash table key */ u32 iZero; /* One less than frame number of aPgno[1] */ volatile u32 *aPgno; /* Page number array */ volatile HASHTABLE_DATATYPE *aHash; /* Hash table */ int idx; /* Value to write to hash-table slot */ TESTONLY( int nCollide = 0; /* Number of hash collisions */ ) walHashFind(pWal, iFrame, &aHash, &aPgno, &iZero); idx = iFrame - iZero; if( idx==1 ) memset((void*)aHash, 0, HASHTABLE_NBYTE); assert( idx <= HASHTABLE_NSLOT/2 + 1 ); aPgno[iFrame] = iPage; for(iKey=walHash(iPage); aHash[iKey]; iKey=walNextHash(iKey)){ assert( nCollide++ < idx ); } aHash[iKey] = idx; } return rc; } /* ** Recover the wal-index by reading the write-ahead log file. ** The caller must hold RECOVER lock on the wal-index file. */ static int walIndexRecover(Wal *pWal){ int rc; /* Return Code */ i64 nSize; /* Size of log file */ WalIndexHdr hdr; /* Recovered wal-index header */ assert( pWal->lockState>SQLITE_SHM_READ ); memset(&hdr, 0, sizeof(hdr)); rc = sqlite3OsFileSize(pWal->pWalFd, &nSize); if( rc!=SQLITE_OK ){ return rc; } if( nSize>WAL_FRAME_HDRSIZE ){ u8 aBuf[WAL_HDRSIZE]; /* Buffer to load first frame header into */ u8 *aFrame = 0; /* Malloc'd buffer to load entire frame */ int szFrame; /* Number of bytes in buffer aFrame[] */ u8 *aData; /* Pointer to data part of aFrame buffer */ int iFrame; /* Index of last frame read */ i64 iOffset; /* Next offset to read from log file */ int szPage; /* Page size according to the log */ /* Read in the first frame header in the file (to determine the ** database page size). */ rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0); if( rc!=SQLITE_OK ){ return rc; } /* If the database page size is not a power of two, or is greater than ** SQLITE_MAX_PAGE_SIZE, conclude that the WAL file contains no valid data. */ szPage = sqlite3Get4byte(&aBuf[8]); if( szPage&(szPage-1) || szPage>SQLITE_MAX_PAGE_SIZE || szPage<512 ){ goto finished; } pWal->szPage = szPage; pWal->nCkpt = sqlite3Get4byte(&aBuf[12]); memcpy(&pWal->hdr.aSalt, &aBuf[16], 8); /* Malloc a buffer to read frames into. */ szFrame = szPage + WAL_FRAME_HDRSIZE; aFrame = (u8 *)sqlite3_malloc(szFrame); if( !aFrame ){ return SQLITE_NOMEM; } aData = &aFrame[WAL_FRAME_HDRSIZE]; /* Read all frames from the log file. */ iFrame = 0; for(iOffset=WAL_HDRSIZE; (iOffset+szFrame)<=nSize; iOffset+=szFrame){ u32 pgno; /* Database page number for frame */ u32 nTruncate; /* dbsize field from frame header */ int isValid; /* True if this frame is valid */ /* Read and decode the next log frame. */ rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset); if( rc!=SQLITE_OK ) break; isValid = walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame); if( !isValid ) break; rc = walIndexAppend(pWal, ++iFrame, pgno); if( rc!=SQLITE_OK ) break; /* If nTruncate is non-zero, this is a commit record. */ if( nTruncate ){ hdr.mxFrame = iFrame; hdr.nPage = nTruncate; hdr.szPage = szPage; } } sqlite3_free(aFrame); }else{ memset(&hdr, 0, sizeof(hdr)); } finished: if( rc==SQLITE_OK && hdr.mxFrame==0 ){ rc = walIndexRemap(pWal, WALINDEX_MMAP_INCREMENT); } if( rc==SQLITE_OK ){ memcpy(&pWal->hdr, &hdr, sizeof(hdr)); walIndexWriteHdr(pWal); } return rc; } /* ** Close an open wal-index. */ static void walIndexClose(Wal *pWal, int isDelete){ if( pWal->isWindexOpen ){ int notUsed; sqlite3OsShmLock(pWal->pDbFd, SQLITE_SHM_UNLOCK, ¬Used); sqlite3OsShmClose(pWal->pDbFd, isDelete); pWal->isWindexOpen = 0; } } /* ** Open a connection to the log file associated with database zDb. The ** database file does not actually have to exist. zDb is used only to ** figure out the name of the log file to open. If the log file does not ** exist it is created by this call. ** ** A SHARED lock should be held on the database file when this function ** is called. The purpose of this SHARED lock is to prevent any other ** client from unlinking the log or wal-index file. If another process ** were to do this just after this client opened one of these files, the ** system would be badly broken. ** ** If the log file is successfully opened, SQLITE_OK is returned and ** *ppWal is set to point to a new WAL handle. If an error occurs, ** an SQLite error code is returned and *ppWal is left unmodified. */ int sqlite3WalOpen( sqlite3_vfs *pVfs, /* vfs module to open wal and wal-index */ sqlite3_file *pDbFd, /* The open database file */ const char *zDbName, /* Name of the database file */ Wal **ppWal /* OUT: Allocated Wal handle */ ){ int rc; /* Return Code */ Wal *pRet; /* Object to allocate and return */ int flags; /* Flags passed to OsOpen() */ char *zWal; /* Name of write-ahead log file */ int nWal; /* Length of zWal in bytes */ assert( zDbName && zDbName[0] ); assert( pDbFd ); /* Allocate an instance of struct Wal to return. */ *ppWal = 0; nWal = sqlite3Strlen30(zDbName) + 5; pRet = (Wal*)sqlite3MallocZero(sizeof(Wal) + pVfs->szOsFile + nWal); if( !pRet ){ return SQLITE_NOMEM; } pRet->pVfs = pVfs; pRet->pWalFd = (sqlite3_file *)&pRet[1]; pRet->pDbFd = pDbFd; sqlite3_randomness(8, &pRet->hdr.aSalt); pRet->zWalName = zWal = pVfs->szOsFile + (char*)pRet->pWalFd; sqlite3_snprintf(nWal, zWal, "%s-wal", zDbName); rc = sqlite3OsShmOpen(pDbFd); /* Open file handle on the write-ahead log file. */ if( rc==SQLITE_OK ){ pRet->isWindexOpen = 1; flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_MAIN_JOURNAL); rc = sqlite3OsOpen(pVfs, zWal, pRet->pWalFd, flags, &flags); } if( rc!=SQLITE_OK ){ walIndexClose(pRet, 0); sqlite3OsClose(pRet->pWalFd); sqlite3_free(pRet); }else{ *ppWal = pRet; } return rc; } /* ** Find the smallest page number out of all pages held in the WAL that ** has not been returned by any prior invocation of this method on the ** same WalIterator object. Write into *piFrame the frame index where ** that page was last written into the WAL. Write into *piPage the page ** number. ** ** Return 0 on success. If there are no pages in the WAL with a page ** number larger than *piPage, then return 1. */ static int walIteratorNext( WalIterator *p, /* Iterator */ u32 *piPage, /* OUT: The page number of the next page */ u32 *piFrame /* OUT: Wal frame index of next page */ ){ u32 iMin; /* Result pgno must be greater than iMin */ u32 iRet = 0xFFFFFFFF; /* 0xffffffff is never a valid page number */ int i; /* For looping through segments */ int nBlock = p->nFinal; /* Number of entries in current segment */ iMin = p->iPrior; assert( iMin<0xffffffff ); for(i=p->nSegment-1; i>=0; i--){ struct WalSegment *pSegment = &p->aSegment[i]; while( pSegment->iNextaPgno[pSegment->aIndex[pSegment->iNext]]; if( iPg>iMin ){ if( iPgaIndex[pSegment->iNext]; } break; } pSegment->iNext++; } nBlock = 256; } *piPage = p->iPrior = iRet; return (iRet==0xFFFFFFFF); } static void walMergesort8( Pgno *aContent, /* Pages in wal */ u8 *aBuffer, /* Buffer of at least *pnList items to use */ u8 *aList, /* IN/OUT: List to sort */ int *pnList /* IN/OUT: Number of elements in aList[] */ ){ int nList = *pnList; if( nList>1 ){ int nLeft = nList / 2; /* Elements in left list */ int nRight = nList - nLeft; /* Elements in right list */ u8 *aLeft = aList; /* Left list */ u8 *aRight = &aList[nLeft]; /* Right list */ int iLeft = 0; /* Current index in aLeft */ int iRight = 0; /* Current index in aright */ int iOut = 0; /* Current index in output buffer */ /* TODO: Change to non-recursive version. */ walMergesort8(aContent, aBuffer, aLeft, &nLeft); walMergesort8(aContent, aBuffer, aRight, &nRight); while( iRight=nRight || aContent[aLeft[iLeft]]=nLeft || aContent[aLeft[iLeft]]>dbpage ); assert( iRight>=nRight || aContent[aRight[iRight]]>dbpage ); } memcpy(aList, aBuffer, sizeof(aList[0])*iOut); *pnList = iOut; } #ifdef SQLITE_DEBUG { int i; for(i=1; i<*pnList; i++){ assert( aContent[aList[i]] > aContent[aList[i-1]] ); } } #endif } /* ** Map the wal-index into memory owned by this thread, if it is not ** mapped already. Then construct a WalInterator object that can be ** used to loop over all pages in the WAL in ascending order. ** ** On success, make *pp point to the newly allocated WalInterator object ** return SQLITE_OK. Otherwise, leave *pp unchanged and return an error ** code. ** ** The calling routine should invoke walIteratorFree() to destroy the ** WalIterator object when it has finished with it. The caller must ** also unmap the wal-index. But the wal-index must not be unmapped ** prior to the WalIterator object being destroyed. */ static int walIteratorInit(Wal *pWal, WalIterator **pp){ u32 *aData; /* Content of the wal-index file */ WalIterator *p; /* Return value */ int nSegment; /* Number of segments to merge */ u32 iLast; /* Last frame in log */ int nByte; /* Number of bytes to allocate */ int i; /* Iterator variable */ int nFinal; /* Number of unindexed entries */ u8 *aTmp; /* Temp space used by merge-sort */ int rc; /* Return code of walIndexMap() */ u8 *aSpace; /* Surplus space on the end of the allocation */ /* Make sure the wal-index is mapped into local memory */ rc = walIndexMap(pWal, walMappingSize(pWal->hdr.mxFrame)); if( rc!=SQLITE_OK ){ return rc; } /* This routine only runs while holding SQLITE_SHM_CHECKPOINT. No other ** thread is able to write to shared memory while this routine is ** running (or, indeed, while the WalIterator object exists). Hence, ** we can cast off the volatile qualifacation from shared memory */ assert( pWal->lockState==SQLITE_SHM_CHECKPOINT ); aData = (u32*)pWal->pWiData; /* Allocate space for the WalIterator object */ iLast = pWal->hdr.mxFrame; nSegment = (iLast >> 8) + 1; nFinal = (iLast & 0x000000FF); nByte = sizeof(WalIterator) + (nSegment+1)*(sizeof(struct WalSegment)+256); p = (WalIterator *)sqlite3_malloc(nByte); if( !p ){ return SQLITE_NOMEM; } memset(p, 0, nByte); /* Initialize the WalIterator object. Each 256-entry segment is ** presorted in order to make iterating through all entries much ** faster. */ p->nSegment = nSegment; aSpace = (u8 *)&p->aSegment[nSegment]; aTmp = &aSpace[nSegment*256]; for(i=0; iaSegment[i].aPgno = &aData[walIndexEntry(i*256+1)]; p->aSegment[i].aIndex = aSpace; for(j=0; jaSegment[i].aPgno, aTmp, aSpace, &nIndex); memset(&aSpace[nIndex], aSpace[nIndex-1], 256-nIndex); aSpace += 256; p->nFinal = nIndex; } /* Return the fully initializd WalIterator object */ *pp = p; return SQLITE_OK ; } /* ** Free an iterator allocated by walIteratorInit(). */ static void walIteratorFree(WalIterator *p){ sqlite3_free(p); } /* ** Checkpoint the contents of the log file. */ static int walCheckpoint( Wal *pWal, /* Wal connection */ int sync_flags, /* Flags for OsSync() (or 0) */ int nBuf, /* Size of zBuf in bytes */ u8 *zBuf /* Temporary buffer to use */ ){ int rc; /* Return code */ int szPage = pWal->hdr.szPage; /* Database page-size */ WalIterator *pIter = 0; /* Wal iterator context */ u32 iDbpage = 0; /* Next database page to write */ u32 iFrame = 0; /* Wal frame containing data for iDbpage */ /* Allocate the iterator */ rc = walIteratorInit(pWal, &pIter); if( rc!=SQLITE_OK || pWal->hdr.mxFrame==0 ){ goto out; } if( pWal->hdr.szPage!=nBuf ){ rc = SQLITE_CORRUPT_BKPT; goto out; } /* Sync the log file to disk */ if( sync_flags ){ rc = sqlite3OsSync(pWal->pWalFd, sync_flags); if( rc!=SQLITE_OK ) goto out; } /* Iterate through the contents of the log, copying data to the db file. */ while( 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){ rc = sqlite3OsRead(pWal->pWalFd, zBuf, szPage, walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE ); if( rc!=SQLITE_OK ) goto out; rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, (iDbpage-1)*szPage); if( rc!=SQLITE_OK ) goto out; } /* Truncate the database file */ rc = sqlite3OsTruncate(pWal->pDbFd, ((i64)pWal->hdr.nPage*(i64)szPage)); if( rc!=SQLITE_OK ) goto out; /* Sync the database file. If successful, update the wal-index. */ if( sync_flags ){ rc = sqlite3OsSync(pWal->pDbFd, sync_flags); if( rc!=SQLITE_OK ) goto out; } pWal->hdr.mxFrame = 0; pWal->nCkpt++; sqlite3Put4byte((u8*)pWal->hdr.aSalt, 1 + sqlite3Get4byte((u8*)pWal->hdr.aSalt)); sqlite3_randomness(4, &pWal->hdr.aSalt[1]); walIndexWriteHdr(pWal); out: walIteratorFree(pIter); return rc; } /* ** Close a connection to a log file. */ int sqlite3WalClose( Wal *pWal, /* Wal to close */ int sync_flags, /* Flags to pass to OsSync() (or 0) */ int nBuf, u8 *zBuf /* Buffer of at least nBuf bytes */ ){ int rc = SQLITE_OK; if( pWal ){ int isDelete = 0; /* True to unlink wal and wal-index files */ /* If an EXCLUSIVE lock can be obtained on the database file (using the ** ordinary, rollback-mode locking methods, this guarantees that the ** connection associated with this log file is the only connection to ** the database. In this case checkpoint the database and unlink both ** the wal and wal-index files. ** ** The EXCLUSIVE lock is not released before returning. */ rc = sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE); if( rc==SQLITE_OK ){ rc = sqlite3WalCheckpoint(pWal, sync_flags, nBuf, zBuf, 0, 0); if( rc==SQLITE_OK ){ isDelete = 1; } walIndexUnmap(pWal); } walIndexClose(pWal, isDelete); sqlite3OsClose(pWal->pWalFd); if( isDelete ){ sqlite3OsDelete(pWal->pVfs, pWal->zWalName, 0); } sqlite3_free(pWal); } return rc; } /* ** Try to read the wal-index header. Return 0 on success and 1 if ** there is a problem. ** ** The wal-index is in shared memory. Another thread or process might ** be writing the header at the same time this procedure is trying to ** read it, which might result in inconsistency. A dirty read is detected ** by verifying a checksum on the header. ** ** If and only if the read is consistent and the header is different from ** pWal->hdr, then pWal->hdr is updated to the content of the new header ** and *pChanged is set to 1. ** ** If the checksum cannot be verified return non-zero. If the header ** is read successfully and the checksum verified, return zero. */ int walIndexTryHdr(Wal *pWal, int *pChanged){ u32 aCksum[2]; /* Checksum on the header content */ WalIndexHdr h1, h2; /* Two copies of the header content */ WalIndexHdr *aHdr; /* Header in shared memory */ assert( pWal->pWiData ); if( pWal->szWIndex==0 ){ /* The wal-index is of size 0 bytes. This is handled in the same way ** as an invalid header. The caller will run recovery to construct ** a valid wal-index file before accessing the database. */ return 1; } /* Read the header. The caller may or may not have an exclusive ** (WRITE, PENDING, CHECKPOINT or RECOVER) lock on the wal-index ** file, meaning it is possible that an inconsistent snapshot is read ** from the file. If this happens, return non-zero. ** ** There are two copies of the header at the beginning of the wal-index. ** When reading, read [0] first then [1]. Writes are in the reverse order. ** Memory barriers are used to prevent the compiler or the hardware from ** reordering the reads and writes. */ aHdr = (WalIndexHdr*)pWal->pWiData; memcpy(&h1, &aHdr[0], sizeof(h1)); sqlite3OsShmBarrier(pWal->pDbFd); memcpy(&h2, &aHdr[1], sizeof(h2)); if( memcmp(&h1, &h2, sizeof(h1))!=0 ){ return 1; /* Dirty read */ } if( h1.szPage==0 ){ return 1; /* Malformed header - probably all zeros */ } walChecksumBytes((u8*)&h1, sizeof(h1)-sizeof(h1.aCksum), 0, aCksum); if( aCksum[0]!=h1.aCksum[0] || aCksum[1]!=h1.aCksum[1] ){ return 1; /* Checksum does not match */ } if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){ *pChanged = 1; memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr)); pWal->szPage = pWal->hdr.szPage; } /* 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 recover the log ** 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. ** ** This routine also maps the wal-index content into memory and assigns ** ownership of that mapping to the current thread. In some implementations, ** only one thread at a time can hold a mapping of the wal-index. Hence, ** the caller should strive to invoke walIndexUnmap() as soon as possible ** after this routine returns. ** ** 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 lockState; /* pWal->lockState before running recovery */ assert( pWal->lockState>=SQLITE_SHM_READ ); assert( pChanged ); rc = walIndexMap(pWal, -1); if( rc!=SQLITE_OK ){ return rc; } /* First attempt to read the wal-index header. This may fail for one ** of two reasons: (a) the wal-index does not yet exist or has been ** corrupted and needs to be constructed by running recovery, or (b) ** the caller is only holding a READ lock and made a dirty read of ** the wal-index header. ** ** A dirty read of the wal-index header occurs if another thread or ** process happens to be writing to the wal-index header at roughly ** the same time as this thread is reading it. In this case it is ** possible that an inconsistent header is read (which is detected ** using the header checksum mechanism). */ if( walIndexTryHdr(pWal, pChanged)==0 ){ return SQLITE_OK; } /* If the first attempt to read the header failed, lock the wal-index ** file with an exclusive lock and try again. If the header checksum ** verification fails again, we can be sure that it is not simply a ** dirty read, but that the wal-index really does need to be ** reconstructed by running log recovery. ** ** In the paragraph above, an "exclusive lock" may be any of WRITE, ** PENDING, CHECKPOINT or RECOVER. If any of these are already held, ** no locking operations are required. If the caller currently holds ** a READ lock, then upgrade to a RECOVER lock before re-reading the ** wal-index header and revert to a READ lock before returning. */ lockState = pWal->lockState; if( lockState>SQLITE_SHM_READ || SQLITE_OK==(rc = walSetLock(pWal, SQLITE_SHM_RECOVER)) ){ if( walIndexTryHdr(pWal, pChanged) ){ *pChanged = 1; rc = walIndexRecover(pWal); } if( lockState==SQLITE_SHM_READ ){ walSetLock(pWal, SQLITE_SHM_READ); } } return rc; } /* ** Take a snapshot of the state of the WAL and wal-index for the current ** instant in time. The current thread will continue to use this snapshot. ** Other threads might containing appending to the WAL and wal-index but ** the extra content appended will be ignored by the current thread. ** ** A snapshot is like a read transaction. ** ** No other threads are allowed to run a checkpoint while this thread is ** holding the snapshot since a checkpoint would remove data out from under ** this thread. ** ** If this call obtains a new read-lock and the database contents have been ** modified since the most recent call to WalCloseSnapshot() on this Wal ** connection, then *pChanged is set to 1 before returning. Otherwise, it ** is left unmodified. This is used by the pager layer to determine whether ** or not any cached pages may be safely reused. */ int sqlite3WalOpenSnapshot(Wal *pWal, int *pChanged){ int rc; /* Return code */ rc = walSetLock(pWal, SQLITE_SHM_READ); assert( rc!=SQLITE_OK || pWal->lockState==SQLITE_SHM_READ ); if( rc==SQLITE_OK ){ rc = walIndexReadHdr(pWal, pChanged); if( rc!=SQLITE_OK ){ /* An error occured while attempting log recovery. */ sqlite3WalCloseSnapshot(pWal); } } walIndexUnmap(pWal); return rc; } /* ** Unlock the current snapshot. */ void sqlite3WalCloseSnapshot(Wal *pWal){ assert( pWal->lockState==SQLITE_SHM_READ || pWal->lockState==SQLITE_SHM_UNLOCK ); walSetLock(pWal, SQLITE_SHM_UNLOCK); } /* ** Read a page from the log, if it is present. */ int sqlite3WalRead( Wal *pWal, /* WAL handle */ Pgno pgno, /* Database page number to read data for */ int *pInWal, /* OUT: True if data is read from WAL */ int nOut, /* Size of buffer pOut in bytes */ u8 *pOut /* Buffer to write page data to */ ){ int rc; /* Return code */ u32 iRead = 0; /* If !=0, WAL frame to return data from */ u32 iLast = pWal->hdr.mxFrame; /* Last page in WAL for this reader */ int iHash; /* Used to loop through N hash tables */ /* 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 to avoid the walIndexMap/Unmap overhead. */ if( iLast==0 ){ *pInWal = 0; return SQLITE_OK; } /* Ensure the wal-index is mapped. */ assert( pWal->lockState==SQLITE_SHM_READ||pWal->lockState==SQLITE_SHM_WRITE ); rc = walIndexMap(pWal, walMappingSize(iLast)); if( rc!=SQLITE_OK ){ return rc; } /* 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 may 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 ** opened remain unmodified. ** ** For the reasons above, the if(...) condition featured in the inner ** loop of the following block is more stringent that would be required ** if we had exclusive access to the hash-table: ** ** (aPgno[iFrame]==pgno): ** This condition filters out normal hash-table collisions. ** ** (iFrame<=iLast): ** This condition filters out entries that were added to the hash ** table after the current read-transaction had started. ** ** (iFrame>iRead): ** This filters out a dangerous class of garbage data. The ** garbage hash slot may refer to a frame with the correct page ** number, but not the most recent version of the frame. For ** example, if at the start of the read-transaction the WAL ** contains three copies of the desired page in frames 2, 3 and 4, ** the hash table may contain the following: ** ** { ..., 2, 3, 4, 99, 99, ..... } ** ** The correct answer is to read data from frame 4. But a ** dirty-read may potentially cause the hash-table to appear as ** follows to the reader: ** ** { ..., 2, 3, 4, 3, 99, ..... } ** ** Without this part of the if(...) clause, the reader might ** incorrectly read data from frame 3 instead of 4. This would be ** an error. ** ** It is not actually clear to the developers that such a dirty-read ** can occur. But if it does, it should not cause any problems. */ for(iHash=iLast; iHash>0 && iRead==0; iHash-=HASHTABLE_NPAGE){ volatile HASHTABLE_DATATYPE *aHash; /* Pointer to hash table */ volatile u32 *aPgno; /* Pointer to array of page numbers */ u32 iZero; /* Frame number corresponding to aPgno[0] */ int iKey; /* Hash slot index */ int mxHash; /* upper bound on aHash[] values */ walHashFind(pWal, iHash, &aHash, &aPgno, &iZero); mxHash = iLast - iZero; if( mxHash > HASHTABLE_NPAGE ) mxHash = HASHTABLE_NPAGE; for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){ u32 iFrame = aHash[iKey] + iZero; if( iFrame<=iLast && aPgno[iFrame]==pgno && iFrame>iRead ){ iRead = iFrame; } } } assert( iRead==0 || pWal->pWiData[walIndexEntry(iRead)]==pgno ); #ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT /* If expensive assert() statements are available, do a linear search ** of the wal-index file content. Make sure the results agree with the ** result obtained using the hash indexes above. */ { u32 iRead2 = 0; u32 iTest; for(iTest=iLast; iTest>0; iTest--){ if( pWal->pWiData[walIndexEntry(iTest)]==pgno ){ iRead2 = iTest; break; } } assert( iRead==iRead2 ); } #endif /* If iRead is non-zero, then it is the log frame number that contains the ** required page. Read and return data from the log file. */ walIndexUnmap(pWal); if( iRead ){ i64 iOffset = walFrameOffset(iRead, pWal->hdr.szPage) + WAL_FRAME_HDRSIZE; *pInWal = 1; return sqlite3OsRead(pWal->pWalFd, pOut, nOut, iOffset); } *pInWal = 0; return SQLITE_OK; } /* ** Set *pPgno to the size of the database file (or zero, if unknown). */ void sqlite3WalDbsize(Wal *pWal, Pgno *pPgno){ assert( pWal->lockState==SQLITE_SHM_READ || pWal->lockState==SQLITE_SHM_WRITE ); *pPgno = pWal->hdr.nPage; } /* ** This function returns SQLITE_OK if the caller may write to the database. ** Otherwise, if the caller is operating on a snapshot that has already ** been overwritten by another writer, SQLITE_BUSY is returned. */ int sqlite3WalWriteLock(Wal *pWal, int op){ int rc = SQLITE_OK; if( op ){ assert( pWal->lockState==SQLITE_SHM_READ ); rc = walSetLock(pWal, SQLITE_SHM_WRITE); /* If this connection is not reading the most recent database snapshot, ** it is not possible to write to the database. In this case release ** the write locks and return SQLITE_BUSY. */ if( rc==SQLITE_OK ){ rc = walIndexMap(pWal, sizeof(WalIndexHdr)); if( rc==SQLITE_OK && memcmp(&pWal->hdr, (void*)pWal->pWiData, sizeof(WalIndexHdr)) ){ rc = SQLITE_BUSY; } walIndexUnmap(pWal); if( rc!=SQLITE_OK ){ walSetLock(pWal, SQLITE_SHM_READ); } } }else if( pWal->lockState==SQLITE_SHM_WRITE ){ rc = walSetLock(pWal, SQLITE_SHM_READ); } return rc; } /* ** Remove entries from zero or more hash-table indexes in the wal-index ** file. ** ** This function is called when rolling back a transaction or savepoint ** transaction in WAL mode. Argument iNewMx is the value that ** Wal.hdr.mxFrame will be set to following the rollback. Argument iOldMx ** is the value that it had before the rollback. This function removes ** entries that refer to frames with frame numbers greater than iNewMx ** from the hash table that contains the entry associated with iNewMx. ** It is not necessary to remove any entries from any subsequent hash ** tables, as they will be zeroed by walIndexAppend() before they are ** next used. */ static void walClearHash(Wal *pWal, u32 iOldMx, u32 iNewMx){ if( iOldMx>iNewMx ){ volatile HASHTABLE_DATATYPE *aHash; /* Pointer to hash table to clear */ volatile u32 *unused1; /* Only to satisfy walHashFind() */ u32 iZero; /* frame == (aHash[x]+iZero) */ int iLimit; /* Zero values greater than this */ walHashFind(pWal, iNewMx+1, &aHash, &unused1, &iZero); iLimit = iNewMx - iZero; if( iLimit>0 ){ int i; /* Used to iterate through aHash[] */ for(i=1; i<=HASHTABLE_NPAGE; i++){ if( aHash[i]>iLimit ){ aHash[i] = 0; } } } } } /* ** If any data has been written (but not committed) to the log file, this ** function moves the write-pointer back to the start of the transaction. ** ** Additionally, the callback function is invoked for each frame written ** to the log since the start of the transaction. If the callback returns ** other than SQLITE_OK, it is not invoked again and the error code is ** returned to the caller. ** ** Otherwise, if the callback function does not return an error, this ** function returns SQLITE_OK. */ int sqlite3WalUndo(Wal *pWal, int (*xUndo)(void *, Pgno), void *pUndoCtx){ int rc = SQLITE_OK; if( pWal->lockState==SQLITE_SHM_WRITE ){ int unused; Pgno iMax = pWal->hdr.mxFrame; Pgno iFrame; assert( pWal->pWiData==0 ); rc = walIndexReadHdr(pWal, &unused); for(iFrame=pWal->hdr.mxFrame+1; rc==SQLITE_OK && iFrame<=iMax; iFrame++){ assert( pWal->lockState==SQLITE_SHM_WRITE ); rc = xUndo(pUndoCtx, pWal->pWiData[walIndexEntry(iFrame)]); } if( rc==SQLITE_OK ){ walClearHash(pWal, iMax, pWal->hdr.mxFrame); } walIndexUnmap(pWal); } return rc; } /* Return an integer that records the current (uncommitted) write ** position in the WAL */ u32 sqlite3WalSavepoint(Wal *pWal){ assert( pWal->lockState==SQLITE_SHM_WRITE ); return pWal->hdr.mxFrame; } /* Move the write position of the WAL back to iFrame. Called in ** response to a ROLLBACK TO command. */ int sqlite3WalSavepointUndo(Wal *pWal, u32 iFrame){ int rc = SQLITE_OK; assert( pWal->lockState==SQLITE_SHM_WRITE ); rc = walIndexMap(pWal, walMappingSize(pWal->hdr.mxFrame)); if( rc==SQLITE_OK ){ walClearHash(pWal, pWal->hdr.mxFrame, iFrame); walIndexUnmap(pWal); } pWal->hdr.mxFrame = iFrame; return rc; } /* ** Write a set of frames to the log. The caller must hold the write-lock ** on the log file (obtained using sqlite3WalWriteLock()). */ int sqlite3WalFrames( Wal *pWal, /* Wal handle to write to */ int szPage, /* Database page-size in bytes */ PgHdr *pList, /* List of dirty pages to write */ Pgno nTruncate, /* Database size after this commit */ int isCommit, /* True if this is a commit */ int sync_flags /* Flags to pass to OsSync() (or 0) */ ){ int rc; /* Used to catch return codes */ u32 iFrame; /* Next frame address */ u8 aFrame[WAL_FRAME_HDRSIZE]; /* Buffer to assemble frame-header in */ PgHdr *p; /* Iterator to run through pList with. */ PgHdr *pLast = 0; /* Last frame in list */ int nLast = 0; /* Number of extra copies of last page */ assert( pList ); assert( pWal->lockState==SQLITE_SHM_WRITE ); assert( pWal->pWiData==0 ); /* If this is the first frame written into the log, write the WAL ** header to the start of the WAL file. See comments at the top of ** this source file for a description of the WAL header format. */ iFrame = pWal->hdr.mxFrame; if( iFrame==0 ){ u8 aWalHdr[WAL_HDRSIZE]; /* Buffer to assembly wal-header in */ sqlite3Put4byte(&aWalHdr[0], 0x377f0682); sqlite3Put4byte(&aWalHdr[4], 3007000); sqlite3Put4byte(&aWalHdr[8], szPage); pWal->szPage = szPage; sqlite3Put4byte(&aWalHdr[12], pWal->nCkpt); memcpy(&aWalHdr[16], pWal->hdr.aSalt, 8); rc = sqlite3OsWrite(pWal->pWalFd, aWalHdr, sizeof(aWalHdr), 0); if( rc!=SQLITE_OK ){ return rc; } } assert( pWal->szPage==szPage ); /* Write the log file. */ for(p=pList; p; p=p->pDirty){ u32 nDbsize; /* Db-size field for frame header */ i64 iOffset; /* Write offset in log file */ iOffset = walFrameOffset(++iFrame, szPage); /* Populate and write the frame header */ nDbsize = (isCommit && p->pDirty==0) ? nTruncate : 0; walEncodeFrame(pWal, p->pgno, nDbsize, p->pData, aFrame); rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOffset); if( rc!=SQLITE_OK ){ return rc; } /* Write the page data */ rc = sqlite3OsWrite(pWal->pWalFd, p->pData, szPage, iOffset+sizeof(aFrame)); if( rc!=SQLITE_OK ){ return rc; } pLast = p; } /* Sync the log file if the 'isSync' flag was specified. */ if( sync_flags ){ i64 iSegment = sqlite3OsSectorSize(pWal->pWalFd); i64 iOffset = walFrameOffset(iFrame+1, szPage); assert( isCommit ); assert( iSegment>0 ); iSegment = (((iOffset+iSegment-1)/iSegment) * iSegment); while( iOffsetpgno, nTruncate, pLast->pData, aFrame); rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOffset); if( rc!=SQLITE_OK ){ return rc; } iOffset += WAL_FRAME_HDRSIZE; rc = sqlite3OsWrite(pWal->pWalFd, pLast->pData, szPage, iOffset); if( rc!=SQLITE_OK ){ return rc; } nLast++; iOffset += szPage; } rc = sqlite3OsSync(pWal->pWalFd, sync_flags); } assert( pWal->pWiData==0 ); /* Append data to the wal-index. It is not necessary to lock the ** wal-index to do this as the SQLITE_SHM_WRITE lock held on the wal-index ** guarantees that there are no other writers, and no data that may ** be in use by existing readers is being overwritten. */ iFrame = pWal->hdr.mxFrame; for(p=pList; p && rc==SQLITE_OK; p=p->pDirty){ iFrame++; rc = walIndexAppend(pWal, iFrame, p->pgno); } while( nLast>0 && rc==SQLITE_OK ){ iFrame++; nLast--; rc = walIndexAppend(pWal, iFrame, pLast->pgno); } if( rc==SQLITE_OK ){ /* Update the private copy of the header. */ pWal->hdr.szPage = szPage; pWal->hdr.mxFrame = iFrame; if( isCommit ){ pWal->hdr.iChange++; pWal->hdr.nPage = nTruncate; } /* If this is a commit, update the wal-index header too. */ if( isCommit ){ walIndexWriteHdr(pWal); pWal->iCallback = iFrame; } } walIndexUnmap(pWal); return rc; } /* ** Checkpoint the database: ** ** 1. Acquire a CHECKPOINT lock ** 2. Copy the contents of the log into the database file. ** 3. Zero the wal-index header (so new readers will ignore the log). ** 4. Drop the CHECKPOINT lock. */ int sqlite3WalCheckpoint( Wal *pWal, /* Wal connection */ int sync_flags, /* Flags to sync db file with (or 0) */ int nBuf, /* Size of temporary buffer */ u8 *zBuf, /* Temporary buffer to use */ int (*xBusyHandler)(void *), /* Pointer to busy-handler function */ void *pBusyHandlerArg /* Argument to pass to xBusyHandler */ ){ int rc; /* Return code */ int isChanged = 0; /* True if a new wal-index header is loaded */ assert( pWal->pWiData==0 ); /* Get the CHECKPOINT lock. ** ** Normally, the connection will be in UNLOCK state at this point. But ** if the connection is in exclusive-mode it may still be in READ state ** even though the upper layer has no active read-transaction (because ** WalCloseSnapshot() is not called in exclusive mode). The state will ** be set to UNLOCK when this function returns. This is Ok. */ assert( (pWal->lockState==SQLITE_SHM_UNLOCK) || (pWal->lockState==SQLITE_SHM_READ) ); walSetLock(pWal, SQLITE_SHM_UNLOCK); do { rc = walSetLock(pWal, SQLITE_SHM_CHECKPOINT); }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) ); if( rc!=SQLITE_OK ){ walSetLock(pWal, SQLITE_SHM_UNLOCK); return rc; } /* Copy data from the log to the database file. */ rc = walIndexReadHdr(pWal, &isChanged); if( rc==SQLITE_OK ){ rc = walCheckpoint(pWal, sync_flags, nBuf, zBuf); } if( isChanged ){ /* If a new wal-index header was loaded before the checkpoint was ** performed, then the pager-cache associated with pWal is now ** out of date. So zero the cached wal-index header to ensure that ** next time the pager opens a snapshot on this database it knows that ** the cache needs to be reset. */ memset(&pWal->hdr, 0, sizeof(WalIndexHdr)); } /* Release the locks. */ walIndexUnmap(pWal); walSetLock(pWal, SQLITE_SHM_UNLOCK); return rc; } /* Return the value to pass to a sqlite3_wal_hook callback, the ** number of frames in the WAL at the point of the last commit since ** sqlite3WalCallback() was called. If no commits have occurred since ** the last call, then return 0. */ int sqlite3WalCallback(Wal *pWal){ u32 ret = 0; if( pWal ){ ret = pWal->iCallback; pWal->iCallback = 0; } return (int)ret; } /* ** This function is called to set or query the exclusive-mode flag ** associated with the WAL connection passed as the first argument. The ** exclusive-mode flag should be set to indicate that the caller is ** holding an EXCLUSIVE lock on the database file (it does this in ** locking_mode=exclusive mode). If the EXCLUSIVE lock is to be dropped, ** the flag set by this function should be cleared before doing so. ** ** The value of the exclusive-mode flag may only be modified when ** the WAL connection is in READ state. ** ** When the flag is set, this module does not call the VFS xShmLock() ** method to obtain any locks on the wal-index (as it assumes it ** has exclusive access to the wal and wal-index files anyhow). It ** continues to hold (and does not drop) the existing READ lock on ** the wal-index. ** ** To set or clear the flag, the "op" parameter is passed 1 or 0, ** respectively. To query the flag, pass -1. In all cases, the value ** returned is the value of the exclusive-mode flag (after its value ** has been modified, if applicable). */ int sqlite3WalExclusiveMode(Wal *pWal, int op){ if( op>=0 ){ assert( pWal->lockState==SQLITE_SHM_READ ); pWal->exclusiveMode = (u8)op; } return pWal->exclusiveMode; } #endif /* #ifndef SQLITE_OMIT_WAL */