src/memjournal.c

   1 /*
   2 ** 2008 October 7
   3 **
   4 ** The author disclaims copyright to this source code.  In place of
   5 ** a legal notice, here is a blessing:
   6 **
   7 **    May you do good and not evil.
   8 **    May you find forgiveness for yourself and forgive others.
   9 **    May you share freely, never taking more than you give.
  10 **
  11 *************************************************************************
  12 **
  13 ** This file contains code use to implement an in-memory rollback journal.
  14 ** The in-memory rollback journal is used to journal transactions for
  15 ** ":memory:" databases and when the journal_mode=MEMORY pragma is used.
  16 */
  17 #include "sqliteInt.h"
  18
  19 /* Forward references to internal structures */
  20 typedef struct MemJournal MemJournal;
  21 typedef struct FilePoint FilePoint;
  22 typedef struct FileChunk FileChunk;
  23
  24 /* Space to hold the rollback journal is allocated in increments of
  25 ** this many bytes.
  26 **
  27 ** The size chosen is a little less than a power of two.  That way,
  28 ** the FileChunk object will have a size that almost exactly fills
  29 ** a power-of-two allocation.  This mimimizes wasted space in power-of-two
  30 ** memory allocators.
  31 */
  32 #define JOURNAL_CHUNKSIZE ((int)(1024-sizeof(FileChunk*)))
  33
  34 /* Macro to find the minimum of two numeric values.
  35 */
  36 #ifndef MIN
  37 # define MIN(x,y) ((x)<(y)?(x):(y))
  38 #endif
  39
  40 /*
  41 ** The rollback journal is composed of a linked list of these structures.
  42 */
  43 struct FileChunk {
  44   FileChunk *pNext;               /* Next chunk in the journal */
  45   u8 zChunk[JOURNAL_CHUNKSIZE];   /* Content of this chunk */
  46 };
  47
  48 /*
  49 ** An instance of this object serves as a cursor into the rollback journal.
  50 ** The cursor can be either for reading or writing.
  51 */
  52 struct FilePoint {
  53   sqlite3_int64 iOffset;          /* Offset from the beginning of the file */
  54   FileChunk *pChunk;              /* Specific chunk into which cursor points */
  55 };
  56
  57 /*
  58 ** This subclass is a subclass of sqlite3_file.  Each open memory-journal
  59 ** is an instance of this class.
  60 */
  61 struct MemJournal {
  62   sqlite3_io_methods *pMethod;    /* Parent class. MUST BE FIRST */
  63   FileChunk *pFirst;              /* Head of in-memory chunk-list */
  64   FilePoint endpoint;             /* Pointer to the end of the file */
  65   FilePoint readpoint;            /* Pointer to the end of the last xRead() */
  66 };
  67
  68 /*
  69 ** Read data from the in-memory journal file.  This is the implementation
  70 ** of the sqlite3_vfs.xRead method.
  71 */
  72 static int memjrnlRead(
  73   sqlite3_file *pJfd,    /* The journal file from which to read */
  74   void *zBuf,            /* Put the results here */
  75   int iAmt,              /* Number of bytes to read */
  76   sqlite_int64 iOfst     /* Begin reading at this offset */
  77 ){
  78   MemJournal *p = (MemJournal *)pJfd;
  79   u8 *zOut = zBuf;
  80   int nRead = iAmt;
  81   int iChunkOffset;
  82   FileChunk *pChunk;
  83
  84   /* SQLite never tries to read past the end of a rollback journal file */
  85   assert( iOfst+iAmt<=p->endpoint.iOffset );
  86
  87   if( p->readpoint.iOffset!=iOfst || iOfst==0 ){
  88     sqlite3_int64 iOff = 0;
  89     for(pChunk=p->pFirst;
  90         ALWAYS(pChunk) && (iOff+JOURNAL_CHUNKSIZE)<=iOfst;
  91         pChunk=pChunk->pNext
  92     ){
  93       iOff += JOURNAL_CHUNKSIZE;
  94     }
  95   }else{
  96     pChunk = p->readpoint.pChunk;
  97   }
  98
  99   iChunkOffset = (int)(iOfst%JOURNAL_CHUNKSIZE);
 100   do {
 101     int iSpace = JOURNAL_CHUNKSIZE - iChunkOffset;
 102     int nCopy = MIN(nRead, (JOURNAL_CHUNKSIZE - iChunkOffset));
 103     memcpy(zOut, &pChunk->zChunk[iChunkOffset], nCopy);
 104     zOut += nCopy;
 105     nRead -= iSpace;
 106     iChunkOffset = 0;
 107   } while( nRead>=0 && (pChunk=pChunk->pNext)!=0 && nRead>0 );
 108   p->readpoint.iOffset = iOfst+iAmt;
 109   p->readpoint.pChunk = pChunk;
 110
 111   return SQLITE_OK;
 112 }
 113
 114 /*
 115 ** Write data to the file.
 116 */
 117 static int memjrnlWrite(
 118   sqlite3_file *pJfd,    /* The journal file into which to write */
 119   const void *zBuf,      /* Take data to be written from here */
 120   int iAmt,              /* Number of bytes to write */
 121   sqlite_int64 iOfst     /* Begin writing at this offset into the file */
 122 ){
 123   MemJournal *p = (MemJournal *)pJfd;
 124   int nWrite = iAmt;
 125   u8 *zWrite = (u8 *)zBuf;
 126
 127   /* An in-memory journal file should only ever be appended to. Random
 128   ** access writes are not required by sqlite.
 129   */
 130   assert( iOfst==p->endpoint.iOffset );
 131   UNUSED_PARAMETER(iOfst);
 132
 133   while( nWrite>0 ){
 134     FileChunk *pChunk = p->endpoint.pChunk;
 135     int iChunkOffset = (int)(p->endpoint.iOffset%JOURNAL_CHUNKSIZE);
 136     int iSpace = MIN(nWrite, JOURNAL_CHUNKSIZE - iChunkOffset);
 137
 138     if( iChunkOffset==0 ){
 139       /* New chunk is required to extend the file. */
 140       FileChunk *pNew = sqlite3_malloc(sizeof(FileChunk));
 141       if( !pNew ){
 142         return SQLITE_IOERR_NOMEM;
 143       }
 144       pNew->pNext = 0;
 145       if( pChunk ){
 146         assert( p->pFirst );
 147         pChunk->pNext = pNew;
 148       }else{
 149         assert( !p->pFirst );
 150         p->pFirst = pNew;
 151       }
 152       p->endpoint.pChunk = pNew;
 153     }
 154
 155     memcpy(&p->endpoint.pChunk->zChunk[iChunkOffset], zWrite, iSpace);
 156     zWrite += iSpace;
 157     nWrite -= iSpace;
 158     p->endpoint.iOffset += iSpace;
 159   }
 160
 161   return SQLITE_OK;
 162 }
 163
 164 /*
 165 ** Truncate the file.
 166 */
 167 static int memjrnlTruncate(sqlite3_file *pJfd, sqlite_int64 size){
 168   MemJournal *p = (MemJournal *)pJfd;
 169   FileChunk *pChunk;
 170   assert(size==0);
 171   UNUSED_PARAMETER(size);
 172   pChunk = p->pFirst;
 173   while( pChunk ){
 174     FileChunk *pTmp = pChunk;
 175     pChunk = pChunk->pNext;
 176     sqlite3_free(pTmp);
 177   }
 178   sqlite3MemJournalOpen(pJfd);
 179   return SQLITE_OK;
 180 }
 181
 182 /*
 183 ** Close the file.
 184 */
 185 static int memjrnlClose(sqlite3_file *pJfd){
 186   memjrnlTruncate(pJfd, 0);
 187   return SQLITE_OK;
 188 }
 189
 190
 191 /*
 192 ** Sync the file.
 193 **
 194 ** Syncing an in-memory journal is a no-op.  And, in fact, this routine
 195 ** is never called in a working implementation.  This implementation
 196 ** exists purely as a contingency, in case some malfunction in some other
 197 ** part of SQLite causes Sync to be called by mistake.
 198 */
 199 static int memjrnlSync(sqlite3_file *NotUsed, int NotUsed2){
 200   UNUSED_PARAMETER2(NotUsed, NotUsed2);
 201   return SQLITE_OK;
 202 }
 203
 204 /*
 205 ** Query the size of the file in bytes.
 206 */
 207 static int memjrnlFileSize(sqlite3_file *pJfd, sqlite_int64 *pSize){
 208   MemJournal *p = (MemJournal *)pJfd;
 209   *pSize = (sqlite_int64) p->endpoint.iOffset;
 210   return SQLITE_OK;
 211 }
 212
 213 /*
 214 ** Table of methods for MemJournal sqlite3_file object.
 215 */
 216 static const struct sqlite3_io_methods MemJournalMethods = {
 217   1,                /* iVersion */
 218   memjrnlClose,     /* xClose */
 219   memjrnlRead,      /* xRead */
 220   memjrnlWrite,     /* xWrite */
 221   memjrnlTruncate,  /* xTruncate */
 222   memjrnlSync,      /* xSync */
 223   memjrnlFileSize,  /* xFileSize */
 224   0,                /* xLock */
 225   0,                /* xUnlock */
 226   0,                /* xCheckReservedLock */
 227   0,                /* xFileControl */
 228   0,                /* xSectorSize */
 229   0,                /* xDeviceCharacteristics */
 230   0,                /* xShmMap */
 231   0,                /* xShmLock */
 232   0,                /* xShmBarrier */
 233   0                 /* xShmUnlock */
 234 };
 235
 236 /*
 237 ** Open a journal file.
 238 */
 239 void sqlite3MemJournalOpen(sqlite3_file *pJfd){
 240   MemJournal *p = (MemJournal *)pJfd;
 241   assert( EIGHT_BYTE_ALIGNMENT(p) );
 242   memset(p, 0, sqlite3MemJournalSize());
 243   p->pMethod = (sqlite3_io_methods*)&MemJournalMethods;
 244 }
 245
 246 /*
 247 ** Return true if the file-handle passed as an argument is
 248 ** an in-memory journal
 249 */
 250 int sqlite3IsMemJournal(sqlite3_file *pJfd){
 251   return pJfd->pMethods==&MemJournalMethods;
 252 }
 253
 254 /*
 255 ** Return the number of bytes required to store a MemJournal file descriptor.
 256 */
 257 int sqlite3MemJournalSize(void){
 258   return sizeof(MemJournal);
 259 }