SQLite

*************************************************************************
** This is the implementation of the page cache subsystem.
** 
** The page cache is used to access a database file.  The pager journals
** all writes in order to support rollback.  Locking is used to limit
** access to one or more reader or on writer.
**
** @(#) $Id: pager.c,v 1.2 2001/04/14 16:38:23 drh Exp $
*/
#include "pager.h"
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <assert.h>

/*
** The page cache as a whole is always in one of the following
** states:
**
**   SQLITE_UNLOCK       The page cache is not currently reading or 
**                       writing the database file.  There is no
**                       data held in memory.  This is the initial
**                       state.
**
**   SQLITE_READLOCK     The page cache is reading the database.
**                       Writing is not permitted.  There can be
**                       multiple readers accessing the same database
**                       file at the same time.
**
**   SQLITE_WRITELOCK    The page cache is writing the database.
**                       Access is exclusive.  No other processes or
**                       threads can be reading or writing while one
**                       process is writing.
**
** The page cache comes up in PCS_UNLOCK.  The first time a

/*
** Each in-memory image of a page begins with the following header.
*/
struct PgHdr {
  Pager *pPager;                 /* The pager to which this page belongs */
  Pgno pgno;                     /* The page number for this page */
  PgHdr *pNextHash, *pPrevHash;  /* Hash collision chain for PgHdr.pgno */
  int nRef;                      /* Number of users of this page */
  PgHdr *pNext, *pPrev;          /* Freelist of pages where nRef==0 */
  char inJournal;                /* TRUE if has been written to journal */
  char dirty;                    /* TRUE if we need to write back changes */
  /* SQLITE_PAGE_SIZE bytes of page data follow this header */
};

/*
** Convert a pointer to a PgHdr into a pointer to its data
** and back again.
*/
#define PGHDR_TO_DATA(P)  ((void*)(&(P)[1]))
#define DATA_TO_PGHDR(D)  (&((PgHdr*)(D))[-1])

/*
** The number of page numbers that will fit on one page.
*/

*/
struct Pager {
  char *zFilename;            /* Name of the database file */
  char *zJournal;             /* Name of the journal file */
  int fd, jfd;                /* File descriptors for database and journal */
  int nRef;                   /* Sum of PgHdr.nRef */
  int dbSize;                 /* Number of pages in the file */
  int origDbSize;             /* dbSize before the current change */
  int jSize;                  /* Number of pages in the journal */
  int nIdx;                   /* Number of entries in aIdx[] */
  int nPage;                  /* Total number of in-memory pages */
  int mxPage;                 /* Maximum number of pages to hold in cache */

  char state;                 /* SQLITE_UNLOCK, _READLOCK or _WRITELOCK */
  char ioErr;                 /* True if an I/O error has occurred */
  PgHdr *pFirst, *pLast;      /* List of free pages */
  PgHdr *aHash[N_PG_HASH];    /* Hash table to map page number of PgHdr */
  Pgno aIdx[SQLITE_INDEX_SIZE];  /* Current journal index page */
};

/*
** Hash a page number
*/
#define sqlite_pager_hash(PN)  ((PN)%N_PG_HASH)

** The first time this routine is called, the pager creates a new
** journal and acquires a write lock on the database.  If the write
** lock could not be acquired, this routine returns SQLITE_BUSY.  The
** calling routine must check for that routine and be careful not to
** change any page data until this routine returns SQLITE_OK.
*/
int sqlite_pager_write(void *pData){
  PgHdr *pPg = DATA_TO_PGHDR(pData);
  Pager *pPager = pPg->pPager;
  int rc;

  if( pPg->inJournal ){ return SQLITE_OK; }
  if( pPager->state==SQLITE_UNLOCK ){ return SQLITE_PROTOCOL; }
  if( pPager->state==SQLITE_READLOCK ){
    pPager->jfd = open(pPager->zJournal, O_RDWR|O_CREAT, 0644);
    if( pPager->jfd<0 ){
      return SQLITE_CANTOPEN;
    }
    if( sqlite_pager_lock(pPager->jfd, 1) ){
      close(pPager->jfd);
      pPager->jfd = -1;
      return SQLITE_BUSY;
    }
    sqlite_pager_unlock(pPager->fd);
    if( sqlite_pager_lock(pPager->fd, 1) ){
      close(pPager->jfd);
      pPager->jfd = -1;
      pPager->state = SQLITE_UNLOCK;
      sqlite_pager_reset(pPager);
      return SQLITE_PROTOCOL;
    }
    pPager->state = SQLITE_WRITELOCK;
    pPager->jSize = 1;
    pPager->aIdx[0] = pPager->dbSize;
    pPager->origDbSize = pPager->dbSize;
    pPager->nIdx = 1;
  }
  /* Write this page to the journal */
  assert( pPager->jfd>=0 );
  if( pPg->pgno >= pPager->origDbSize ){
    sqlite_pager_seekpage(pPager->fd, pPg->pgno, SEEK_SET);
    rc = sqlite_pager_writepage(pPager->fd, pData);
    pPg->inJournal = 1;
    return rc;
  }
  pPager->aIdx[pPager->nIdx++] = pPg->pgno;
  sqlite_pager_seekpage(pPager->jfd, pPager->jSize++, SEEK_SET);
  rc = sqlite_pager_write(pPager->jfd, pData);
  pPg->inJournal = 1;
  if( pPager->nIdx==SQLITE_INDEX_SIZE ){
    sqlite_pager_seekpage(pPager->jfd, pPager->idxPgno, SEEK_SET);
    rc = sqlite_pager_writepage(pPager->jfd, &pPager->aIdx);
    pPager->nIdx = 0;
    pPager->jSize++;
  }
  return rc;
}

/*
** Commit all changes to the database and release the write lock.
*/
int sqlite_pager_commit(Pager*){
  int i, rc;
  PgHdr *pPg;
  assert( pPager->state==SQLITE_WRITELOCK );
  assert( pPager->jfd>=0 );
  memset(&pPager->aIdx[&pPager->nIdx], 0, 
          (SQLITE_INDEX_SIZE - pPager->nIdx)*sizeof(Pgno));
  sqlite_pager_seekpage(pPager->jfd, pPager->idxPgno, SEEK_SET);
  rc = sqlite_pager_writepage(pPager->jfd, &pPager->aIdx);
  if( fsync(pPager->jfd) ){
    return SQLITE_IOERR;
  }
  for(i=0; i<N_PG_HASH; i++){
    for(pPg=pPager->aHash[i]; pPg; pPg=pPg->pNextHash){
      if( pPg->dirty==0 ) continue;
      rc = sqlite_pager_seekpage(pPager->fd, pPg->pgno, SEEK_SET);

** Rollback all changes.  The database falls back to read-only mode.
** All in-memory cache pages revert to their original data contents.
** The journal is deleted.
*/
int sqlite_pager_rollback(Pager *pPager){
  int rc;
  if( pPager->state!=SQLITE_WRITELOCK ) return SQLITE_OK;
  memset(&pPager->aIdx[&pPager->nIdx], 0, 
          (SQLITE_INDEX_SIZE - pPager->nIdx)*sizeof(Pgno));
  sqlite_pager_seekpage(pPager->jfd, pPager->idxPgno, SEEK_SET);
  rc = sqlite_pager_writepage(pPager->jfd, &pPager->aIdx);
  rc = sqlite_pager_playback(pPager);
  if( rc!=SQLITE_OK ){
    rc = sqlite_pager_unwritelock(pPager);
  }
  return rc;
};