SQLite

** 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 */

    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;
}

    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

      }
    }
  }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

  
    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()).

    PRAGMA locking_mode = exclusive;
    SELECT * FROM t2;
  }
} {normal exclusive I II III IV}
do_test wal2-6.5.3 {
  execsql { PRAGMA wal_checkpoint }
} {}
db close

#-------------------------------------------------------------------------
# Test a theory about the checksum algorithm. Theory was false and this
# test did not provoke a bug.
file delete -force test.db test.db-wal test.db-journal
do_test wal2-7.1.1 {
  sqlite3 db test.db
  execsql {
    PRAGMA page_size = 4096;
    PRAGMA journal_mode = WAL;
    CREATE TABLE t1(a, b);
  }
  file size test.db
} {4096}
do_test wal2-7.1.2 {
  file copy -force test.db test2.db
  file copy -force test.db-wal test2.db-wal
  hexio_write test2.db-wal 48 FF
} {1}
do_test wal2-7.1.3 {
  sqlite3 db2 test2.db
  execsql { PRAGMA wal_checkpoint } db2
  execsql { SELECT * FROM sqlite_master } db2
} {}
db2 close

finish_test