SQLite

  Wal *pWal,                      /* WAL handle */
  Pgno pgno,                      /* Database page number to read data for */
  u32 *piRead                     /* OUT: Frame number (or zero) */
){
  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 */
  u32 iFirst;
  int iMinHash;

  /* 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 
   || iLast==(iFirst = walCkptInfo(pWal)->nBackfill)
  ){
    *piRead = 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).

  **   (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.
  */
  iMinHash = walFramePage(iFirst);
  for(iHash=walFramePage(iLast); iHash>=iMinHash && iRead==0; iHash--){
    volatile ht_slot *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 nCollide;                 /* Number of hash collisions remaining */
    int rc;                       /* Error code */

    rc = walHashGet(pWal, iHash, &aHash, &aPgno, &iZero);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    nCollide = HASHTABLE_NSLOT;
    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = aHash[iKey] + iZero;
      if( iFrame<=iLast && iFrame>iFirst && aPgno[aHash[iKey]]==pgno ){
        assert( iFrame>iRead || CORRUPT_DB );
        iRead = iFrame;
      }
      if( (nCollide--)==0 ){
        return SQLITE_CORRUPT_BKPT;
      }
    }

} {}

db eval {SELECT test4('3.3.2')}

do_test 3.x {
  db2 close
} {}

#-------------------------------------------------------------------------
# Check that if a wal file has been partially checkpointed, no frames are
# read from the checkpointed part.
#
reset_db
do_execsql_test 4.1 {
  PRAGMA page_size = 1024;
  PRAGMA journal_mode = wal;
  CREATE TABLE t1(a, b);
  CREATE TABLE t2(a, b);
  PRAGMA wal_checkpoint = truncate;
} {wal 0 0 0}

do_test 4.2 {
  execsql { INSERT INTO t1 VALUES(1, 2) }
  file size test.db-wal
} [wal_file_size 1 1024]

do_test 4.3 {
  sqlite3 db2 test.db
  execsql { 
    BEGIN;
    INSERT INTO t2 VALUES(3, 4);
  }
  execsql { PRAGMA wal_checkpoint = passive } db2
} {0 1 1}

do_test 4.3 {
  execsql { COMMIT }
  db2 close
  hexio_write test.db-wal 0 [string repeat 00 2000]
  sqlite3 db2 test.db
} {}

do_test 4.4.1 { 
  catchsql { SELECT * FROM t1 } db2 
} {0 {1 2}}
do_test 4.4.2 { 
  catchsql { SELECT * FROM t2 } db2 
} {1 {database disk image is malformed}}


finish_test