** 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.
**
*************************************************************************
** $Id: btree.c,v 1.569 2009/02/24 18:57:32 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** See the header comment on "btreeInt.h" for additional information.
** Including a description of file format and an overview of operation.
*/
#include "btreeInt.h"

................................................................................
}
#endif


/*
** Forward declaration
*/
static int checkReadLocks(Btree*, Pgno, BtCursor*, i64);


#ifdef SQLITE_OMIT_SHARED_CACHE
  /*
  ** The functions querySharedCacheTableLock(), setSharedCacheTableLock(),
  ** and clearAllSharedCacheTableLocks()
  ** manipulate entries in the BtShared.pLock linked list used to store
................................................................................
  assert( sqlite3BtreeHoldsMutex(p) );
  assert( wrFlag==0 || wrFlag==1 );
  if( wrFlag ){
    assert( !pBt->readOnly );
    if( NEVER(pBt->readOnly) ){
      return SQLITE_READONLY;
    }
    if( checkReadLocks(p, iTable, 0, 0) ){
      return SQLITE_LOCKED;
    }
  }

  if( pBt->pPage1==0 ){
    rc = lockBtreeWithRetry(p);
    if( rc!=SQLITE_OK ){
................................................................................
** This routine checks all cursors that point to table pgnoRoot.
** If any of those cursors were opened with wrFlag==0 in a different
** database connection (a database connection that shares the pager
** cache with the current connection) and that other connection 
** is not in the ReadUncommmitted state, then this routine returns 
** SQLITE_LOCKED.
**
** As well as cursors with wrFlag==0, cursors with wrFlag==1 and 
** isIncrblobHandle==1 are also considered 'read' cursors. Incremental 
** blob cursors are used for both reading and writing.
**
** When pgnoRoot is the root page of an intkey table, this function is also
** responsible for invalidating incremental blob cursors when the table row
** on which they are opened is deleted or modified. Cursors are invalidated
** according to the following rules:
**
**   1) When BtreeClearTable() is called to completely delete the contents
................................................................................
**      to the integer row id of the B-Tree entry being modified. Unless
**      pExclude is itself an incremental blob cursor, then all incremental
**      blob cursors open on row iRow of the B-Tree are invalidated.
**
**   3) If both pExclude and iRow are set to zero, no incremental blob 
**      cursors are invalidated.
*/
static int checkReadLocks(
  Btree *pBtree, 
  Pgno pgnoRoot, 
  BtCursor *pExclude,
  i64 iRow
){
  BtCursor *p;
  BtShared *pBt = pBtree->pBt;
  sqlite3 *db = pBtree->db;
  assert( sqlite3BtreeHoldsMutex(pBtree) );
  for(p=pBt->pCursor; p; p=p->pNext){
    if( p==pExclude ) continue;
................................................................................
  unsigned char *oldCell;
  unsigned char *newCell = 0;

  assert( cursorHoldsMutex(pCur) );
  assert( pBt->inTransaction==TRANS_WRITE );
  assert( !pBt->readOnly );
  assert( pCur->wrFlag );
  if( checkReadLocks(pCur->pBtree, pCur->pgnoRoot, pCur, nKey) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  if( pCur->eState==CURSOR_FAULT ){
    return pCur->skip;
  }

  /* Save the positions of any other cursors open on this table */
................................................................................
  if( pCur->eState==CURSOR_FAULT ){
    return pCur->skip;
  }
  if( NEVER(pCur->aiIdx[pCur->iPage]>=pPage->nCell) ){
    return SQLITE_ERROR;  /* The cursor is not pointing to anything */
  }
  assert( pCur->wrFlag );
  if( checkReadLocks(pCur->pBtree, pCur->pgnoRoot, pCur, pCur->info.nKey) ){


    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }

  /* Restore the current cursor position (a no-op if the cursor is not in 
  ** CURSOR_REQUIRESEEK state) and save the positions of any other cursors 
  ** open on the same table. Then call sqlite3PagerWrite() on the page
  ** that the entry will be deleted from.
................................................................................
*/
int sqlite3BtreeClearTable(Btree *p, int iTable, int *pnChange){
  int rc;
  BtShared *pBt = p->pBt;
  sqlite3BtreeEnter(p);
  pBt->db = p->db;
  assert( p->inTrans==TRANS_WRITE );
  if( (rc = checkReadLocks(p, iTable, 0, 1))!=SQLITE_OK ){
    /* nothing to do */
  }else if( SQLITE_OK!=(rc = saveAllCursors(pBt, iTable, 0)) ){
    /* nothing to do */
  }else{
    rc = clearDatabasePage(pBt, (Pgno)iTable, 0, pnChange);
  }
  sqlite3BtreeLeave(p);
................................................................................
  **   (c) the cursor points at a valid row of an intKey table.
  */
  if( !pCsr->wrFlag ){
    return SQLITE_READONLY;
  }
  assert( !pCsr->pBt->readOnly 
          && pCsr->pBt->inTransaction==TRANS_WRITE );
  if( checkReadLocks(pCsr->pBtree, pCsr->pgnoRoot, pCsr, 0) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  if( pCsr->eState==CURSOR_INVALID || !pCsr->apPage[pCsr->iPage]->intKey ){
    return SQLITE_ERROR;
  }

  return accessPayload(pCsr, offset, amt, (unsigned char *)z, 0, 1);

** 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.
**
*************************************************************************
** $Id: btree.c,v 1.570 2009/02/24 19:21:41 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** See the header comment on "btreeInt.h" for additional information.
** Including a description of file format and an overview of operation.
*/
#include "btreeInt.h"

................................................................................
}
#endif


/*
** Forward declaration
*/
static int checkForReadConflicts(Btree*, Pgno, BtCursor*, i64);


#ifdef SQLITE_OMIT_SHARED_CACHE
  /*
  ** The functions querySharedCacheTableLock(), setSharedCacheTableLock(),
  ** and clearAllSharedCacheTableLocks()
  ** manipulate entries in the BtShared.pLock linked list used to store
................................................................................
  assert( sqlite3BtreeHoldsMutex(p) );
  assert( wrFlag==0 || wrFlag==1 );
  if( wrFlag ){
    assert( !pBt->readOnly );
    if( NEVER(pBt->readOnly) ){
      return SQLITE_READONLY;
    }
    if( checkForReadConflicts(p, iTable, 0, 0) ){
      return SQLITE_LOCKED;
    }
  }

  if( pBt->pPage1==0 ){
    rc = lockBtreeWithRetry(p);
    if( rc!=SQLITE_OK ){
................................................................................
** This routine checks all cursors that point to table pgnoRoot.
** If any of those cursors were opened with wrFlag==0 in a different
** database connection (a database connection that shares the pager
** cache with the current connection) and that other connection 
** is not in the ReadUncommmitted state, then this routine returns 
** SQLITE_LOCKED.
**
** As well as cursors with wrFlag==0, cursors with 
** isIncrblobHandle==1 are also considered 'read' cursors because
** incremental blob cursors are used for both reading and writing.
**
** When pgnoRoot is the root page of an intkey table, this function is also
** responsible for invalidating incremental blob cursors when the table row
** on which they are opened is deleted or modified. Cursors are invalidated
** according to the following rules:
**
**   1) When BtreeClearTable() is called to completely delete the contents
................................................................................
**      to the integer row id of the B-Tree entry being modified. Unless
**      pExclude is itself an incremental blob cursor, then all incremental
**      blob cursors open on row iRow of the B-Tree are invalidated.
**
**   3) If both pExclude and iRow are set to zero, no incremental blob 
**      cursors are invalidated.
*/
static int checkForReadConflicts(
  Btree *pBtree,          /* The database file to check */
  Pgno pgnoRoot,          /* Look for read cursors on this btree */
  BtCursor *pExclude,     /* Ignore this cursor */
  i64 iRow                /* The rowid that might be changing */
){
  BtCursor *p;
  BtShared *pBt = pBtree->pBt;
  sqlite3 *db = pBtree->db;
  assert( sqlite3BtreeHoldsMutex(pBtree) );
  for(p=pBt->pCursor; p; p=p->pNext){
    if( p==pExclude ) continue;
................................................................................
  unsigned char *oldCell;
  unsigned char *newCell = 0;

  assert( cursorHoldsMutex(pCur) );
  assert( pBt->inTransaction==TRANS_WRITE );
  assert( !pBt->readOnly );
  assert( pCur->wrFlag );
  if( checkForReadConflicts(pCur->pBtree, pCur->pgnoRoot, pCur, nKey) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  if( pCur->eState==CURSOR_FAULT ){
    return pCur->skip;
  }

  /* Save the positions of any other cursors open on this table */
................................................................................
  if( pCur->eState==CURSOR_FAULT ){
    return pCur->skip;
  }
  if( NEVER(pCur->aiIdx[pCur->iPage]>=pPage->nCell) ){
    return SQLITE_ERROR;  /* The cursor is not pointing to anything */
  }
  assert( pCur->wrFlag );
  if( checkForReadConflicts(pCur->pBtree, pCur->pgnoRoot,
                            pCur, pCur->info.nKey)
  ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }

  /* Restore the current cursor position (a no-op if the cursor is not in 
  ** CURSOR_REQUIRESEEK state) and save the positions of any other cursors 
  ** open on the same table. Then call sqlite3PagerWrite() on the page
  ** that the entry will be deleted from.
................................................................................
*/
int sqlite3BtreeClearTable(Btree *p, int iTable, int *pnChange){
  int rc;
  BtShared *pBt = p->pBt;
  sqlite3BtreeEnter(p);
  pBt->db = p->db;
  assert( p->inTrans==TRANS_WRITE );
  if( (rc = checkForReadConflicts(p, iTable, 0, 1))!=SQLITE_OK ){
    /* nothing to do */
  }else if( SQLITE_OK!=(rc = saveAllCursors(pBt, iTable, 0)) ){
    /* nothing to do */
  }else{
    rc = clearDatabasePage(pBt, (Pgno)iTable, 0, pnChange);
  }
  sqlite3BtreeLeave(p);
................................................................................
  **   (c) the cursor points at a valid row of an intKey table.
  */
  if( !pCsr->wrFlag ){
    return SQLITE_READONLY;
  }
  assert( !pCsr->pBt->readOnly 
          && pCsr->pBt->inTransaction==TRANS_WRITE );
  if( checkForReadConflicts(pCsr->pBtree, pCsr->pgnoRoot, pCsr, 0) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  if( pCsr->eState==CURSOR_INVALID || !pCsr->apPage[pCsr->iPage]->intKey ){
    return SQLITE_ERROR;
  }

  return accessPayload(pCsr, offset, amt, (unsigned char *)z, 0, 1);