SQLite

/*
** 2004 April 6
**
** The author disclaims copyright to this source code.  In place of
** 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 1.252 2005/03/14 02:01:50 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** For a detailed discussion of BTrees, refer to
**
**     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
**     "Sorting And Searching", pages 473-480. Addison-Wesley
**     Publishing Company, Reading, Massachusetts.

  u16 pageSize;         /* Total number of bytes on a page */
  u16 psAligned;        /* pageSize rounded up to a multiple of 8 */
  u16 usableSize;       /* Number of usable bytes on each page */
  int maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
  int minLocal;         /* Minimum local payload in non-LEAFDATA tables */
  int maxLeaf;          /* Maximum local payload in a LEAFDATA table */
  int minLeaf;          /* Minimum local payload in a LEAFDATA table */
  BusyHandler *pBusyHandler;   /* Callback for when there is lock contention */
};
typedef Btree Bt;

/*
** Btree.inTrans may take one of the following values.
*/
#define TRANS_NONE  0

  return SQLITE_OK;
}

/*
** Change the busy handler callback function.
*/
int sqlite3BtreeSetBusyHandler(Btree *pBt, BusyHandler *pHandler){
  pBt->pBusyHandler = pHandler;
  sqlite3pager_set_busyhandler(pBt->pPager, pHandler);
  return SQLITE_OK;
}

/*
** Change the limit on the number of pages allowed in the cache.
**

page1_init_failed:
  releasePage(pPage1);
  pBt->pPage1 = 0;
  return rc;
}

/*
** This routine works like lockBtree() except that it also invokes the
** busy callback if there is lock contention.
*/
static int lockBtreeWithRetry(Btree *pBt){
  int rc = SQLITE_OK;
  if( pBt->inTrans==TRANS_NONE ){
    rc = sqlite3BtreeBeginTrans(pBt, 0);
    pBt->inTrans = TRANS_NONE;
  }
  return rc;
}
       

/*
** If there are no outstanding cursors and we are not in the middle
** of a transaction but there is a read lock on the database, then
** this routine unrefs the first page of the database file which 
** has the effect of releasing the read lock.
**
** If there are any outstanding cursors, this routine is a no-op.

/*
** Attempt to start a new transaction. A write-transaction
** is started if the second argument is nonzero, otherwise a read-
** transaction.  If the second argument is 2 or more and exclusive
** transaction is started, meaning that no other process is allowed
** to access the database.  A preexisting transaction may not be
** upgraded to exclusive by calling this routine a second time - the
** exclusivity flag only works for a new transaction.
**
** A write-transaction must be started before attempting any 
** changes to the database.  None of the following routines 
** will work unless a transaction is started first:
**
**      sqlite3BtreeCreateTable()
**      sqlite3BtreeCreateIndex()
**      sqlite3BtreeClearTable()
**      sqlite3BtreeDropTable()
**      sqlite3BtreeInsert()
**      sqlite3BtreeDelete()
**      sqlite3BtreeUpdateMeta()
**
** If an initial attempt to acquire the lock fails because of lock contention
** and the database was previously unlocked, then invoke the busy handler
** if there is one.  But if there was previously a read-lock, do not
** invoke the busy handler - just return SQLITE_BUSY.  SQLITE_BUSY is 
** returned when there is already a read-lock in order to avoid a deadlock.
**
** Suppose there are two processes A and B.  A has a read lock and B has
** a reserved lock.  B tries to promote to exclusive but is blocked because
** of A's read lock.  A tries to promote to reserved but is blocked by B.
** One or the other of the two processes must give way or there can be
** no progress.  By returning SQLITE_BUSY and not invoking the busy callback
** when A already has a read lock, we encourage A to give up and let B
** proceed.
*/
int sqlite3BtreeBeginTrans(Btree *pBt, int wrflag){
  int rc = SQLITE_OK;
  int busy = 0;
  BusyHandler *pH;

  /* If the btree is already in a write-transaction, or it
  ** is already in a read-transaction and a read-transaction
  ** is requested, this is a no-op.
  */
  if( pBt->inTrans==TRANS_WRITE || (pBt->inTrans==TRANS_READ && !wrflag) ){

    return SQLITE_OK;
  }

  /* Write transactions are not possible on a read-only database */
  if( pBt->readOnly && wrflag ){
    return SQLITE_READONLY;
  }

  do {
    if( pBt->pPage1==0 ){
      rc = lockBtree(pBt);
    }
  
    if( rc==SQLITE_OK && wrflag ){
      rc = sqlite3pager_begin(pBt->pPage1->aData, wrflag>1);
      if( rc==SQLITE_OK ){
        rc = newDatabase(pBt);
      }
    }
  
    if( rc==SQLITE_OK ){
      pBt->inTrans = (wrflag?TRANS_WRITE:TRANS_READ);
      if( wrflag ) pBt->inStmt = 0;
    }else{
      unlockBtreeIfUnused(pBt);
    }
  }while( rc==SQLITE_BUSY && pBt->inTrans==TRANS_NONE &&
      (pH = pBt->pBusyHandler)!=0 && 
      pH->xFunc && pH->xFunc(pH->pArg, busy++)
  );
  return rc;
}

#ifndef SQLITE_OMIT_AUTOVACUUM

/*
** Set the pointer-map entries for all children of page pPage. Also, if

      return SQLITE_READONLY;
    }
    if( checkReadLocks(pBt, iTable, 0) ){
      return SQLITE_LOCKED;
    }
  }
  if( pBt->pPage1==0 ){
    rc = lockBtreeWithRetry(pBt);
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }
  pCur = sqliteMallocRaw( sizeof(*pCur) );
  if( pCur==0 ){
    rc = SQLITE_NOMEM;

*/
char *sqlite3BtreeIntegrityCheck(Btree *pBt, int *aRoot, int nRoot){
  int i;
  int nRef;
  IntegrityCk sCheck;

  nRef = *sqlite3pager_stats(pBt->pPager);
  if( lockBtreeWithRetry(pBt)!=SQLITE_OK ){
    return sqliteStrDup("Unable to acquire a read lock on the database");
  }
  sCheck.pBt = pBt;
  sCheck.pPager = pBt->pPager;
  sCheck.nPage = sqlite3pager_pagecount(sCheck.pPager);
  if( sCheck.nPage==0 ){
    unlockBtreeIfUnused(pBt);

** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.193 2005/03/14 02:01:50 drh Exp $
*/
#include "sqliteInt.h"
#include "os.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

  assert( PAGER_SHARED==SHARED_LOCK );
  assert( PAGER_RESERVED==RESERVED_LOCK );
  assert( PAGER_EXCLUSIVE==EXCLUSIVE_LOCK );
  if( pPager->state>=locktype ){
    rc = SQLITE_OK;
  }else{
    int busy = 1;
    BusyHandler *pH;
    do {
      rc = sqlite3OsLock(&pPager->fd, locktype);
    }while( rc==SQLITE_BUSY && 
        (pH = pPager->pBusyHandler)!=0 && 
        pH->xFunc && pH->xFunc(pH->pArg, busy++)

    );
    if( rc==SQLITE_OK ){
      pPager->state = locktype;
    }
  }
  return rc;
}

  assert( pPager->state!=PAGER_UNLOCK );
  if( pPager->state==PAGER_SHARED ){
    assert( pPager->aInJournal==0 );
    if( MEMDB ){
      pPager->state = PAGER_EXCLUSIVE;
      pPager->origDbSize = pPager->dbSize;
    }else{



      rc = sqlite3OsLock(&pPager->fd, RESERVED_LOCK);

      if( rc==SQLITE_OK ){
        pPager->state = PAGER_RESERVED;
        if( exFlag ){
          rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
        }
      }
      if( rc!=SQLITE_OK ){

# 2001 September 15
#
# The author disclaims copyright to this source code.  In place of
# 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.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this script is database locks.
#
# $Id: lock.test,v 1.31 2005/03/14 02:01:50 drh Exp $


set testdir [file dirname $argv0]
source $testdir/tester.tcl

# Create an alternative connection to the database
#

# A thread can read when another has a RESERVED lock.
#
do_test lock-2.2 {
  catchsql {SELECT * FROM t2} db2
} {0 {9 8}}

# If the other thread (the one that does not hold the transaction with
# a RESERVED lock) tries to get a RESERVED lock, we do get a busy callback
# as long as we were not orginally holding a READ lock.
#
do_test lock-2.3.1 {
  proc callback {count} {
    set ::callback_value $count
    break
  }
  set ::callback_value {}
  db2 busy callback
  # db2 does not hold a lock so we should get a busy callback here
  set r [catch {execsql {UPDATE t1 SET a=b, b=a} db2} msg]
  lappend r $msg
  lappend r $::callback_value
} {1 {database is locked} 0}
do_test lock-2.3.2 {
  set ::callback_value {}
  execsql {BEGIN; SELECT rowid FROM sqlite_master LIMIT 1} db2
  # This time db2 does hold a read lock.  No busy callback this time.
  set r [catch {execsql {UPDATE t1 SET a=b, b=a} db2} msg]
  lappend r $msg
  lappend r $::callback_value
} {1 {database is locked} {}}
catch {execsql {ROLLBACK} db2}
do_test lock-2.4.1 {
  proc callback {count} {
    lappend ::callback_value $count
    if {$count>4} break
  }
  set ::callback_value {}
  db2 busy callback
  # We get a busy callback because db2 is not holding a lock
  set r [catch {execsql {UPDATE t1 SET a=b, b=a} db2} msg]
  lappend r $msg
  lappend r $::callback_value
} {1 {database is locked} {0 1 2 3 4 5}}
do_test lock-2.4.2 {
  proc callback {count} {
    lappend ::callback_value $count
    if {$count>4} break
  }
  set ::callback_value {}
  db2 busy callback
  execsql {BEGIN; SELECT rowid FROM sqlite_master LIMIT 1} db2
  # No busy callback this time because we are holding a lock
  set r [catch {execsql {UPDATE t1 SET a=b, b=a} db2} msg]
  lappend r $msg
  lappend r $::callback_value
} {1 {database is locked} {}}
catch {execsql {ROLLBACK} db2}
do_test lock-2.5 {
  proc callback {count} {
    lappend ::callback_value $count
    if {$count>4} break
  }
  set ::callback_value {}
  db2 busy callback

  proc callback {count} {
    lappend ::callback_value $count
    if {$count>4} break
  }
  db2 busy callback
  set rc [catch {db2 eval {UPDATE t1 SET a=0}} msg]
  lappend rc $msg $::callback_value
} {1 {database is locked} {0 1 2 3 4 5}}
execsql {ROLLBACK}

# When one thread is writing, other threads cannot read.  Except if the
# writing thread is writing to its temporary tables, the other threads
# can still read.  -> Not so in 3.0.  One thread can read while another
# holds a RESERVED lock.
#

set rcsid {$Id: capi3ref.tcl,v 1.20 2005/03/14 02:01:50 drh Exp $}
source common.tcl
header {C/C++ Interface For SQLite Version 3}
puts {
<h2>C/C++ Interface For SQLite Version 3</h2>
}

proc api {name prototype desc {notused x}} {

 second argument is the number of prior calls to the busy callback
 for the same lock.  If the
 busy callback returns 0, then no additional attempts are made to
 access the database and SQLITE_BUSY is returned.
 If the callback returns non-zero, then another attempt is made to open the
 database for reading and the cycle repeats.

 The presence of a busy handler does not guarantee that
 it will be invoked when there is lock contention.
 If SQLite determines that invoking the busy handler could result in
 a deadlock, it will return SQLITE_BUSY instead.
 Consider a scenario where one process is holding a read lock that
 it is trying to promote to a reserved lock and
 a second process is holding a reserved lock that it is trying
 to promote to an exclusive lock.  The first process cannot proceed