** to select one of the aReadMark[] entries that is closest to
  ** but not exceeding pWal->hdr.mxFrame and lock that entry.
  */
  mxReadMark = 0;
  mxI = 0;
  mxFrame = pWal->hdr.mxFrame;
#ifdef SQLITE_ENABLE_SNAPSHOT

  if( pWal->pSnapshot ) mxFrame = pWal->pSnapshot->mxFrame;

#endif
  for(i=1; i<WAL_NREADER; i++){
    u32 thisMark = pInfo->aReadMark[i];
    if( mxReadMark<=thisMark && thisMark<=mxFrame ){
      assert( thisMark!=READMARK_NOT_USED );
      mxReadMark = thisMark;
      mxI = i;

  ** to select one of the aReadMark[] entries that is closest to
  ** but not exceeding pWal->hdr.mxFrame and lock that entry.
  */
  mxReadMark = 0;
  mxI = 0;
  mxFrame = pWal->hdr.mxFrame;
#ifdef SQLITE_ENABLE_SNAPSHOT
  if( pWal->pSnapshot && pWal->pSnapshot->mxFrame<mxFrame ){
    mxFrame = pWal->pSnapshot->mxFrame;
  }
#endif
  for(i=1; i<WAL_NREADER; i++){
    u32 thisMark = pInfo->aReadMark[i];
    if( mxReadMark<=thisMark && thisMark<=mxFrame ){
      assert( thisMark!=READMARK_NOT_USED );
      mxReadMark = thisMark;
      mxI = i;

# 2015 December 7
#
# 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 file is testing the SELECT statement.
#
# $Id: select1.test,v 1.70 2009/05/28 01:00:56 drh Exp $

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

#-------------------------------------------------------------------------
# Check some error conditions in snapshot_get(). It is an error if:
#
#  1) snapshot_get() is called on a non-WAL database.
#  2) there is no open read transaction on the database, or
#  3) there is an open write transaction on the database.
#
do_execsql_test 1.0 {
  CREATE TABLE t1(a, b);
  INSERT INTO t1 VALUES(1, 2);
  INSERT INTO t1 VALUES(3, 4);
}

do_test 1.1.1 {
  execsql { BEGIN; SELECT * FROM t1; }
  list [catch { sqlite3_snapshot_get db main } msg] $msg
} {1 SQLITE_ERROR}
do_execsql_test 1.1.2 COMMIT

do_test 1.2.1 {
  execsql {
    PRAGMA journal_mode = wal;
    INSERT INTO t1 VALUES(5, 6);
  }
  list [catch { sqlite3_snapshot_get db main } msg] $msg
} {1 SQLITE_ERROR}

do_test 1.3.1 {
  execsql {
    BEGIN;
      INSERT INTO t1 VALUES(7, 8);
  }
  list [catch { sqlite3_snapshot_get db main } msg] $msg
} {1 SQLITE_ERROR}
do_execsql_test 1.3.2 COMMIT

#-------------------------------------------------------------------------
# Check that a simple case works. Reuse the database created by the
# block of tests above.
#
do_execsql_test 2.0 {
  BEGIN;
    SELECT * FROM t1;
} {1 2 3 4 5 6 7 8}

do_test 2.1 {
  set snapshot [sqlite3_snapshot_get db main]
  execsql {
    COMMIT;
    INSERT INTO t1 VALUES(9, 10);
    SELECT * FROM t1;
  }
} {1 2 3 4 5 6 7 8 9 10}

do_test 2.2 {
  execsql BEGIN
  sqlite3_snapshot_open db main $snapshot
  execsql { SELECT * FROM t1 }
} {1 2 3 4 5 6 7 8}

do_test 2.3 {
  sqlite3_snapshot_free $snapshot
  execsql COMMIT
} {}

#-------------------------------------------------------------------------
# Check some errors in sqlite3_snapshot_open(). It is an error if:
#
#   1) the db is in auto-commit mode,
#   2) the db has an open (read or write) transaction,
#   3) the db is not a wal database,
#
# Reuse the database created by earlier tests.
#
do_execsql_test 3.0.0 {
  CREATE TABLE t2(x, y);
  INSERT INTO t2 VALUES('a', 'b');
  INSERT INTO t2 VALUES('c', 'd');
  BEGIN;
    SELECT * FROM t2;
} {a b c d}
do_test 3.0.1 {
  set snapshot [sqlite3_snapshot_get db main]
  execsql { COMMIT }
  execsql { INSERT INTO t2 VALUES('e', 'f'); }
} {}

do_test 3.1 {
  list [catch {sqlite3_snapshot_open db main $snapshot } msg] $msg
} {1 SQLITE_ERROR}

do_test 3.2.1 {
  execsql {
    BEGIN;
      SELECT * FROM t2;
  }
} {a b c d e f}
do_test 3.2.2 {
  list [catch {sqlite3_snapshot_open db main $snapshot } msg] $msg
} {1 SQLITE_ERROR}

do_test 3.2.3 {
  execsql {
    COMMIT;
    BEGIN;
      INSERT INTO t2 VALUES('g', 'h');
  }
  list [catch {sqlite3_snapshot_open db main $snapshot } msg] $msg
} {1 SQLITE_ERROR}
do_execsql_test 3.2.4 COMMIT

do_test 3.3.1 {
  execsql { PRAGMA journal_mode = DELETE }
  execsql { BEGIN }
  list [catch {sqlite3_snapshot_open db main $snapshot } msg] $msg
} {1 SQLITE_ERROR}

do_test 3.3.2 {
  sqlite3_snapshot_free $snapshot
  execsql COMMIT
} {}

#-------------------------------------------------------------------------
# Check that SQLITE_BUSY_SNAPSHOT is returned if the specified snapshot
# no longer exists because the wal file has been checkpointed.
#
#   1. Reading a snapshot from the middle of a wal file is not possible
#      after the wal file has been checkpointed.
#
#   2. That a snapshot from the end of a wal file can not be read once
#      the wal file has been wrapped.
#
do_execsql_test 4.1.0 {
  PRAGMA journal_mode = wal;
  CREATE TABLE t3(i, j);
  INSERT INTO t3 VALUES('o', 't');
  INSERT INTO t3 VALUES('t', 'f');
  BEGIN;
    SELECT * FROM t3;
} {wal o t t f}

do_test 4.1.1 {
  set snapshot [sqlite3_snapshot_get db main]
  execsql COMMIT
} {}
do_test 4.1.2 {
  execsql { 
    INSERT INTO t3 VALUES('f', 's'); 
    BEGIN;
  }
  sqlite3_snapshot_open db main $snapshot
  execsql { SELECT * FROM t3 }
} {o t t f}

do_test 4.1.3 {
  execsql { 
    COMMIT;
    PRAGMA wal_checkpoint;
    BEGIN;
  }
  list [catch {sqlite3_snapshot_open db main $snapshot} msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}
do_test 4.1.4 {
  sqlite3_snapshot_free $snapshot
  execsql COMMIT
} {}

do_test 4.2.1 {
  execsql {
    INSERT INTO t3 VALUES('s', 'e');
    INSERT INTO t3 VALUES('n', 't');
    BEGIN;
      SELECT * FROM t3;
  }
} {o t t f f s s e n t}
do_test 4.2.2 {
  set snapshot [sqlite3_snapshot_get db main]
  execsql {
    COMMIT;
    PRAGMA wal_checkpoint;
    BEGIN;
  }
  sqlite3_snapshot_open db main $snapshot
  execsql { SELECT * FROM t3 }
} {o t t f f s s e n t}
do_test 4.2.3 {
  execsql {
    COMMIT;
    INSERT INTO t3 VALUES('e', 't');
    BEGIN;
  }
  list [catch {sqlite3_snapshot_open db main $snapshot} msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}
do_test 4.2.4 {
  sqlite3_snapshot_free $snapshot
} {}

finish_test