}

  assert( pWal->nWiData>0 );
  assert( pWal->apWiData[0]!=0 );
  pInfo = walCkptInfo(pWal);
  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
#ifdef SQLITE_ENABLE_SNAPSHOT
   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0
     || 0==memcmp(&pWal->hdr, pWal->pSnapshot, sizeof(WalIndexHdr)))
#endif
  ){
    /* The WAL has been completely backfilled (or it is empty).
    ** and can be safely ignored.
    */
    rc = walLockShared(pWal, WAL_READ_LOCK(0));
    walShmBarrier(pWal);

  }

  assert( pWal->nWiData>0 );
  assert( pWal->apWiData[0]!=0 );
  pInfo = walCkptInfo(pWal);
  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame
#ifdef SQLITE_ENABLE_SNAPSHOT
   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0)

#endif
  ){
    /* The WAL has been completely backfilled (or it is empty).
    ** and can be safely ignored.
    */
    rc = walLockShared(pWal, WAL_READ_LOCK(0));
    walShmBarrier(pWal);

# 2016 September 23
#
# 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 the sqlite3_snapshot_xxx() APIs.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
ifcapable !snapshot {finish_test; return}
set testprefix snapshot3

# This test does not work with the inmemory_journal permutation. The reason
# is that each connection opened as part of this permutation executes
# "PRAGMA journal_mode=memory", which fails if the database is in wal mode
# and there are one or more existing connections.
if {[permutation]=="inmemory_journal"} {
  finish_test
  return
}

#-------------------------------------------------------------------------
# This block of tests verifies that it is not possible to wrap the wal
# file - using a writer or a "PRAGMA wal_checkpoint = TRUNCATE" - while
# there is an open snapshot transaction (transaction opened using
# sqlite3_snapshot_open()).
#
do_execsql_test 1.0 {
  CREATE TABLE t1(y);
  PRAGMA journal_mode = wal;
  INSERT INTO t1 VALUES(1);
  INSERT INTO t1 VALUES(2);
  INSERT INTO t1 VALUES(3);
  INSERT INTO t1 VALUES(4);
} {wal}

do_test 1.1 {
  sqlite3 db2 test.db
  sqlite3 db3 test.db

  execsql {SELECT * FROM sqlite_master} db2
  execsql {SELECT * FROM sqlite_master} db3

  db2 trans { set snap [sqlite3_snapshot_get_blob db2 main] }
  db2 eval { SELECT * FROM t1 }
} {1 2 3 4}

do_test 1.2 {
  execsql BEGIN db2
  sqlite3_snapshot_open_blob db2 main $snap
  db2 eval { SELECT * FROM t1 }
} {1 2 3 4}

do_test 1.2 {
  execsql END db2
  execsql { PRAGMA wal_checkpoint }

  execsql BEGIN db2
  sqlite3_snapshot_open_blob db2 main $snap
  db2 eval { SELECT * FROM t1 }
} {1 2 3 4}

set sz [file size test.db-wal]
do_test 1.3 {
  execsql { PRAGMA wal_checkpoint = truncate }
  file size test.db-wal
} $sz

do_test 1.4 {
  execsql BEGIN db3
  list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
} {0 {}}

do_test 1.5 {
  db3 eval { SELECT * FROM t1; END }
} {1 2 3 4}

do_test 1.6 {
  db2 eval { SELECT * FROM t1; END }
} {1 2 3 4}

do_test 1.7 {
  execsql { PRAGMA wal_checkpoint = truncate }
  file size test.db-wal
} 0

do_test 1.8 {
  execsql BEGIN db3
  list [catch { sqlite3_snapshot_open_blob db3 main $snap } msg] $msg
} {1 SQLITE_BUSY_SNAPSHOT}

finish_test