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Comment:Ensure that when the source of a backup is a database that is zero bytes in size, the final destination database consists of at least one page. Truncating it to zero bytes is equivalent to zeroing the schema cookie and change counter, which can cause problems for existing clients.
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SHA1:ca86138bda26c5ac5a7e67351acbd19fbd39b0ae
User & Date: drh 2012-10-13 23:16:00
Context
2012-10-15
11:28
Fix a failing assert() in wal.c. check-in: 8745530b user: dan tags: trunk
2012-10-13
23:16
Ensure that when the source of a backup is a database that is zero bytes in size, the final destination database consists of at least one page. Truncating it to zero bytes is equivalent to zeroing the schema cookie and change counter, which can cause problems for existing clients. check-in: ca86138b user: drh tags: trunk
20:55
Mark an always-true conditional as such. Closed-Leaf check-in: ae025cae user: drh tags: zero-byte-backup-fix
09:31
Allow the showdb tool to be compiled with MSVC. check-in: dce391fc user: mistachkin tags: trunk
Changes
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Changes to src/backup.c.

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    /* Update the schema version field in the destination database. This
    ** is to make sure that the schema-version really does change in
    ** the case where the source and destination databases have the
    ** same schema version.
    */
    if( rc==SQLITE_DONE ){





      rc = sqlite3BtreeUpdateMeta(p->pDest,1,p->iDestSchema+1);

      if( rc==SQLITE_OK ){
        if( p->pDestDb ){
          sqlite3ResetAllSchemasOfConnection(p->pDestDb);
        }
        if( destMode==PAGER_JOURNALMODE_WAL ){
          rc = sqlite3BtreeSetVersion(p->pDest, 2);
        }
................................................................................
          nDestTruncate = (nSrcPage+ratio-1)/ratio;
          if( nDestTruncate==(int)PENDING_BYTE_PAGE(p->pDest->pBt) ){
            nDestTruncate--;
          }
        }else{
          nDestTruncate = nSrcPage * (pgszSrc/pgszDest);
        }

        sqlite3PagerTruncateImage(pDestPager, nDestTruncate);

        if( pgszSrc<pgszDest ){
          /* If the source page-size is smaller than the destination page-size,
          ** two extra things may need to happen:
          **
          **   * The destination may need to be truncated, and
................................................................................
          */
          const i64 iSize = (i64)pgszSrc * (i64)nSrcPage;
          sqlite3_file * const pFile = sqlite3PagerFile(pDestPager);
          i64 iOff;
          i64 iEnd;

          assert( pFile );

          assert( (i64)nDestTruncate*(i64)pgszDest >= iSize || (
                nDestTruncate==(int)(PENDING_BYTE_PAGE(p->pDest->pBt)-1)
             && iSize>=PENDING_BYTE && iSize<=PENDING_BYTE+pgszDest
          ));

          /* This call ensures that all data required to recreate the original
          ** database has been stored in the journal for pDestPager and the
          ** journal synced to disk. So at this point we may safely modify







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    /* Update the schema version field in the destination database. This
    ** is to make sure that the schema-version really does change in
    ** the case where the source and destination databases have the
    ** same schema version.
    */
    if( rc==SQLITE_DONE ){
      if( nSrcPage==0 ){
        rc = sqlite3BtreeNewDb(p->pDest);
        nSrcPage = 1;
      }
      if( rc==SQLITE_OK || rc==SQLITE_DONE ){
        rc = sqlite3BtreeUpdateMeta(p->pDest,1,p->iDestSchema+1);
      }
      if( rc==SQLITE_OK ){
        if( p->pDestDb ){
          sqlite3ResetAllSchemasOfConnection(p->pDestDb);
        }
        if( destMode==PAGER_JOURNALMODE_WAL ){
          rc = sqlite3BtreeSetVersion(p->pDest, 2);
        }
................................................................................
          nDestTruncate = (nSrcPage+ratio-1)/ratio;
          if( nDestTruncate==(int)PENDING_BYTE_PAGE(p->pDest->pBt) ){
            nDestTruncate--;
          }
        }else{
          nDestTruncate = nSrcPage * (pgszSrc/pgszDest);
        }
        assert( nDestTruncate>0 );
        sqlite3PagerTruncateImage(pDestPager, nDestTruncate);

        if( pgszSrc<pgszDest ){
          /* If the source page-size is smaller than the destination page-size,
          ** two extra things may need to happen:
          **
          **   * The destination may need to be truncated, and
................................................................................
          */
          const i64 iSize = (i64)pgszSrc * (i64)nSrcPage;
          sqlite3_file * const pFile = sqlite3PagerFile(pDestPager);
          i64 iOff;
          i64 iEnd;

          assert( pFile );
          assert( nDestTruncate==0 
              || (i64)nDestTruncate*(i64)pgszDest >= iSize || (
                nDestTruncate==(int)(PENDING_BYTE_PAGE(p->pDest->pBt)-1)
             && iSize>=PENDING_BYTE && iSize<=PENDING_BYTE+pgszDest
          ));

          /* This call ensures that all data required to recreate the original
          ** database has been stored in the journal for pDestPager and the
          ** journal synced to disk. So at this point we may safely modify

Changes to src/btree.c.

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  put4byte(&data[36 + 4*4], pBt->autoVacuum);
  put4byte(&data[36 + 7*4], pBt->incrVacuum);
#endif
  pBt->nPage = 1;
  data[31] = 1;
  return SQLITE_OK;
}















/*
** 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







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  put4byte(&data[36 + 4*4], pBt->autoVacuum);
  put4byte(&data[36 + 7*4], pBt->incrVacuum);
#endif
  pBt->nPage = 1;
  data[31] = 1;
  return SQLITE_OK;
}

/*
** Initialize the first page of the database file (creating a database
** consisting of a single page and no schema objects). Return SQLITE_OK
** if successful, or an SQLite error code otherwise.
*/
int sqlite3BtreeNewDb(Btree *p){
  int rc;
  sqlite3BtreeEnter(p);
  p->pBt->nPage = 0;
  rc = newDatabase(p->pBt);
  sqlite3BtreeLeave(p);
  return rc;
}

/*
** 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

Changes to src/btree.h.

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int sqlite3BtreeDropTable(Btree*, int, int*);
int sqlite3BtreeClearTable(Btree*, int, int*);
void sqlite3BtreeTripAllCursors(Btree*, int);

void sqlite3BtreeGetMeta(Btree *pBtree, int idx, u32 *pValue);
int sqlite3BtreeUpdateMeta(Btree*, int idx, u32 value);



/*
** The second parameter to sqlite3BtreeGetMeta or sqlite3BtreeUpdateMeta
** should be one of the following values. The integer values are assigned 
** to constants so that the offset of the corresponding field in an
** SQLite database header may be found using the following formula:
**







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int sqlite3BtreeDropTable(Btree*, int, int*);
int sqlite3BtreeClearTable(Btree*, int, int*);
void sqlite3BtreeTripAllCursors(Btree*, int);

void sqlite3BtreeGetMeta(Btree *pBtree, int idx, u32 *pValue);
int sqlite3BtreeUpdateMeta(Btree*, int idx, u32 value);

int sqlite3BtreeNewDb(Btree *p);

/*
** The second parameter to sqlite3BtreeGetMeta or sqlite3BtreeUpdateMeta
** should be one of the following values. The integer values are assigned 
** to constants so that the offset of the corresponding field in an
** SQLite database header may be found using the following formula:
**

Changes to src/pager.c.

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# define DIRECT_MODE 0
  assert( isDirectMode==0 );
  UNUSED_PARAMETER(isDirectMode);
#else
# define DIRECT_MODE isDirectMode
#endif

  if( !pPager->changeCountDone && pPager->dbSize>0 ){
    PgHdr *pPgHdr;                /* Reference to page 1 */

    assert( !pPager->tempFile && isOpen(pPager->fd) );

    /* Open page 1 of the file for writing. */
    rc = sqlite3PagerGet(pPager, 1, &pPgHdr);
    assert( pPgHdr==0 || rc==SQLITE_OK );







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# define DIRECT_MODE 0
  assert( isDirectMode==0 );
  UNUSED_PARAMETER(isDirectMode);
#else
# define DIRECT_MODE isDirectMode
#endif

  if( !pPager->changeCountDone && ALWAYS(pPager->dbSize>0) ){
    PgHdr *pPgHdr;                /* Reference to page 1 */

    assert( !pPager->tempFile && isOpen(pPager->fd) );

    /* Open page 1 of the file for writing. */
    rc = sqlite3PagerGet(pPager, 1, &pPgHdr);
    assert( pPgHdr==0 || rc==SQLITE_OK );

Added test/backup4.test.





















































































































































































































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# 2012 October 13
#
# 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.
#
#***********************************************************************
#
# The tests in this file verify that if an empty database (zero bytes in 
# size) is used as the source of a backup operation, the final destination
# database is one page in size.
#
# The destination must consist of at least one page as truncating a 
# database file to zero bytes is equivalent to resetting the database
# schema cookie and change counter. Doing that could cause other clients
# to become confused and continue using out-of-date cache data.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
set testprefix backup4

#-------------------------------------------------------------------------
# At one point this test was failing because [db] was using an out of
# date schema in test case 1.2.
#
do_execsql_test 1.0 {
  CREATE TABLE t1(x, y, UNIQUE(x, y));
  INSERT INTO t1 VALUES('one', 'two');
  SELECT * FROM t1 WHERE x='one';
  PRAGMA integrity_check;
} {one two ok}

do_test 1.1 {
  sqlite3 db1 :memory:
  db1 backup test.db
  sqlite3 db1 test.db
  db1 eval {
    CREATE TABLE t1(x, y);
    INSERT INTO t1 VALUES('one', 'two');
  }
  db1 close
} {}

do_execsql_test 1.2 {
  SELECT * FROM t1 WHERE x='one';
  PRAGMA integrity_check;
} {one two ok}

db close
forcedelete test.db
forcedelete test.db2
sqlite3 db test.db

#-------------------------------------------------------------------------
# Test that if the source is zero bytes, the destination database 
# consists of a single page only.
#
do_execsql_test 2.1 {
  CREATE TABLE t1(a, b);
  CREATE INDEX i1 ON t1(a, b);
}

do_test 2.2 { file size test.db } 3072

do_test 2.3 {
  sqlite3 db1 test.db2
  db1 backup test.db
  db1 close
  file size test.db
} {1024}

do_test 2.4 { file size test.db2 } 0

db close
forcedelete test.db
forcedelete test.db2
sqlite3 db test.db

#-------------------------------------------------------------------------
# Test that if the destination has a page-size larger than the implicit
# page-size of the source, the final destination database still consists
# of a single page.
#
do_execsql_test 3.1 {
  PRAGMA page_size = 4096;
  CREATE TABLE t1(a, b);
  CREATE INDEX i1 ON t1(a, b);
}

do_test 3.2 { file size test.db } 12288

do_test 3.3 {
  sqlite3 db1 test.db2
  db1 backup test.db
  db1 close
  file size test.db
} {1024}

do_test 3.4 { file size test.db2 } 0

finish_test

Changes to test/pager1.test.

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    CREATE TABLE t2(a, b);
  } db2
  sqlite3_backup B db2 main db main
  list [B step 10000] [B finish]
} {SQLITE_DONE SQLITE_OK}
do_test pager1-9.4.2 {
  list [file size test.db2] [file size test.db]
} {0 0}
db2 close

#-------------------------------------------------------------------------
# Test that regardless of the value returned by xSectorSize(), the
# minimum effective sector-size is 512 and the maximum 65536 bytes.
#
testvfs tv -default 1







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    CREATE TABLE t2(a, b);
  } db2
  sqlite3_backup B db2 main db main
  list [B step 10000] [B finish]
} {SQLITE_DONE SQLITE_OK}
do_test pager1-9.4.2 {
  list [file size test.db2] [file size test.db]
} {1024 0}
db2 close

#-------------------------------------------------------------------------
# Test that regardless of the value returned by xSectorSize(), the
# minimum effective sector-size is 512 and the maximum 65536 bytes.
#
testvfs tv -default 1