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Overview
Comment:All WAL frame overwrites even if there are active savepoints. This is safe because a ROLLBACK TO will cause all reverted pages to be rewritten to the WAL file prior to COMMIT.
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SHA1: 99b31a6b491c1c51227f478d3713b020d37a17cf
User & Date: drh 2016-01-09 23:55:47.044
Context
2016-01-11
00:52
Improved testability. (check-in: e83d3a2a4e user: drh tags: wal-overwrite-frames)
2016-01-09
23:55
All WAL frame overwrites even if there are active savepoints. This is safe because a ROLLBACK TO will cause all reverted pages to be rewritten to the WAL file prior to COMMIT. (check-in: 99b31a6b49 user: drh tags: wal-overwrite-frames)
18:57
If it is known that checksums will be recalculated on transaction commit, skip calculating checksums when appending frames to the wal file. When recalculating checksums, recalculate them starting with the first overwritten frame - not the first frame in the transaction. (check-in: 16b34f2537 user: dan tags: wal-overwrite-frames)
Changes
Unified Diff Ignore Whitespace Patch
Changes to src/pager.c.
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      pPager->pWal = 0;
      pagerFixMaplimit(pPager);
    }
  }
  return rc;
}

/*
** Return the number of open savepoints.
**/
int sqlite3PagerSavepointCount(Pager *pPager){
  return pPager->nSavepoint;
}

#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** If this is a WAL database, obtain a snapshot handle for the snapshot
** currently open. Otherwise, return an error.
*/
int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){
  int rc = SQLITE_ERROR;








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      pPager->pWal = 0;
      pagerFixMaplimit(pPager);
    }
  }
  return rc;
}








#ifdef SQLITE_ENABLE_SNAPSHOT
/*
** If this is a WAL database, obtain a snapshot handle for the snapshot
** currently open. Otherwise, return an error.
*/
int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){
  int rc = SQLITE_ERROR;
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Changes to src/pager.h.
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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager);
  int sqlite3PagerSavepointCount(Pager *pPager);
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
# endif
#endif

#ifdef SQLITE_ENABLE_ZIPVFS








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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager);

# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
# endif
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
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Changes to src/wal.c.
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  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  pLive = (WalIndexHdr*)walIndexHdr(pWal);
  if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0
   && (isCommit || sqlite3PagerSavepointCount(pList->pPager)==0)
  ){
    iFirst = pLive->mxFrame+1;
  }

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){








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  { int cnt; for(cnt=0, p=pList; p; p=p->pDirty, cnt++){}
    WALTRACE(("WAL%p: frame write begin. %d frames. mxFrame=%d. %s\n",
              pWal, cnt, pWal->hdr.mxFrame, isCommit ? "Commit" : "Spill"));
  }
#endif

  pLive = (WalIndexHdr*)walIndexHdr(pWal);
  if( memcmp(&pWal->hdr, (void *)pLive, sizeof(WalIndexHdr))!=0 ){


    iFirst = pLive->mxFrame+1;
  }

  /* See if it is possible to write these frames into the start of the
  ** log file, instead of appending to it at pWal->hdr.mxFrame.
  */
  if( SQLITE_OK!=(rc = walRestartLog(pWal)) ){
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Changes to test/wal.test.
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  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [wal_file_size 40 1024]]
do_test wal-11.9 {
  db close
  list [expr [file size test.db]/1024] [log_deleted test.db-wal]
} {37 1}
sqlite3_wal db test.db





set nWal 39
if {[permutation]!="mmap"} {set nWal 37}
ifcapable !mmap {set nWal 37}


do_test wal-11.10 {
  execsql {
    PRAGMA cache_size = 10;
    BEGIN;
      INSERT INTO t1 SELECT blob(900) FROM t1;   -- 32
      SELECT count(*) FROM t1;
  }








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  list [expr [file size test.db]/1024] [file size test.db-wal]
} [list 37 [wal_file_size 40 1024]]
do_test wal-11.9 {
  db close
  list [expr [file size test.db]/1024] [log_deleted test.db-wal]
} {37 1}
sqlite3_wal db test.db

# After adding the capability of WAL to overwrite prior uncommitted
# frame in the WAL-file with revised content, the size of the WAL file
# following cache-spill is smaller.
#
#set nWal 39
#if {[permutation]!="mmap"} {set nWal 37}
#ifcapable !mmap {set nWal 37}
set nWal 34

do_test wal-11.10 {
  execsql {
    PRAGMA cache_size = 10;
    BEGIN;
      INSERT INTO t1 SELECT blob(900) FROM t1;   -- 32
      SELECT count(*) FROM t1;
  }
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