[sql] SQLite patch to implement "smart" auto-vacuum.

third_party/sqlite/src/src/btreeInt.h

 /*
 ** 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.
 **
@@ skipping 394 matching lines @@
 **   This feature is included to help prevent writer-starvation.
 */
 struct BtShared {
   Pager *pPager;        /* The page cache */
   sqlite3 *db;          /* Database connection currently using this Btree */
   BtCursor *pCursor;    /* A list of all open cursors */
   MemPage *pPage1;      /* First page of the database */
   u8 openFlags;         /* Flags to sqlite3BtreeOpen() */
 #ifndef SQLITE_OMIT_AUTOVACUUM
   u8 autoVacuum;        /* True if auto-vacuum is enabled */
+  u8 autoVacuumSlack;   /* Optional pages of slack for auto-vacuum */
   u8 incrVacuum;        /* True if incr-vacuum is enabled */
   u8 bDoTruncate;       /* True to truncate db on commit */
 #endif
   u8 inTransaction;     /* Transaction state */
   u8 max1bytePayload;   /* Maximum first byte of cell for a 1-byte payload */
 #ifdef SQLITE_HAS_CODEC
   u8 optimalReserve;    /* Desired amount of reserved space per page */
 #endif
   u16 btsFlags;         /* Boolean parameters.  See BTS_* macros below */
   u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
@@ skipping 267 matching lines @@
 */
 #if SQLITE_BYTEORDER==4321
 # define get2byteAligned(x)  (*(u16*)(x))
 #elif SQLITE_BYTEORDER==1234 && GCC_VERSION>=4008000
 # define get2byteAligned(x)  __builtin_bswap16(*(u16*)(x))
 #elif SQLITE_BYTEORDER==1234 && MSVC_VERSION>=1300
 # define get2byteAligned(x)  _byteswap_ushort(*(u16*)(x))
 #else
 # define get2byteAligned(x)  ((x)[0]<<8 | (x)[1])
 #endif