Simplify the new syntax to be just "COMMIT AND CONTINUE TRANSACTION".

FossilOrigin-Name: 61797109a2f0fee84bc3e0a0c021544c87e4b985e7e7b549e387e396cf1f86ec

manifest

@@ -1,5 +1,5 @@
-C Enhance\sthe\svfstrace\sextension\ssuch\sthat\sthe\soutput\scan\sbe\scontrolled\susing\nthe\s"PRAGMA\svfstrace('...');"\sstatement.\s\sSee\sheader\scomment\son\sthe\ssource\scode\nfor\sdetails.
-D 2024-11-15T20:39:41.451
+C Simplify\sthe\snew\ssyntax\sto\sbe\sjust\s"COMMIT\sAND\sCONTINUE\sTRANSACTION".
+D 2024-11-16T14:40:11.304
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md e108e1e69ae8e8a59e93c455654b8ac9356a11720d3345df2a4743e9590fb20d
@@ -716,7 +716,7 @@ F src/btmutex.c 79a43670447eacc651519a429f6ece9fd638563cf95b469d6891185ddae2b522
 F src/btree.c 63ca6b647342e8cef643863cd0962a542f133e1069460725ba4461dcda92b03c
 F src/btree.h 18e5e7b2124c23426a283523e5f31a4bff029131b795bb82391f9d2f3136fc50
 F src/btreeInt.h 98aadb6dcb77b012cab2574d6a728fad56b337fc946839b9898c4b4c969e30b6
-F src/build.c c6b09342d870a509529244ed8e19b4175a261f2e3163c199241d69e1d8a57607
+F src/build.c c219c01d3490420addfe5e9673ed136cbf89acf4c72370b548cf68a4ad5cc281
 F src/callback.c db3a45e376deff6a16c0058163fe0ae2b73a2945f3f408ca32cf74960b28d490
 F src/complete.c a3634ab1e687055cd002e11b8f43eb75c17da23e
 F src/ctime.c d35723024b963edce9c0fad5b3303e8bb9266083784844baed10a6dedfe26f3b
@@ -761,9 +761,9 @@ F src/os_setup.h 6011ad7af5db4e05155f385eb3a9b4470688de6f65d6166b8956e58a3d87210
 F src/os_unix.c d2edbd92b07a3f778c2defa8a2e9d75acceb6267bda56948c41e8cdda65224d6
 F src/os_win.c db4baa8f62bbfe3967c71b008cea31a8f2ff337c1667ff4d8a677e697315ff0d
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
-F src/pager.c 9656ad4e8331efb8a4f94f7a0c6440b98caea073950a367ea0c728a53b8e62c9
-F src/pager.h 4b1140d691860de0be1347474c51fee07d5420bd7f802d38cbab8ea4ab9f538a
-F src/parse.y 8ec56598aa0df92428627502267d0d1c9778cc27308f8ffd31dfb2d017a8755f
+F src/pager.c 6d5d8ebe7e0fbaa5ad8c6b327e17271334b99e887b871df667a79b71215acfc7
+F src/pager.h 87f5995a18a11130c52d20c9556da02cdfe40ffd7daaa43ca0fcd922d83b7257
+F src/parse.y f0e72511b10d8456d456706b6e7d1ec68486622f66b5593ac00dff9ee9cd2c99
 F src/pcache.c 588cc3c5ccaaadde689ed35ce5c5c891a1f7b1f4d1f56f6cf0143b74d8ee6484
 F src/pcache.h 1497ce1b823cf00094bb0cf3bac37b345937e6f910890c626b16512316d3abf5
 F src/pcache1.c 49516ad7718a3626f28f710fa7448ef1fce3c07fd169acbb4817341950264319
@@ -779,7 +779,7 @@ F src/shell.c.in c61d7af58e678d479f1450e4ea236d2d6ac3bb16a636030104f210303b30d56
 F src/sqlite.h.in 4d93768709c53b7c653a63817a82d5a8625264ca0d8cdf99967ba147bdcf2aa6
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
 F src/sqlite3ext.h 3f046c04ea3595d6bfda99b781926b17e672fd6d27da2ba6d8d8fc39981dcb54
-F src/sqliteInt.h 77be043f8694f4a8702d0ee882022b2e5a6489a0493e77c5d9a73f1efc5a2cc1
+F src/sqliteInt.h 08735a5db1be299c3478ceb03d63729c1c7c4fe1ba82ac7b724535098c86bfa7
 F src/sqliteLimit.h 6993c9cfe3af5b8169ae0e5f15627fc15596726d4f1dc90a221309f79715ce88
 F src/status.c cb11f8589a6912af2da3bb1ec509a94dd8ef27df4d4c1a97e0bcf2309ece972b
 F src/table.c 0f141b58a16de7e2fbe81c308379e7279f4c6b50eb08efeec5892794a0ba30d1
@@ -843,7 +843,7 @@ F src/upsert.c 215328c3f91623c520ec8672c44323553f12caeb4f01b1090ebdca99fdf7b4f1
 F src/utf.c 8b29d9a5956569ea2700f869669b8ef67a9662ee5e724ff77ab3c387e27094ba
 F src/util.c ceebf912f673247e305f16f97f0bb7285fca1d37413b79680714a553a9021d33
 F src/vacuum.c b763b6457bd058d2072ef9364832351fd8d11e8abf70cbb349657360f7d55c40
-F src/vdbe.c 8a6eb02823b424b273614bae41579392a5c495424592b60423dd2c443a583df0
+F src/vdbe.c d726c4fc8cca5d960d95b61573920a5f8e8ff5b34734c918680e00e3c6fbf07f
 F src/vdbe.h c2549a215898a390de6669cfa32adba56f0d7e17ba5a7f7b14506d6fd5f0c36a
 F src/vdbeInt.h 2da01c73e8e3736a9015d5b04aa04d209bc9023d279d237d4d409205e921ea1e
 F src/vdbeapi.c 6353de05e8e78e497ccb33381ba5662ccc11c0339e5b1455faff01b6dacc3075
@@ -855,8 +855,8 @@ F src/vdbetrace.c fe0bc29ebd4e02c8bc5c1945f1d2e6be5927ec12c06d89b03ef2a4def34bf8
 F src/vdbevtab.c fc46b9cbd759dc013f0b3724549cc0d71379183c667df3a5988f7e2f1bd485f3
 F src/vtab.c 316cd48e9320660db3047cd306cd056e4361180cebb4d0f10a39244e10c11422
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c 8b7e309a8012659ac9275ad8cdcc6acaf73fa04b1090e38a01335f230fd10681
-F src/wal.h ba252daaa94f889f4b2c17c027e823d9be47ce39da1d3799886bbd51f0490452
+F src/wal.c 75a94c7dca2018f5a0cf65022f3c59661c7d327b324de068e0e1ee1f271d9dd7
+F src/wal.h 3289ab8741f440d675f08448e067028b275774cb29548e6fb4d3ab789ab6e30e
 F src/walker.c d5006d6b005e4ea7302ad390957a8d41ed83faa177e412f89bc5600a7462a014
 F src/where.c 4de9e7ca5f49e4a21c1d733e2b2fbbc8b62b1a157a58a562c569da84cfcb005b
 F src/whereInt.h 1e36ec50392f7cc3d93d1152d4338064cd522b87156a0739388b7e273735f0ca
@@ -2198,8 +2198,8 @@ F tool/version-info.c 3b36468a90faf1bbd59c65fd0eb66522d9f941eedd364fabccd7227350
 F tool/warnings-clang.sh bbf6a1e685e534c92ec2bfba5b1745f34fb6f0bc2a362850723a9ee87c1b31a7
 F tool/warnings.sh 49a486c5069de041aedcbde4de178293e0463ae9918ecad7539eedf0ec77a139
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P 0773677b553e032e992266c6c75e10565729238df3ef52b56602a92cf651bea7
-R 1cea5b1a16b6733cdd9ac2dd470ae319
+P 605889b02122d98011e9a8d458ba01010423680dd69446458ffe7230a1d47937
+R 14eb9d9ad1dcefff4fb094ea7a1cb27f
 U drh
-Z 6b0f9afcc9547572803b9a01d3227a70
+Z 7bc09c6cab911404200c67cb0830a94e
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-96105d33597765c23dbd490b3aa0c2273731d1970d7041720e9f043dbe3517b3
+61797109a2f0fee84bc3e0a0c021544c87e4b985e7e7b549e387e396cf1f86ec

src/build.c

@@ -5198,6 +5198,7 @@ void sqlite3BeginTransaction(Parse *pParse, int type){
   int i;
 
   assert( pParse!=0 );
+  assert( type==TK_DEFERRED || type==TK_IMMEDIATE || type==TK_EXCLUSIVE );
   db = pParse->db;
   assert( db!=0 );
   if( sqlite3AuthCheck(pParse, SQLITE_TRANSACTION, "BEGIN", 0, 0) ){
@@ -5228,13 +5229,15 @@ void sqlite3BeginTransaction(Parse *pParse, int type){
 ** Code for ROLLBACK is generated if eType==TK_ROLLBACK.  Otherwise
 ** code is generated for a COMMIT.
 */
-void sqlite3EndTransaction(Parse *pParse, int eType){
+void sqlite3EndTransaction(Parse *pParse, int eType, int eRestart){
   Vdbe *v;
   int isRollback;
 
   assert( pParse!=0 );
   assert( pParse->db!=0 );
   assert( eType==TK_COMMIT || eType==TK_END || eType==TK_ROLLBACK );
+  assert( eRestart==0 || eRestart==TK_DEFERRED || eRestart==TK_IMMEDIATE
+            || eRestart==TK_EXCLUSIVE );
   isRollback = eType==TK_ROLLBACK;
   if( sqlite3AuthCheck(pParse, SQLITE_TRANSACTION,
        isRollback ? "ROLLBACK" : "COMMIT", 0, 0) ){
@@ -5242,7 +5245,10 @@ void sqlite3EndTransaction(Parse *pParse, int eType){
   }
   v = sqlite3GetVdbe(pParse);
   if( v ){
-    sqlite3VdbeAddOp2(v, OP_AutoCommit, 1, isRollback);
+    int p3 = 0;
+    if( eRestart ) p3 = eRestart==TK_DEFERRED ? 1 : 2;
+    sqlite3VdbeAddOp3(v, OP_AutoCommit, 1, isRollback, p3);
+    if( eRestart ) sqlite3BeginTransaction(pParse, eRestart);
   }
 }
 

src/pager.c

@@ -643,6 +643,7 @@ struct Pager {
   */
   u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
   u8 eLock;                   /* Current lock held on database file */
+  u8 eMinLock;                /* Minimum lock */
   u8 changeCountDone;         /* Set after incrementing the change-counter */
   u8 setSuper;                /* Super-jrnl name is written into jrnl */
   u8 doNotSpill;              /* Do not spill the cache when non-zero */
@@ -1145,15 +1146,22 @@ static int pagerUnlockDb(Pager *pPager, int eLock){
   assert( !pPager->exclusiveMode || pPager->eLock==eLock );
   assert( eLock==NO_LOCK || eLock==SHARED_LOCK );
   assert( eLock!=NO_LOCK || pagerUseWal(pPager)==0 );
+  pPager->changeCountDone = pPager->tempFile; /* ticket fb3b3024ea238d5c */
   if( isOpen(pPager->fd) ){
     assert( pPager->eLock>=eLock );
+    if( pPager->eMinLock ){
+      if( eLock<SHARED_LOCK ) eLock = SHARED_LOCK;
+      if( pPager->eMinLock==2 && eLock<EXCLUSIVE_LOCK ) eLock = EXCLUSIVE_LOCK;
+      if( eLock>=pPager->eLock ){
+        return SQLITE_OK;
+      }
+    }
     rc = pPager->noLock ? SQLITE_OK : sqlite3OsUnlock(pPager->fd, eLock);
     if( pPager->eLock!=UNKNOWN_LOCK ){
       pPager->eLock = (u8)eLock;
     }
     IOTRACE(("UNLOCK %p %d\n", pPager, eLock))
   }
-  pPager->changeCountDone = pPager->tempFile; /* ticket fb3b3024ea238d5c */
   return rc;
 }
 
@@ -5365,6 +5373,7 @@ int sqlite3PagerSharedLock(Pager *pPager){
       assert( pPager->eState==PAGER_OPEN );
       assert( (pPager->eLock==SHARED_LOCK)
            || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
+           || (pPager->eMinLock>0 && pPager->eLock>=SHARED_LOCK)
       );
     }
 
@@ -6887,6 +6896,24 @@ int sqlite3PagerIsMemdb(Pager *pPager){
   return pPager->tempFile || pPager->memVfs;
 }
 
+/*
+** Set the minimum locking level on all pagers.
+**
+**    0:  No minimum
+**    1:  Read lock
+**    2:  Write lock
+*/
+void sqlite3PagerMinLock(sqlite3 *db, u8 eMinLock){
+  int iDb;
+  Pager *pPager;
+  for(iDb=0; iDb<db->nDb; iDb++){
+    if( db->aDb[iDb].pBt==0 ) continue;
+    pPager = sqlite3BtreePager(db->aDb[iDb].pBt);
+    pPager->eMinLock = eMinLock;
+    if( pPager->pWal ) sqlite3WalMinLock(pPager->pWal, eMinLock);
+  }
+}
+
 /*
 ** Check that there are at least nSavepoint savepoints open. If there are
 ** currently less than nSavepoints open, then open one or more savepoints

src/pager.h

@@ -218,6 +218,7 @@ void *sqlite3PagerTempSpace(Pager*);
 int sqlite3PagerIsMemdb(Pager*);
 void sqlite3PagerCacheStat(Pager *, int, int, u64*);
 void sqlite3PagerClearCache(Pager*);
+void sqlite3PagerMinLock(sqlite3*,u8);
 int sqlite3SectorSize(sqlite3_file *);
 
 /* Functions used to truncate the database file. */

src/parse.y

@@ -169,8 +169,10 @@ transtype(A) ::= .             {A = TK_DEFERRED;}
 transtype(A) ::= DEFERRED(X).  {A = @X; /*A-overwrites-X*/}
 transtype(A) ::= IMMEDIATE(X). {A = @X; /*A-overwrites-X*/}
 transtype(A) ::= EXCLUSIVE(X). {A = @X; /*A-overwrites-X*/}
-cmd ::= COMMIT|END(X) trans_opt.   {sqlite3EndTransaction(pParse,@X);}
-cmd ::= ROLLBACK(X) trans_opt.     {sqlite3EndTransaction(pParse,@X);}
+cmd ::= ROLLBACK(X) trans_opt.            {sqlite3EndTransaction(pParse,@X,0);}
+cmd ::= COMMIT(X) trans_opt.              {sqlite3EndTransaction(pParse,@X,0);}
+cmd ::= END(X) trans_opt.                 {sqlite3EndTransaction(pParse,@X,0);}
+// See also the COMMIT AND BEGIN section below
 
 savepoint_opt ::= SAVEPOINT.
 savepoint_opt ::= .
@@ -474,6 +476,15 @@ resolvetype(A) ::= raisetype(A).
 resolvetype(A) ::= IGNORE.                   {A = OE_Ignore;}
 resolvetype(A) ::= REPLACE.                  {A = OE_Replace;}
 
+////////////////////////// COMMIT AND BEGIN ///////////////////////////////////
+//
+cmd ::= COMMIT(X) AND ID(Y) TRANSACTION. {
+  if( Y.n!=8  || sqlite3_strnicmp(Y.z,"continue",8)!=0 ){
+    sqlite3ErrorMsg(pParse, "near \"%T\": syntax error", &Y);
+  }
+  sqlite3EndTransaction(pParse, @X, TK_IMMEDIATE);
+}
+
 ////////////////////////// The DROP TABLE /////////////////////////////////////
 //
 cmd ::= DROP TABLE ifexists(E) fullname(X). {

src/sqliteInt.h

@@ -5072,8 +5072,8 @@ void sqlite3PrngRestoreState(void);
 void sqlite3RollbackAll(sqlite3*,int);
 void sqlite3CodeVerifySchema(Parse*, int);
 void sqlite3CodeVerifyNamedSchema(Parse*, const char *zDb);
-void sqlite3BeginTransaction(Parse*, int);
-void sqlite3EndTransaction(Parse*,int);
+void sqlite3BeginTransaction(Parse*,int);
+void sqlite3EndTransaction(Parse*,int,int);
 void sqlite3Savepoint(Parse*, int, Token*);
 void sqlite3CloseSavepoints(sqlite3 *);
 void sqlite3LeaveMutexAndCloseZombie(sqlite3*);

src/vdbe.c

@@ -3934,14 +3934,22 @@ case OP_Savepoint: {
   break;
 }
 
-/* Opcode: AutoCommit P1 P2 * * *
+/* Opcode: AutoCommit P1 P2 P3 * *
 **
-** Set the database auto-commit flag to P1 (1 or 0). If P2 is true, roll
-** back any currently active btree transactions. If there are any active
-** VMs (apart from this one), then a ROLLBACK fails.  A COMMIT fails if
-** there are active writing VMs or active VMs that use shared cache.
+** Set the database auto-commit flag to P1 (1 or 0).  The current trasaction
+** while commit when the VDBE halts if the auto-commit flag is 1.  The
+** cureent transaction will stay in effect if the auto-commit flags is 0.
+** Thus, this opcode implements COMMIT when P1 is 0 and it implements
+** BEGIN when P1 is 1.
 **
-** This instruction causes the VM to halt.
+** If P2 is true, rollback any currently active btree transactions. If there
+** are any active VMs (apart from this one), then a ROLLBACK fails.  A
+** COMMIT fails if there are active writing VMs or active VMs that use
+** shared cache.
+**
+** If P3 is 1 or 2 and P1 is 1, then COMMIT but also start a new transaction
+** atomically.  P3 is 1 to restart a read transaction or 2 to restart a write
+** transaction.
 */
 case OP_AutoCommit: {
   int desiredAutoCommit;
@@ -3959,7 +3967,7 @@ case OP_AutoCommit: {
       assert( desiredAutoCommit==1 );
       sqlite3RollbackAll(db, SQLITE_ABORT_ROLLBACK);
       db->autoCommit = 1;
-    }else if( desiredAutoCommit && db->nVdbeWrite>0 ){
+    }else if( desiredAutoCommit && db->nVdbeWrite>(pOp->p3==2) ){
       /* If this instruction implements a COMMIT and other VMs are writing
       ** return an error indicating that the other VMs must complete first.
       */
@@ -3972,6 +3980,7 @@ case OP_AutoCommit: {
     }else{
       db->autoCommit = (u8)desiredAutoCommit;
     }
+    sqlite3PagerMinLock(db, pOp->p3);
     if( sqlite3VdbeHalt(p)==SQLITE_BUSY ){
       p->pc = (int)(pOp - aOp);
       db->autoCommit = (u8)(1-desiredAutoCommit);
@@ -3980,6 +3989,13 @@ case OP_AutoCommit: {
     }
     sqlite3CloseSavepoints(db);
     if( p->rc==SQLITE_OK ){
+      if( pOp->p3 ){
+        db->nVdbeActive++;
+        db->nVdbeRead++;
+        if( pOp->p3==2 ) db->nVdbeWrite++;
+        p->eVdbeState = VDBE_RUN_STATE;
+        break;
+      }
       rc = SQLITE_DONE;
     }else{
       rc = SQLITE_ERROR;
@@ -9128,6 +9144,7 @@ abort_due_to_error:
     rc = SQLITE_CORRUPT_BKPT;
   }
   assert( rc );
+  sqlite3PagerMinLock(db, 0);
 #ifdef SQLITE_DEBUG
   if( db->flags & SQLITE_VdbeTrace ){
     const char *zTrace = p->zSql;

src/wal.c

@@ -516,6 +516,7 @@ struct Wal {
   i16 readLock;              /* Which read lock is being held.  -1 for none */
   u8 syncFlags;              /* Flags to use to sync header writes */
   u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
+  u8 eMinLock;               /* 0, 1, or 2, for none, read, or write */
   u8 writeLock;              /* True if in a write transaction */
   u8 ckptLock;               /* True if holding a checkpoint lock */
   u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
@@ -2064,7 +2065,7 @@ int sqlite3WalWriteLock(Wal *pWal, int bLock){
       }
       walDisableBlocking(pWal);
     }
-  }else if( pWal->writeLock ){
+  }else if( pWal->writeLock && pWal->eMinLock<2 ){
     walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
     pWal->writeLock = 0;
   }
@@ -2698,7 +2699,7 @@ static int walIndexReadHdr(Wal *pWal, int *pChanged){
             *pChanged = 1;
           }
         }
-        if( bWriteLock==0 ){
+        if( bWriteLock==0 && pWal->eMinLock<2 ){
           pWal->writeLock = 0;
           walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
         }
@@ -3670,7 +3671,7 @@ int sqlite3WalBeginWriteTransaction(Wal *pWal){
   /* Cannot start a write transaction without first holding a read
   ** transaction. */
   assert( pWal->readLock>=0 );
-  assert( pWal->writeLock==0 && pWal->iReCksum==0 );
+  // assert( pWal->writeLock==0 && pWal->iReCksum==0 );
 
   if( pWal->readOnly ){
     return SQLITE_READONLY;
@@ -3679,11 +3680,13 @@ int sqlite3WalBeginWriteTransaction(Wal *pWal){
   /* Only one writer allowed at a time.  Get the write lock.  Return
   ** SQLITE_BUSY if unable.
   */
-  rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
-  if( rc ){
-    return rc;
+  if( pWal->writeLock==0 ){
+    rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
+    if( rc ){
+      return rc;
+    }
+    pWal->writeLock = 1;
   }
-  pWal->writeLock = 1;
 
   /* If another connection has written to the database file since the
   ** time the read transaction on this connection was started, then
@@ -3708,7 +3711,7 @@ int sqlite3WalBeginWriteTransaction(Wal *pWal){
 ** routine merely releases the lock.
 */
 int sqlite3WalEndWriteTransaction(Wal *pWal){
-  if( pWal->writeLock ){
+  if( pWal->writeLock && pWal->eMinLock<2 ){
     walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
     pWal->writeLock = 0;
     pWal->iReCksum = 0;
@@ -4419,7 +4422,7 @@ int sqlite3WalCallback(Wal *pWal){
 */
 int sqlite3WalExclusiveMode(Wal *pWal, int op){
   int rc;
-  assert( pWal->writeLock==0 );
+  // assert( pWal->writeLock==0 );
   assert( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE || op==-1 );
 
   /* pWal->readLock is usually set, but might be -1 if there was a
@@ -4456,6 +4459,13 @@ int sqlite3WalExclusiveMode(Wal *pWal, int op){
   return rc;
 }
 
+/*
+** Set the temporary minimum lock level for the WAL subsystem.
+*/
+void sqlite3WalMinLock(Wal *pWal, int eMinLock){
+  pWal->eMinLock = eMinLock;
+}
+
 /*
 ** Return true if the argument is non-NULL and the WAL module is using
 ** heap-memory for the wal-index. Otherwise, if the argument is NULL or the

src/wal.h

@@ -129,6 +129,10 @@ int sqlite3WalExclusiveMode(Wal *pWal, int op);
 */
 int sqlite3WalHeapMemory(Wal *pWal);
 
+/* Set the transient minimum lock level for the WAL
+*/
+void sqlite3WalMinLock(Wal *pWal, int eMinLock);
+
 #ifdef SQLITE_ENABLE_SNAPSHOT
 int sqlite3WalSnapshotGet(Wal *pWal, sqlite3_snapshot **ppSnapshot);
 void sqlite3WalSnapshotOpen(Wal *pWal, sqlite3_snapshot *pSnapshot);