SQLite

static int saveCursorPosition(BtCursor *pCur){
  int rc;

  assert( CURSOR_VALID==pCur->eState || CURSOR_SKIPNEXT==pCur->eState );
  assert( 0==pCur->pKey );
  assert( cursorHoldsMutex(pCur) );

  if( pCur->curFlags & BTCF_Pinned ){
    return SQLITE_CONSTRAINT_PINNED;
  }
  if( pCur->eState==CURSOR_SKIPNEXT ){
    pCur->eState = CURSOR_VALID;
  }else{
    pCur->skipNext = 0;
  }

  rc = saveCursorKey(pCur);

i64 sqlite3BtreeIntegerKey(BtCursor *pCur){
  assert( cursorHoldsMutex(pCur) );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->curIntKey );
  getCellInfo(pCur);
  return pCur->info.nKey;
}

/*
** Pin or unpin a cursor.
*/
void sqlite3BtreeCursorPin(BtCursor *pCur){
  assert( (pCur->curFlags & BTCF_Pinned)==0 );
  pCur->curFlags |= BTCF_Pinned;
}
void sqlite3BtreeCursorUnpin(BtCursor *pCur){
  assert( (pCur->curFlags & BTCF_Pinned)!=0 );
  pCur->curFlags &= ~BTCF_Pinned;
}

#ifdef SQLITE_ENABLE_OFFSET_SQL_FUNC
/*
** Return the offset into the database file for the start of the
** payload to which the cursor is pointing.
*/
i64 sqlite3BtreeOffset(BtCursor *pCur){

                       int flags, int seekResult);
int sqlite3BtreeFirst(BtCursor*, int *pRes);
int sqlite3BtreeLast(BtCursor*, int *pRes);
int sqlite3BtreeNext(BtCursor*, int flags);
int sqlite3BtreeEof(BtCursor*);
int sqlite3BtreePrevious(BtCursor*, int flags);
i64 sqlite3BtreeIntegerKey(BtCursor*);
void sqlite3BtreeCursorPin(BtCursor*);
void sqlite3BtreeCursorUnpin(BtCursor*);
#ifdef SQLITE_ENABLE_OFFSET_SQL_FUNC
i64 sqlite3BtreeOffset(BtCursor*);
#endif
int sqlite3BtreePayload(BtCursor*, u32 offset, u32 amt, void*);
const void *sqlite3BtreePayloadFetch(BtCursor*, u32 *pAmt);
u32 sqlite3BtreePayloadSize(BtCursor*);
sqlite3_int64 sqlite3BtreeMaxRecordSize(BtCursor*);

*/
#define BTCF_WriteFlag    0x01   /* True if a write cursor */
#define BTCF_ValidNKey    0x02   /* True if info.nKey is valid */
#define BTCF_ValidOvfl    0x04   /* True if aOverflow is valid */
#define BTCF_AtLast       0x08   /* Cursor is pointing ot the last entry */
#define BTCF_Incrblob     0x10   /* True if an incremental I/O handle */
#define BTCF_Multiple     0x20   /* Maybe another cursor on the same btree */
#define BTCF_Pinned       0x40   /* Cursor is busy and cannot be moved */

/*
** Potential values for BtCursor.eState.
**
** CURSOR_INVALID:
**   Cursor does not point to a valid entry. This can happen (for example) 
**   because the table is empty or because BtreeCursorFirst() has not been

        nConflictCk = sqlite3VdbeCurrentAddr(v) - addrConflictCk;
        assert( nConflictCk>0 );
        testcase( nConflictCk>1 );
        if( regTrigCnt ){
          sqlite3MultiWrite(pParse);
          nReplaceTrig++;
        }
        if( pTrigger && isUpdate ){
          sqlite3VdbeAddOp1(v, OP_CursorLock, iDataCur);
        }
        sqlite3GenerateRowDelete(pParse, pTab, pTrigger, iDataCur, iIdxCur,
            regR, nPkField, 0, OE_Replace,
            (pIdx==pPk ? ONEPASS_SINGLE : ONEPASS_OFF), iThisCur);
        if( pTrigger && isUpdate ){
          sqlite3VdbeAddOp1(v, OP_CursorUnlock, iDataCur);
        }
        if( regTrigCnt ){
          int addrBypass;  /* Jump destination to bypass recheck logic */

          sqlite3VdbeAddOp2(v, OP_AddImm, regTrigCnt, 1); /* incr trigger cnt */
          addrBypass = sqlite3VdbeAddOp0(v, OP_Goto);  /* Bypass recheck */
          VdbeComment((v, "bypass recheck"));

#define SQLITE_CONSTRAINT_FUNCTION     (SQLITE_CONSTRAINT | (4<<8))
#define SQLITE_CONSTRAINT_NOTNULL      (SQLITE_CONSTRAINT | (5<<8))
#define SQLITE_CONSTRAINT_PRIMARYKEY   (SQLITE_CONSTRAINT | (6<<8))
#define SQLITE_CONSTRAINT_TRIGGER      (SQLITE_CONSTRAINT | (7<<8))
#define SQLITE_CONSTRAINT_UNIQUE       (SQLITE_CONSTRAINT | (8<<8))
#define SQLITE_CONSTRAINT_VTAB         (SQLITE_CONSTRAINT | (9<<8))
#define SQLITE_CONSTRAINT_ROWID        (SQLITE_CONSTRAINT |(10<<8))
#define SQLITE_CONSTRAINT_PINNED       (SQLITE_CONSTRAINT |(11<<8))
#define SQLITE_NOTICE_RECOVER_WAL      (SQLITE_NOTICE | (1<<8))
#define SQLITE_NOTICE_RECOVER_ROLLBACK (SQLITE_NOTICE | (2<<8))
#define SQLITE_WARNING_AUTOINDEX       (SQLITE_WARNING | (1<<8))
#define SQLITE_AUTH_USER               (SQLITE_AUTH | (1<<8))
#define SQLITE_OK_LOAD_PERMANENTLY     (SQLITE_OK | (1<<8))
#define SQLITE_OK_SYMLINK              (SQLITE_OK | (2<<8))

  if( !pOp->p1 ){
    sqlite3ExpirePreparedStatements(db, pOp->p2);
  }else{
    p->expired = pOp->p2+1;
  }
  break;
}

/* Opcode: CursorLock P1 * * * *
**
** Lock the btree to which cursor P1 is pointing so that the btree cannot be
** written by an other cursor.
*/
case OP_CursorLock: {
  VdbeCursor *pC;
  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
  pC = p->apCsr[pOp->p1];
  assert( pC!=0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  sqlite3BtreeCursorPin(pC->uc.pCursor);
  break;
}

/* Opcode: CursorUnlock P1 * * * *
**
** Unlock the btree to which cursor P1 is pointing so that it can be
** written by other cursors.
*/
case OP_CursorUnlock: {
  VdbeCursor *pC;
  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
  pC = p->apCsr[pOp->p1];
  assert( pC!=0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  sqlite3BtreeCursorUnpin(pC->uc.pCursor);
  break;
}

#ifndef SQLITE_OMIT_SHARED_CACHE
/* Opcode: TableLock P1 P2 P3 P4 *
** Synopsis: iDb=P1 root=P2 write=P3
**
** Obtain a lock on a particular table. This instruction is only used when
** the shared-cache feature is enabled. 

      DELETE FROM t0; 
    END;

    INSERT INTO t0 VALUES(1, NULL);
    INSERT INTO t0 VALUES(0, NULL);
  }

  do_catchsql_test 13.1.1 {
    UPDATE OR REPLACE t0 SET c1 = 1;
  } {1 {constraint failed}}

  integrity_check 13.1.2

  do_execsql_test 13.1.3 {
    SELECT * FROM t0
  } {1 {} 0 {}}

  do_execsql_test 13.2.0 {
    CREATE TABLE t2 (a PRIMARY KEY, b UNIQUE, c UNIQUE) WITHOUT ROWID;
    CREATE TRIGGER tr3 AFTER DELETE ON t2 BEGIN 
      DELETE FROM t2; 
    END;

    INSERT INTO t2 VALUES(1, 1, 1);
    INSERT INTO t2 VALUES(2, 2, 2);
  }

  do_catchsql_test 13.2.1 {
    UPDATE OR REPLACE t2 SET c = 0;
  } {1 {constraint failed}}

  integrity_check 13.2.2

  do_execsql_test 13.2.3 {
    SELECT * FROM t2
  } {1 1 1 2 2 2}

  do_execsql_test 13.3.0 {
    CREATE TABLE t1(a, b);
    CREATE TABLE log(x);
    CREATE INDEX i1 ON t1(a);
    INSERT INTO t1 VALUES(1, 2);

   a TEXT,
   b INTEGER PRIMARY KEY UNIQUE
  ); 
  INSERT INTO t1 VALUES(1,2);
  UPDATE t1 SET a = quote(b) WHERE b>=2;
  SELECT * FROM t1;
} {2 2}

# 2019-12-29 ticket https://www.sqlite.org/src/info/314cc133e5ada126
# REPLACE conflict resolution during an UPDATE causes a DELETE trigger 
# to fire.  If that DELETE trigger subsequently modifies the row
# being updated, bad things can happen.  Prevent this by prohibiting
# triggers from making changes to the table being updated while doing
# REPLACE conflict resolution on the UPDATE.
#
# See also tickets:
#   https://www.sqlite.org/src/info/c1e19e12046d23fe 2019-10-25
#   https://www.sqlite.org/src/info/a8a4847a2d96f5de 2019-10-16 
#
reset_db
do_execsql_test update-20.10 {
  PRAGMA recursive_triggers = true;
  CREATE TABLE t1(a UNIQUE ON CONFLICT REPLACE, b);
  INSERT INTO t1(a,b) VALUES(4,12),(9,13);
  CREATE INDEX i0 ON t1(b);
  CREATE TRIGGER tr0 DELETE ON t1 BEGIN
    UPDATE t1 SET b = a;
  END;
  PRAGMA integrity_check;
} {ok}
do_catchsql_test update-20.20 {
  UPDATE t1 SET a=0;
} {1 {constraint failed}}
do_execsql_test update-20.30 {
  PRAGMA integrity_check;
} {ok}

finish_test