** take care that any prior string is freed by a call to [sqlite3_free()]
** prior to assigning a new string to zErrMsg.  ^After the error message
** is delivered up to the client application, the string will be automatically
** freed by sqlite3_free() and the zErrMsg field will be zeroed.
*/
struct sqlite3_vtab {
  const sqlite3_module *pModule;  /* The module for this virtual table */
  int nRef;                       /* Number of open cursors */
  char *zErrMsg;                  /* Error message from sqlite3_mprintf() */
  /* Virtual table implementations will typically add additional fields */
};

/*
** CAPI3REF: Virtual Table Cursor Object
** KEYWORDS: sqlite3_vtab_cursor {virtual table cursor}

    u8 orphanTrigger;           /* Last statement is orphaned TEMP trigger */
    u8 imposterTable;           /* Building an imposter table */
  } init;
  int nVdbeActive;              /* Number of VDBEs currently running */
  int nVdbeRead;                /* Number of active VDBEs that read or write */
  int nVdbeWrite;               /* Number of active VDBEs that read and write */
  int nVdbeExec;                /* Number of nested calls to VdbeExec() */
  int nVDestroy;                /* Number of active OP_VDestroy operations */
  int nExtension;               /* Number of loaded extensions */
  void **aExtension;            /* Array of shared library handles */
  void (*xTrace)(void*,const char*);        /* Trace function */
  void *pTraceArg;                          /* Argument to the trace function */
  void (*xProfile)(void*,const char*,u64);  /* Profiling function */
  void *pProfileArg;                        /* Argument to profile function */
  void *pCommitArg;                 /* Argument to xCommitCallback() */   

** the last one in the database) then a zero is stored in register P2.
** If AUTOVACUUM is disabled then a zero is stored in register P2.
**
** See also: Clear
*/
case OP_Destroy: {     /* out2-prerelease */
  int iMoved;


  int iDb;

  assert( p->readOnly==0 );












  pOut->flags = MEM_Null;
  if( db->nVdbeRead > db->nVDestroy+1 ){
    rc = SQLITE_LOCKED;
    p->errorAction = OE_Abort;
  }else{
    iDb = pOp->p3;

    assert( DbMaskTest(p->btreeMask, iDb) );
    iMoved = 0;  /* Not needed.  Only to silence a warning. */
    rc = sqlite3BtreeDropTable(db->aDb[iDb].pBt, pOp->p1, &iMoved);
    pOut->flags = MEM_Int;
    pOut->u.i = iMoved;
#ifndef SQLITE_OMIT_AUTOVACUUM
    if( rc==SQLITE_OK && iMoved!=0 ){

#ifndef SQLITE_OMIT_VIRTUALTABLE
/* Opcode: VDestroy P1 * * P4 *
**
** P4 is the name of a virtual table in database P1.  Call the xDestroy method
** of that table.
*/
case OP_VDestroy: {
  db->nVDestroy++;
  rc = sqlite3VtabCallDestroy(db, pOp->p1, pOp->p4.z);
  db->nVDestroy--;
  break;
}
#endif /* SQLITE_OMIT_VIRTUALTABLE */

#ifndef SQLITE_OMIT_VIRTUALTABLE
/* Opcode: VOpen P1 * * P4 *
**
** P4 is a pointer to a virtual table object, an sqlite3_vtab structure.
** P1 is a cursor number.  This opcode opens a cursor to the virtual
** table and stores that cursor in P1.
*/
case OP_VOpen: {
  VdbeCursor *pCur;
  sqlite3_vtab_cursor *pVtabCursor;
  sqlite3_vtab *pVtab;
  const sqlite3_module *pModule;

  assert( p->bIsReader );
  pCur = 0;
  pVtabCursor = 0;
  pVtab = pOp->p4.pVtab->pVtab;
  if( pVtab==0 || NEVER(pVtab->pModule==0) ){
    rc = SQLITE_LOCKED;
    break;
  }
  pModule = pVtab->pModule;
  rc = pModule->xOpen(pVtab, &pVtabCursor);
  sqlite3VtabImportErrmsg(p, pVtab);
  if( SQLITE_OK==rc ){
    /* Initialize sqlite3_vtab_cursor base class */
    pVtabCursor->pVtab = pVtab;

    /* Initialize vdbe cursor object */
    pCur = allocateCursor(p, pOp->p1, 0, -1, 0);
    if( pCur ){
      pCur->pVtabCursor = pVtabCursor;
      pVtab->nRef++;
    }else{
      db->mallocFailed = 1;
      pModule->xClose(pVtabCursor);
    }
  }
  break;
}

  {
    res = 0;
    apArg = p->apArg;
    for(i = 0; i<nArg; i++){
      apArg[i] = &pArgc[i+1];
    }


    rc = pModule->xFilter(pVtabCursor, iQuery, pOp->p4.z, nArg, apArg);

    sqlite3VtabImportErrmsg(p, pVtab);
    if( rc==SQLITE_OK ){
      res = pModule->xEof(pVtabCursor);
    }
    VdbeBranchTaken(res!=0,2);
    if( res ){
      pc = pOp->p2 - 1;

  /* Invoke the xNext() method of the module. There is no way for the
  ** underlying implementation to return an error if one occurs during
  ** xNext(). Instead, if an error occurs, true is returned (indicating that 
  ** data is available) and the error code returned when xColumn or
  ** some other method is next invoked on the save virtual table cursor.
  */

  rc = pModule->xNext(pCur->pVtabCursor);

  sqlite3VtabImportErrmsg(p, pVtab);
  if( rc==SQLITE_OK ){
    res = pModule->xEof(pCur->pVtabCursor);
  }
  VdbeBranchTaken(!res,2);
  if( !res ){
    /* If there is data, jump to P2 */

** is set to the value of the rowid for the row just inserted.
**
** P5 is the error actions (OE_Replace, OE_Fail, OE_Ignore, etc) to
** apply in the case of a constraint failure on an insert or update.
*/
case OP_VUpdate: {
  sqlite3_vtab *pVtab;
  const sqlite3_module *pModule;
  int nArg;
  int i;
  sqlite_int64 rowid;
  Mem **apArg;
  Mem *pX;

  assert( pOp->p2==1        || pOp->p5==OE_Fail   || pOp->p5==OE_Rollback 
       || pOp->p5==OE_Abort || pOp->p5==OE_Ignore || pOp->p5==OE_Replace
  );
  assert( p->readOnly==0 );
  pVtab = pOp->p4.pVtab->pVtab;
  if( pVtab==0 || NEVER(pVtab->pModule==0) ){
    rc = SQLITE_LOCKED;
    break;
  }
  pModule = pVtab->pModule;
  nArg = pOp->p2;
  assert( pOp->p4type==P4_VTAB );
  if( ALWAYS(pModule->xUpdate) ){
    u8 vtabOnConflict = db->vtabOnConflict;
    apArg = p->apArg;
    pX = &aMem[pOp->p3];
    for(i=0; i<nArg; i++){

/*
** An instance of the virtual machine.  This structure contains the complete
** state of the virtual machine.
**
** The "sqlite3_stmt" structure pointer that is returned by sqlite3_prepare()
** is really a pointer to an instance of this structure.








*/
struct Vdbe {
  sqlite3 *db;            /* The database connection that owns this statement */
  Op *aOp;                /* Space to hold the virtual machine's program */
  Mem *aMem;              /* The memory locations */
  Mem **apArg;            /* Arguments to currently executing user function */
  Mem *aColName;          /* Column names to return */

#ifdef SQLITE_DEBUG
  int rcApp;              /* errcode set by sqlite3_result_error_code() */
#endif
  u16 nResColumn;         /* Number of columns in one row of the result set */
  u8 errorAction;         /* Recovery action to do in case of an error */
  u8 minWriteFileFormat;  /* Minimum file format for writable database files */
  bft explain:2;          /* True if EXPLAIN present on SQL command */

  bft changeCntOn:1;      /* True to update the change-counter */
  bft expired:1;          /* True if the VM needs to be recompiled */
  bft runOnlyOnce:1;      /* Automatically expire on reset */
  bft usesStmtJournal:1;  /* True if uses a statement journal */
  bft readOnly:1;         /* True for statements that do not write */
  bft bIsReader:1;        /* True for statements that read */
  bft isPrepareV2:1;      /* True if prepared with prepare_v2() */

        zP4 = "(blob)";
      }
      break;
    }
#ifndef SQLITE_OMIT_VIRTUALTABLE
    case P4_VTAB: {
      sqlite3_vtab *pVtab = pOp->p4.pVtab->pVtab;
      sqlite3_snprintf(nTemp, zTemp, "vtab:%p:%p",
                       pVtab, pVtab ? pVtab->pModule : (sqlite3_module*)0);
      break;
    }
#endif
    case P4_INTARRAY: {
      sqlite3_snprintf(nTemp, zTemp, "intarray");
      break;
    }

  }else if( pCx->pCursor ){
    sqlite3BtreeCloseCursor(pCx->pCursor);
  }
#ifndef SQLITE_OMIT_VIRTUALTABLE
  else if( pCx->pVtabCursor ){
    sqlite3_vtab_cursor *pVtabCursor = pCx->pVtabCursor;
    const sqlite3_module *pModule = pVtabCursor->pVtab->pModule;
    assert( pVtabCursor->pVtab->nRef>0 );
    pVtabCursor->pVtab->nRef--;
    pModule->xClose(pVtabCursor);

  }
#endif
}

/*
** Copy the values stored in the VdbeFrame structure to its Vdbe. This
** is used, for example, when a trigger sub-program is halted to restore

*/
int sqlite3VtabCallDestroy(sqlite3 *db, int iDb, const char *zTab){
  int rc = SQLITE_OK;
  Table *pTab;

  pTab = sqlite3FindTable(db, zTab, db->aDb[iDb].zName);
  if( ALWAYS(pTab!=0 && pTab->pVTable!=0) ){
    VTable *p;
    for(p=pTab->pVTable; p; p=p->pNext){
      assert( p->pVtab );
      if( p->pVtab->nRef>0 ){
        return SQLITE_LOCKED;
      }


    }
    p = vtabDisconnectAll(db, pTab);
    rc = p->pMod->pModule->xDestroy(p->pVtab);
    /* Remove the sqlite3_vtab* from the aVTrans[] array, if applicable */
    if( rc==SQLITE_OK ){
      assert( pTab->pVTable==p && p->pNext==0 );
      p->pVtab = 0;
      pTab->pVTable = 0;
      sqlite3VtabUnlock(p);
    }

  } {SQLITE_DONE}
  
  do_test 22.4.2 {
    sqlite3_finalize $stmt
  } {SQLITE_OK}
}


#-------------------------------------------------------------------------
# The following tests verify that a DROP TABLE command on a virtual
# table does not cause other operations to crash.
#
#   23.1: Dropping a vtab while a SELECT is running on it.
#
#   23.2: Dropping a vtab while a SELECT that will, but has not yet,
#         open a cursor on the vtab, is running. In this case the
#         DROP TABLE succeeds and the SELECT hits an error.
#   
#   23.3: Dropping a vtab from within a user-defined-function callback
#         in the middle of an "INSERT INTO vtab SELECT ..." statement.
#
reset_db
load_static_extension db wholenumber
load_static_extension db eval
register_echo_module db

do_test 23.1 {
  execsql { CREATE VIRTUAL TABLE t1 USING wholenumber }
  set res ""
  db eval { SELECT value FROM t1 WHERE value<10 } {
    if {$value == 5} {
      set res [catchsql { DROP TABLE t1 }]
    }
  }
  set res
} {1 {database table is locked}}

do_test 23.2 {
  execsql { 
    CREATE TABLE t2(value);
    INSERT INTO t2 VALUES(1), (2), (3);
  }

  set res2 [list [catch {
    db eval {
      SELECT value FROM t2 UNION ALL 
      SELECT value FROM t1 WHERE value<10
    } {
      if {$value == 2} { set res1 [catchsql { DROP TABLE t1 }] }
    }
  } msg] $msg]
  list $res1 $res2
} {{0 {}} {1 {database table is locked}}}

do_test 23.3.1 {
  execsql { CREATE VIRTUAL TABLE t1e USING echo(t2) }
  execsql { INSERT INTO t1e SELECT 4 }
  catchsql { INSERT INTO t1e SELECT eval('DROP TABLE t1e') }
} {1 {database table is locked}}
do_execsql_test 23.3.2 { SELECT * FROM t1e } {1 2 3 4}

finish_test