src/test_thread.c

   1 /*
   2 ** 2007 September 9
   3 **
   4 ** The author disclaims copyright to this source code.  In place of
   5 ** a legal notice, here is a blessing:
   6 **
   7 **    May you do good and not evil.
   8 **    May you find forgiveness for yourself and forgive others.
   9 **    May you share freely, never taking more than you give.
  10 **
  11 *************************************************************************
  12 **
  13 ** This file contains the implementation of some Tcl commands used to
  14 ** test that sqlite3 database handles may be concurrently accessed by
  15 ** multiple threads. Right now this only works on unix.
  16 */
  17
  18 #include "sqliteInt.h"
  19 #if defined(INCLUDE_SQLITE_TCL_H)
  20 #  include "sqlite_tcl.h"
  21 #else
  22 #  include "tcl.h"
  23 #endif
  24
  25 #if SQLITE_THREADSAFE
  26
  27 #include <errno.h>
  28
  29 #if !defined(_MSC_VER)
  30 #include <unistd.h>
  31 #endif
  32
  33 /*
  34 ** One of these is allocated for each thread created by [sqlthread spawn].
  35 */
  36 typedef struct SqlThread SqlThread;
  37 struct SqlThread {
  38   Tcl_ThreadId parent;     /* Thread id of parent thread */
  39   Tcl_Interp *interp;      /* Parent interpreter */
  40   char *zScript;           /* The script to execute. */
  41   char *zVarname;          /* Varname in parent script */
  42 };
  43
  44 /*
  45 ** A custom Tcl_Event type used by this module. When the event is
  46 ** handled, script zScript is evaluated in interpreter interp. If
  47 ** the evaluation throws an exception (returns TCL_ERROR), then the
  48 ** error is handled by Tcl_BackgroundError(). If no error occurs,
  49 ** the result is simply discarded.
  50 */
  51 typedef struct EvalEvent EvalEvent;
  52 struct EvalEvent {
  53   Tcl_Event base;          /* Base class of type Tcl_Event */
  54   char *zScript;           /* The script to execute. */
  55   Tcl_Interp *interp;      /* The interpreter to execute it in. */
  56 };
  57
  58 static Tcl_ObjCmdProc sqlthread_proc;
  59 static Tcl_ObjCmdProc clock_seconds_proc;
  60 #if SQLITE_OS_UNIX && defined(SQLITE_ENABLE_UNLOCK_NOTIFY)
  61 static Tcl_ObjCmdProc blocking_step_proc;
  62 static Tcl_ObjCmdProc blocking_prepare_v2_proc;
  63 #endif
  64 int Sqlitetest1_Init(Tcl_Interp *);
  65 int Sqlite3_Init(Tcl_Interp *);
  66
  67 /* Functions from main.c */
  68 extern const char *sqlite3ErrName(int);
  69
  70 /* Functions from test1.c */
  71 extern void *sqlite3TestTextToPtr(const char *);
  72 extern int getDbPointer(Tcl_Interp *, const char *, sqlite3 **);
  73 extern int sqlite3TestMakePointerStr(Tcl_Interp *, char *, void *);
  74 extern int sqlite3TestErrCode(Tcl_Interp *, sqlite3 *, int);
  75
  76 /*
  77 ** Handler for events of type EvalEvent.
  78 */
  79 static int SQLITE_TCLAPI tclScriptEvent(Tcl_Event *evPtr, int flags){
  80   int rc;
  81   EvalEvent *p = (EvalEvent *)evPtr;
  82   rc = Tcl_Eval(p->interp, p->zScript);
  83   if( rc!=TCL_OK ){
  84     Tcl_BackgroundError(p->interp);
  85   }
  86   UNUSED_PARAMETER(flags);
  87   return 1;
  88 }
  89
  90 /*
  91 ** Register an EvalEvent to evaluate the script pScript in the
  92 ** parent interpreter/thread of SqlThread p.
  93 */
  94 static void postToParent(SqlThread *p, Tcl_Obj *pScript){
  95   EvalEvent *pEvent;
  96   char *zMsg;
  97   int nMsg;
  98
  99   zMsg = Tcl_GetStringFromObj(pScript, &nMsg);
 100   pEvent = (EvalEvent *)ckalloc(sizeof(EvalEvent)+nMsg+1);
 101   pEvent->base.nextPtr = 0;
 102   pEvent->base.proc = tclScriptEvent;
 103   pEvent->zScript = (char *)&pEvent[1];
 104   memcpy(pEvent->zScript, zMsg, nMsg+1);
 105   pEvent->interp = p->interp;
 106
 107   Tcl_ThreadQueueEvent(p->parent, (Tcl_Event *)pEvent, TCL_QUEUE_TAIL);
 108   Tcl_ThreadAlert(p->parent);
 109 }
 110
 111 /*
 112 ** The main function for threads created with [sqlthread spawn].
 113 */
 114 static Tcl_ThreadCreateType tclScriptThread(ClientData pSqlThread){
 115   Tcl_Interp *interp;
 116   Tcl_Obj *pRes;
 117   Tcl_Obj *pList;
 118   int rc;
 119   SqlThread *p = (SqlThread *)pSqlThread;
 120   extern int Sqlitetest_mutex_Init(Tcl_Interp*);
 121
 122   interp = Tcl_CreateInterp();
 123   Tcl_CreateObjCommand(interp, "clock_seconds", clock_seconds_proc, 0, 0);
 124   Tcl_CreateObjCommand(interp, "sqlthread", sqlthread_proc, pSqlThread, 0);
 125 #if SQLITE_OS_UNIX && defined(SQLITE_ENABLE_UNLOCK_NOTIFY)
 126   Tcl_CreateObjCommand(interp, "sqlite3_blocking_step", blocking_step_proc,0,0);
 127   Tcl_CreateObjCommand(interp,
 128       "sqlite3_blocking_prepare_v2", blocking_prepare_v2_proc, (void *)1, 0);
 129   Tcl_CreateObjCommand(interp,
 130       "sqlite3_nonblocking_prepare_v2", blocking_prepare_v2_proc, 0, 0);
 131 #endif
 132   Sqlitetest1_Init(interp);
 133   Sqlitetest_mutex_Init(interp);
 134   Sqlite3_Init(interp);
 135
 136   rc = Tcl_Eval(interp, p->zScript);
 137   pRes = Tcl_GetObjResult(interp);
 138   pList = Tcl_NewObj();
 139   Tcl_IncrRefCount(pList);
 140   Tcl_IncrRefCount(pRes);
 141
 142   if( rc!=TCL_OK ){
 143     Tcl_ListObjAppendElement(interp, pList, Tcl_NewStringObj("error", -1));
 144     Tcl_ListObjAppendElement(interp, pList, pRes);
 145     postToParent(p, pList);
 146     Tcl_DecrRefCount(pList);
 147     pList = Tcl_NewObj();
 148   }
 149
 150   Tcl_ListObjAppendElement(interp, pList, Tcl_NewStringObj("set", -1));
 151   Tcl_ListObjAppendElement(interp, pList, Tcl_NewStringObj(p->zVarname, -1));
 152   Tcl_ListObjAppendElement(interp, pList, pRes);
 153   postToParent(p, pList);
 154
 155   ckfree((void *)p);
 156   Tcl_DecrRefCount(pList);
 157   Tcl_DecrRefCount(pRes);
 158   Tcl_DeleteInterp(interp);
 159   while( Tcl_DoOneEvent(TCL_ALL_EVENTS|TCL_DONT_WAIT) );
 160   Tcl_ExitThread(0);
 161   TCL_THREAD_CREATE_RETURN;
 162 }
 163
 164 /*
 165 ** sqlthread spawn VARNAME SCRIPT
 166 **
 167 **     Spawn a new thread with its own Tcl interpreter and run the
 168 **     specified SCRIPT(s) in it. The thread terminates after running
 169 **     the script. The result of the script is stored in the variable
 170 **     VARNAME.
 171 **
 172 **     The caller can wait for the script to terminate using [vwait VARNAME].
 173 */
 174 static int SQLITE_TCLAPI sqlthread_spawn(
 175   ClientData clientData,
 176   Tcl_Interp *interp,
 177   int objc,
 178   Tcl_Obj *CONST objv[]
 179 ){
 180   Tcl_ThreadId x;
 181   SqlThread *pNew;
 182   int rc;
 183
 184   int nVarname; char *zVarname;
 185   int nScript; char *zScript;
 186
 187   /* Parameters for thread creation */
 188   const int nStack = TCL_THREAD_STACK_DEFAULT;
 189   const int flags = TCL_THREAD_NOFLAGS;
 190
 191   assert(objc==4);
 192   UNUSED_PARAMETER(clientData);
 193   UNUSED_PARAMETER(objc);
 194
 195   zVarname = Tcl_GetStringFromObj(objv[2], &nVarname);
 196   zScript = Tcl_GetStringFromObj(objv[3], &nScript);
 197
 198   pNew = (SqlThread *)ckalloc(sizeof(SqlThread)+nVarname+nScript+2);
 199   pNew->zVarname = (char *)&pNew[1];
 200   pNew->zScript = (char *)&pNew->zVarname[nVarname+1];
 201   memcpy(pNew->zVarname, zVarname, nVarname+1);
 202   memcpy(pNew->zScript, zScript, nScript+1);
 203   pNew->parent = Tcl_GetCurrentThread();
 204   pNew->interp = interp;
 205
 206   rc = Tcl_CreateThread(&x, tclScriptThread, (void *)pNew, nStack, flags);
 207   if( rc!=TCL_OK ){
 208     Tcl_AppendResult(interp, "Error in Tcl_CreateThread()", 0);
 209     ckfree((char *)pNew);
 210     return TCL_ERROR;
 211   }
 212
 213   return TCL_OK;
 214 }
 215
 216 /*
 217 ** sqlthread parent SCRIPT
 218 **
 219 **     This can be called by spawned threads only. It sends the specified
 220 **     script back to the parent thread for execution. The result of
 221 **     evaluating the SCRIPT is returned. The parent thread must enter
 222 **     the event loop for this to work - otherwise the caller will
 223 **     block indefinitely.
 224 **
 225 **     NOTE: At the moment, this doesn't work. FIXME.
 226 */
 227 static int SQLITE_TCLAPI sqlthread_parent(
 228   ClientData clientData,
 229   Tcl_Interp *interp,
 230   int objc,
 231   Tcl_Obj *CONST objv[]
 232 ){
 233   EvalEvent *pEvent;
 234   char *zMsg;
 235   int nMsg;
 236   SqlThread *p = (SqlThread *)clientData;
 237
 238   assert(objc==3);
 239   UNUSED_PARAMETER(objc);
 240
 241   if( p==0 ){
 242     Tcl_AppendResult(interp, "no parent thread", 0);
 243     return TCL_ERROR;
 244   }
 245
 246   zMsg = Tcl_GetStringFromObj(objv[2], &nMsg);
 247   pEvent = (EvalEvent *)ckalloc(sizeof(EvalEvent)+nMsg+1);
 248   pEvent->base.nextPtr = 0;
 249   pEvent->base.proc = tclScriptEvent;
 250   pEvent->zScript = (char *)&pEvent[1];
 251   memcpy(pEvent->zScript, zMsg, nMsg+1);
 252   pEvent->interp = p->interp;
 253   Tcl_ThreadQueueEvent(p->parent, (Tcl_Event *)pEvent, TCL_QUEUE_TAIL);
 254   Tcl_ThreadAlert(p->parent);
 255
 256   return TCL_OK;
 257 }
 258
 259 static int xBusy(void *pArg, int nBusy){
 260   UNUSED_PARAMETER(pArg);
 261   UNUSED_PARAMETER(nBusy);
 262   sqlite3_sleep(50);
 263   return 1;             /* Try again... */
 264 }
 265
 266 /*
 267 ** sqlthread open
 268 **
 269 **     Open a database handle and return the string representation of
 270 **     the pointer value.
 271 */
 272 static int SQLITE_TCLAPI sqlthread_open(
 273   ClientData clientData,
 274   Tcl_Interp *interp,
 275   int objc,
 276   Tcl_Obj *CONST objv[]
 277 ){
 278   int sqlite3TestMakePointerStr(Tcl_Interp *interp, char *zPtr, void *p);
 279
 280   const char *zFilename;
 281   sqlite3 *db;
 282   char zBuf[100];
 283   extern int Md5_Register(sqlite3*,char**,const sqlite3_api_routines*);
 284
 285   UNUSED_PARAMETER(clientData);
 286   UNUSED_PARAMETER(objc);
 287
 288   zFilename = Tcl_GetString(objv[2]);
 289   sqlite3_open(zFilename, &db);
 290 #ifdef SQLITE_HAS_CODEC
 291   if( db && objc>=4 ){
 292     const char *zKey;
 293     int nKey;
 294     int rc;
 295     zKey = Tcl_GetStringFromObj(objv[3], &nKey);
 296     rc = sqlite3_key(db, zKey, nKey);
 297     if( rc!=SQLITE_OK ){
 298       char *zErrMsg = sqlite3_mprintf("error %d: %s", rc, sqlite3_errmsg(db));
 299       sqlite3_close(db);
 300       Tcl_AppendResult(interp, zErrMsg, (char*)0);
 301       sqlite3_free(zErrMsg);
 302       return TCL_ERROR;
 303     }
 304   }
 305 #endif
 306   Md5_Register(db, 0, 0);
 307   sqlite3_busy_handler(db, xBusy, 0);
 308
 309   if( sqlite3TestMakePointerStr(interp, zBuf, db) ) return TCL_ERROR;
 310   Tcl_AppendResult(interp, zBuf, 0);
 311
 312   return TCL_OK;
 313 }
 314
 315
 316 /*
 317 ** sqlthread open
 318 **
 319 **     Return the current thread-id (Tcl_GetCurrentThread()) cast to
 320 **     an integer.
 321 */
 322 static int SQLITE_TCLAPI sqlthread_id(
 323   ClientData clientData,
 324   Tcl_Interp *interp,
 325   int objc,
 326   Tcl_Obj *CONST objv[]
 327 ){
 328   Tcl_ThreadId id = Tcl_GetCurrentThread();
 329   Tcl_SetObjResult(interp, Tcl_NewIntObj(SQLITE_PTR_TO_INT(id)));
 330   UNUSED_PARAMETER(clientData);
 331   UNUSED_PARAMETER(objc);
 332   UNUSED_PARAMETER(objv);
 333   return TCL_OK;
 334 }
 335
 336
 337 /*
 338 ** Dispatch routine for the sub-commands of [sqlthread].
 339 */
 340 static int SQLITE_TCLAPI sqlthread_proc(
 341   ClientData clientData,
 342   Tcl_Interp *interp,
 343   int objc,
 344   Tcl_Obj *CONST objv[]
 345 ){
 346   struct SubCommand {
 347     char *zName;
 348     Tcl_ObjCmdProc *xProc;
 349     int nArg;
 350     char *zUsage;
 351   } aSub[] = {
 352     {"parent", sqlthread_parent, 1, "SCRIPT"},
 353     {"spawn",  sqlthread_spawn,  2, "VARNAME SCRIPT"},
 354     {"open",   sqlthread_open,   1, "DBNAME"},
 355     {"id",     sqlthread_id,     0, ""},
 356     {0, 0, 0}
 357   };
 358   struct SubCommand *pSub;
 359   int rc;
 360   int iIndex;
 361
 362   if( objc<2 ){
 363     Tcl_WrongNumArgs(interp, 1, objv, "SUB-COMMAND");
 364     return TCL_ERROR;
 365   }
 366
 367   rc = Tcl_GetIndexFromObjStruct(
 368       interp, objv[1], aSub, sizeof(aSub[0]), "sub-command", 0, &iIndex
 369   );
 370   if( rc!=TCL_OK ) return rc;
 371   pSub = &aSub[iIndex];
 372
 373   if( objc<(pSub->nArg+2) ){
 374     Tcl_WrongNumArgs(interp, 2, objv, pSub->zUsage);
 375     return TCL_ERROR;
 376   }
 377
 378   return pSub->xProc(clientData, interp, objc, objv);
 379 }
 380
 381 /*
 382 ** The [clock_seconds] command. This is more or less the same as the
 383 ** regular tcl [clock seconds], except that it is available in testfixture
 384 ** when linked against both Tcl 8.4 and 8.5. Because [clock seconds] is
 385 ** implemented as a script in Tcl 8.5, it is not usually available to
 386 ** testfixture.
 387 */
 388 static int SQLITE_TCLAPI clock_seconds_proc(
 389   ClientData clientData,
 390   Tcl_Interp *interp,
 391   int objc,
 392   Tcl_Obj *CONST objv[]
 393 ){
 394   Tcl_Time now;
 395   Tcl_GetTime(&now);
 396   Tcl_SetObjResult(interp, Tcl_NewIntObj(now.sec));
 397   UNUSED_PARAMETER(clientData);
 398   UNUSED_PARAMETER(objc);
 399   UNUSED_PARAMETER(objv);
 400   return TCL_OK;
 401 }
 402
 403 /*************************************************************************
 404 ** This block contains the implementation of the [sqlite3_blocking_step]
 405 ** command available to threads created by [sqlthread spawn] commands. It
 406 ** is only available on UNIX for now. This is because pthread condition
 407 ** variables are used.
 408 **
 409 ** The source code for the C functions sqlite3_blocking_step(),
 410 ** blocking_step_notify() and the structure UnlockNotification is
 411 ** automatically extracted from this file and used as part of the
 412 ** documentation for the sqlite3_unlock_notify() API function. This
 413 ** should be considered if these functions are to be extended (i.e. to
 414 ** support windows) in the future.
 415 */
 416 #if SQLITE_OS_UNIX && defined(SQLITE_ENABLE_UNLOCK_NOTIFY)
 417
 418 /* BEGIN_SQLITE_BLOCKING_STEP */
 419 /* This example uses the pthreads API */
 420 #include <pthread.h>
 421
 422 /*
 423 ** A pointer to an instance of this structure is passed as the user-context
 424 ** pointer when registering for an unlock-notify callback.
 425 */
 426 typedef struct UnlockNotification UnlockNotification;
 427 struct UnlockNotification {
 428   int fired;                         /* True after unlock event has occurred */
 429   pthread_cond_t cond;               /* Condition variable to wait on */
 430   pthread_mutex_t mutex;             /* Mutex to protect structure */
 431 };
 432
 433 /*
 434 ** This function is an unlock-notify callback registered with SQLite.
 435 */
 436 static void unlock_notify_cb(void **apArg, int nArg){
 437   int i;
 438   for(i=0; i<nArg; i++){
 439     UnlockNotification *p = (UnlockNotification *)apArg[i];
 440     pthread_mutex_lock(&p->mutex);
 441     p->fired = 1;
 442     pthread_cond_signal(&p->cond);
 443     pthread_mutex_unlock(&p->mutex);
 444   }
 445 }
 446
 447 /*
 448 ** This function assumes that an SQLite API call (either sqlite3_prepare_v2()
 449 ** or sqlite3_step()) has just returned SQLITE_LOCKED. The argument is the
 450 ** associated database connection.
 451 **
 452 ** This function calls sqlite3_unlock_notify() to register for an
 453 ** unlock-notify callback, then blocks until that callback is delivered
 454 ** and returns SQLITE_OK. The caller should then retry the failed operation.
 455 **
 456 ** Or, if sqlite3_unlock_notify() indicates that to block would deadlock
 457 ** the system, then this function returns SQLITE_LOCKED immediately. In
 458 ** this case the caller should not retry the operation and should roll
 459 ** back the current transaction (if any).
 460 */
 461 static int wait_for_unlock_notify(sqlite3 *db){
 462   int rc;
 463   UnlockNotification un;
 464
 465   /* Initialize the UnlockNotification structure. */
 466   un.fired = 0;
 467   pthread_mutex_init(&un.mutex, 0);
 468   pthread_cond_init(&un.cond, 0);
 469
 470   /* Register for an unlock-notify callback. */
 471   rc = sqlite3_unlock_notify(db, unlock_notify_cb, (void *)&un);
 472   assert( rc==SQLITE_LOCKED || rc==SQLITE_OK );
 473
 474   /* The call to sqlite3_unlock_notify() always returns either SQLITE_LOCKED
 475   ** or SQLITE_OK.
 476   **
 477   ** If SQLITE_LOCKED was returned, then the system is deadlocked. In this
 478   ** case this function needs to return SQLITE_LOCKED to the caller so
 479   ** that the current transaction can be rolled back. Otherwise, block
 480   ** until the unlock-notify callback is invoked, then return SQLITE_OK.
 481   */
 482   if( rc==SQLITE_OK ){
 483     pthread_mutex_lock(&un.mutex);
 484     if( !un.fired ){
 485       pthread_cond_wait(&un.cond, &un.mutex);
 486     }
 487     pthread_mutex_unlock(&un.mutex);
 488   }
 489
 490   /* Destroy the mutex and condition variables. */
 491   pthread_cond_destroy(&un.cond);
 492   pthread_mutex_destroy(&un.mutex);
 493
 494   return rc;
 495 }
 496
 497 /*
 498 ** This function is a wrapper around the SQLite function sqlite3_step().
 499 ** It functions in the same way as step(), except that if a required
 500 ** shared-cache lock cannot be obtained, this function may block waiting for
 501 ** the lock to become available. In this scenario the normal API step()
 502 ** function always returns SQLITE_LOCKED.
 503 **
 504 ** If this function returns SQLITE_LOCKED, the caller should rollback
 505 ** the current transaction (if any) and try again later. Otherwise, the
 506 ** system may become deadlocked.
 507 */
 508 int sqlite3_blocking_step(sqlite3_stmt *pStmt){
 509   int rc;
 510   while( SQLITE_LOCKED==(rc = sqlite3_step(pStmt)) ){
 511     rc = wait_for_unlock_notify(sqlite3_db_handle(pStmt));
 512     if( rc!=SQLITE_OK ) break;
 513     sqlite3_reset(pStmt);
 514   }
 515   return rc;
 516 }
 517
 518 /*
 519 ** This function is a wrapper around the SQLite function sqlite3_prepare_v2().
 520 ** It functions in the same way as prepare_v2(), except that if a required
 521 ** shared-cache lock cannot be obtained, this function may block waiting for
 522 ** the lock to become available. In this scenario the normal API prepare_v2()
 523 ** function always returns SQLITE_LOCKED.
 524 **
 525 ** If this function returns SQLITE_LOCKED, the caller should rollback
 526 ** the current transaction (if any) and try again later. Otherwise, the
 527 ** system may become deadlocked.
 528 */
 529 int sqlite3_blocking_prepare_v2(
 530   sqlite3 *db,              /* Database handle. */
 531   const char *zSql,         /* UTF-8 encoded SQL statement. */
 532   int nSql,                 /* Length of zSql in bytes. */
 533   sqlite3_stmt **ppStmt,    /* OUT: A pointer to the prepared statement */
 534   const char **pz           /* OUT: End of parsed string */
 535 ){
 536   int rc;
 537   while( SQLITE_LOCKED==(rc = sqlite3_prepare_v2(db, zSql, nSql, ppStmt, pz)) ){
 538     rc = wait_for_unlock_notify(db);
 539     if( rc!=SQLITE_OK ) break;
 540   }
 541   return rc;
 542 }
 543 /* END_SQLITE_BLOCKING_STEP */
 544
 545 /*
 546 ** Usage: sqlite3_blocking_step STMT
 547 **
 548 ** Advance the statement to the next row.
 549 */
 550 static int SQLITE_TCLAPI blocking_step_proc(
 551   void * clientData,
 552   Tcl_Interp *interp,
 553   int objc,
 554   Tcl_Obj *CONST objv[]
 555 ){
 556
 557   sqlite3_stmt *pStmt;
 558   int rc;
 559
 560   if( objc!=2 ){
 561     Tcl_WrongNumArgs(interp, 1, objv, "STMT");
 562     return TCL_ERROR;
 563   }
 564
 565   pStmt = (sqlite3_stmt*)sqlite3TestTextToPtr(Tcl_GetString(objv[1]));
 566   rc = sqlite3_blocking_step(pStmt);
 567
 568   Tcl_SetResult(interp, (char *)sqlite3ErrName(rc), 0);
 569   return TCL_OK;
 570 }
 571
 572 /*
 573 ** Usage: sqlite3_blocking_prepare_v2 DB sql bytes ?tailvar?
 574 ** Usage: sqlite3_nonblocking_prepare_v2 DB sql bytes ?tailvar?
 575 */
 576 static int SQLITE_TCLAPI blocking_prepare_v2_proc(
 577   void * clientData,
 578   Tcl_Interp *interp,
 579   int objc,
 580   Tcl_Obj *CONST objv[]
 581 ){
 582   sqlite3 *db;
 583   const char *zSql;
 584   int bytes;
 585   const char *zTail = 0;
 586   sqlite3_stmt *pStmt = 0;
 587   char zBuf[50];
 588   int rc;
 589   int isBlocking = !(clientData==0);
 590
 591   if( objc!=5 && objc!=4 ){
 592     Tcl_AppendResult(interp, "wrong # args: should be \"",
 593        Tcl_GetString(objv[0]), " DB sql bytes tailvar", 0);
 594     return TCL_ERROR;
 595   }
 596   if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) ) return TCL_ERROR;
 597   zSql = Tcl_GetString(objv[2]);
 598   if( Tcl_GetIntFromObj(interp, objv[3], &bytes) ) return TCL_ERROR;
 599
 600   if( isBlocking ){
 601     rc = sqlite3_blocking_prepare_v2(db, zSql, bytes, &pStmt, &zTail);
 602   }else{
 603     rc = sqlite3_prepare_v2(db, zSql, bytes, &pStmt, &zTail);
 604   }
 605
 606   assert(rc==SQLITE_OK || pStmt==0);
 607   if( zTail && objc>=5 ){
 608     if( bytes>=0 ){
 609       bytes = bytes - (zTail-zSql);
 610     }
 611     Tcl_ObjSetVar2(interp, objv[4], 0, Tcl_NewStringObj(zTail, bytes), 0);
 612   }
 613   if( rc!=SQLITE_OK ){
 614     assert( pStmt==0 );
 615     sqlite3_snprintf(sizeof(zBuf), zBuf, "%s ", (char *)sqlite3ErrName(rc));
 616     Tcl_AppendResult(interp, zBuf, sqlite3_errmsg(db), 0);
 617     return TCL_ERROR;
 618   }
 619
 620   if( pStmt ){
 621     if( sqlite3TestMakePointerStr(interp, zBuf, pStmt) ) return TCL_ERROR;
 622     Tcl_AppendResult(interp, zBuf, 0);
 623   }
 624   return TCL_OK;
 625 }
 626
 627 #endif /* SQLITE_OS_UNIX && SQLITE_ENABLE_UNLOCK_NOTIFY */
 628 /*
 629 ** End of implementation of [sqlite3_blocking_step].
 630 ************************************************************************/
 631
 632 /*
 633 ** Register commands with the TCL interpreter.
 634 */
 635 int SqlitetestThread_Init(Tcl_Interp *interp){
 636   Tcl_CreateObjCommand(interp, "sqlthread", sqlthread_proc, 0, 0);
 637   Tcl_CreateObjCommand(interp, "clock_seconds", clock_seconds_proc, 0, 0);
 638 #if SQLITE_OS_UNIX && defined(SQLITE_ENABLE_UNLOCK_NOTIFY)
 639   Tcl_CreateObjCommand(interp, "sqlite3_blocking_step", blocking_step_proc,0,0);
 640   Tcl_CreateObjCommand(interp,
 641       "sqlite3_blocking_prepare_v2", blocking_prepare_v2_proc, (void *)1, 0);
 642   Tcl_CreateObjCommand(interp,
 643       "sqlite3_nonblocking_prepare_v2", blocking_prepare_v2_proc, 0, 0);
 644 #endif
 645   return TCL_OK;
 646 }
 647 #else
 648 int SqlitetestThread_Init(Tcl_Interp *interp){
 649   return TCL_OK;
 650 }
 651 #endif