/*

** 2007 August 28

**

** 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.

**

*************************************************************************

** This file contains the C functions that implement mutexes for pthreads

**

** $Id: mutex_unix.c,v 1.13 2008/07/16 12:33:24 drh Exp $

*/

#include "sqliteInt.h"

/*

** The code in this file is only used if we are compiling threadsafe

** under unix with pthreads.

**

** Note that this implementation requires a version of pthreads that

** supports recursive mutexes.

*/

#ifdef SQLITE_MUTEX_PTHREADS

#include <pthread.h>

/*

** Each recursive mutex is an instance of the following structure.

*/

struct sqlite3_mutex {

  pthread_mutex_t mutex;     /* Mutex controlling the lock */

  int id;                    /* Mutex type */

  int nRef;                  /* Number of entrances */

  pthread_t owner;           /* Thread that is within this mutex */

#ifdef SQLITE_DEBUG

  int trace;                 /* True to trace changes */

#endif

};

#ifdef SQLITE_DEBUG

#define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, 0, 0, (pthread_t)0, 0 }

#else

#define SQLITE3_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, 0, 0, (pthread_t)0 }

#endif

/*

** The sqlite3_mutex_held() and sqlite3_mutex_notheld() routine are

** intended for use only inside assert() statements.  On some platforms,

** there might be race conditions that can cause these routines to

** deliver incorrect results.  In particular, if pthread_equal() is

** not an atomic operation, then these routines might delivery

** incorrect results.  On most platforms, pthread_equal() is a 

** comparison of two integers and is therefore atomic.  But we are

** told that HPUX is not such a platform.  If so, then these routines

** will not always work correctly on HPUX.

**

** On those platforms where pthread_equal() is not atomic, SQLite

** should be compiled without -DSQLITE_DEBUG and with -DNDEBUG to

** make sure no assert() statements are evaluated and hence these

** routines are never called.

*/

#ifndef NDEBUG

static int pthreadMutexHeld(sqlite3_mutex *p){

  return (p->nRef!=0 && pthread_equal(p->owner, pthread_self()));

}

static int pthreadMutexNotheld(sqlite3_mutex *p){

  return p->nRef==0 || pthread_equal(p->owner, pthread_self())==0;

}

#endif

/*

** Initialize and deinitialize the mutex subsystem.

*/

static int pthreadMutexInit(void){ return SQLITE_OK; }

static int pthreadMutexEnd(void){ return SQLITE_OK; }

/*

** The sqlite3_mutex_alloc() routine allocates a new

** mutex and returns a pointer to it.  If it returns NULL

** that means that a mutex could not be allocated.  SQLite

** will unwind its stack and return an error.  The argument

** to sqlite3_mutex_alloc() is one of these integer constants:

**

** <ul>

** <li>  SQLITE_MUTEX_FAST

** <li>  SQLITE_MUTEX_RECURSIVE

** <li>  SQLITE_MUTEX_STATIC_MASTER

** <li>  SQLITE_MUTEX_STATIC_MEM

** <li>  SQLITE_MUTEX_STATIC_MEM2

** <li>  SQLITE_MUTEX_STATIC_PRNG

** <li>  SQLITE_MUTEX_STATIC_LRU

** </ul>

**

** The first two constants cause sqlite3_mutex_alloc() to create

** a new mutex.  The new mutex is recursive when SQLITE_MUTEX_RECURSIVE

** is used but not necessarily so when SQLITE_MUTEX_FAST is used.

** The mutex implementation does not need to make a distinction

** between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does

** not want to.  But SQLite will only request a recursive mutex in

** cases where it really needs one.  If a faster non-recursive mutex

** implementation is available on the host platform, the mutex subsystem

** might return such a mutex in response to SQLITE_MUTEX_FAST.

**

** The other allowed parameters to sqlite3_mutex_alloc() each return

** a pointer to a static preexisting mutex.  Three static mutexes are

** used by the current version of SQLite.  Future versions of SQLite

** may add additional static mutexes.  Static mutexes are for internal

** use by SQLite only.  Applications that use SQLite mutexes should

** use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or

** SQLITE_MUTEX_RECURSIVE.

**

** Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST

** or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()

** returns a different mutex on every call.  But for the static 

** mutex types, the same mutex is returned on every call that has

** the same type number.

*/

static sqlite3_mutex *pthreadMutexAlloc(int iType){

  static sqlite3_mutex staticMutexes[] = {

    SQLITE3_MUTEX_INITIALIZER,

    SQLITE3_MUTEX_INITIALIZER,

    SQLITE3_MUTEX_INITIALIZER,

    SQLITE3_MUTEX_INITIALIZER,

    SQLITE3_MUTEX_INITIALIZER,

    SQLITE3_MUTEX_INITIALIZER

  };

  sqlite3_mutex *p;

  switch( iType ){

    case SQLITE_MUTEX_RECURSIVE: {

      p = sqlite3MallocZero( sizeof(*p) );

      if( p ){

#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX

        /* If recursive mutexes are not available, we will have to

        ** build our own.  See below. */

        pthread_mutex_init(&p->mutex, 0);

#else

        /* Use a recursive mutex if it is available */

        pthread_mutexattr_t recursiveAttr;

        pthread_mutexattr_init(&recursiveAttr);

        pthread_mutexattr_settype(&recursiveAttr, PTHREAD_MUTEX_RECURSIVE);

        pthread_mutex_init(&p->mutex, &recursiveAttr);

        pthread_mutexattr_destroy(&recursiveAttr);

#endif

        p->id = iType;

      }

      break;

    }

    case SQLITE_MUTEX_FAST: {

      p = sqlite3MallocZero( sizeof(*p) );

      if( p ){

        p->id = iType;

        pthread_mutex_init(&p->mutex, 0);

      }

      break;

    }

    default: {

      assert( iType-2 >= 0 );

      assert( iType-2 < sizeof(staticMutexes)/sizeof(staticMutexes[0]) );

      p = &staticMutexes[iType-2];

      p->id = iType;

      break;

    }

  }

  return p;

}

/*

** This routine deallocates a previously

** allocated mutex.  SQLite is careful to deallocate every

** mutex that it allocates.

*/

static void pthreadMutexFree(sqlite3_mutex *p){

  assert( p->nRef==0 );

  assert( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE );

  pthread_mutex_destroy(&p->mutex);

  sqlite3_free(p);

}

/*

** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt

** to enter a mutex.  If another thread is already within the mutex,

** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return

** SQLITE_BUSY.  The sqlite3_mutex_try() interface returns SQLITE_OK

** upon successful entry.  Mutexes created using SQLITE_MUTEX_RECURSIVE can

** be entered multiple times by the same thread.  In such cases the,

** mutex must be exited an equal number of times before another thread

** can enter.  If the same thread tries to enter any other kind of mutex

** more than once, the behavior is undefined.

*/

static void pthreadMutexEnter(sqlite3_mutex *p){

  assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );

#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX

  /* If recursive mutexes are not available, then we have to grow

  ** our own.  This implementation assumes that pthread_equal()

  ** is atomic - that it cannot be deceived into thinking self

  ** and p->owner are equal if p->owner changes between two values

  ** that are not equal to self while the comparison is taking place.

  ** This implementation also assumes a coherent cache - that 

  ** separate processes cannot read different values from the same

  ** address at the same time.  If either of these two conditions

  ** are not met, then the mutexes will fail and problems will result.

  */

  {

    pthread_t self = pthread_self();

    if( p->nRef>0 && pthread_equal(p->owner, self) ){

      p->nRef++;

    }else{

      pthread_mutex_lock(&p->mutex);

      assert( p->nRef==0 );

      p->owner = self;

      p->nRef = 1;

    }

  }

#else

  /* Use the built-in recursive mutexes if they are available.

  */

  pthread_mutex_lock(&p->mutex);

  p->owner = pthread_self();

  p->nRef++;

#endif

#ifdef SQLITE_DEBUG

  if( p->trace ){

    printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);

  }

#endif

}

static int pthreadMutexTry(sqlite3_mutex *p){

  int rc;

  assert( p->id==SQLITE_MUTEX_RECURSIVE || pthreadMutexNotheld(p) );

#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX

  /* If recursive mutexes are not available, then we have to grow

  ** our own.  This implementation assumes that pthread_equal()

  ** is atomic - that it cannot be deceived into thinking self

  ** and p->owner are equal if p->owner changes between two values

  ** that are not equal to self while the comparison is taking place.

  ** This implementation also assumes a coherent cache - that 

  ** separate processes cannot read different values from the same

  ** address at the same time.  If either of these two conditions

  ** are not met, then the mutexes will fail and problems will result.

  */

  {

    pthread_t self = pthread_self();

    if( p->nRef>0 && pthread_equal(p->owner, self) ){

      p->nRef++;

      rc = SQLITE_OK;

    }else if( pthread_mutex_trylock(&p->mutex)==0 ){

      assert( p->nRef==0 );

      p->owner = self;

      p->nRef = 1;

      rc = SQLITE_OK;

    }else{

      rc = SQLITE_BUSY;

    }

  }

#else

  /* Use the built-in recursive mutexes if they are available.

  */

  if( pthread_mutex_trylock(&p->mutex)==0 ){

    p->owner = pthread_self();

    p->nRef++;

    rc = SQLITE_OK;

  }else{

    rc = SQLITE_BUSY;

  }

#endif

#ifdef SQLITE_DEBUG

  if( rc==SQLITE_OK && p->trace ){

    printf("enter mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);

  }

#endif

  return rc;

}

/*

** The sqlite3_mutex_leave() routine exits a mutex that was

** previously entered by the same thread.  The behavior

** is undefined if the mutex is not currently entered or

** is not currently allocated.  SQLite will never do either.

*/

static void pthreadMutexLeave(sqlite3_mutex *p){

  assert( pthreadMutexHeld(p) );

  p->nRef--;

  assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );

#ifdef SQLITE_HOMEGROWN_RECURSIVE_MUTEX

  if( p->nRef==0 ){

    pthread_mutex_unlock(&p->mutex);

  }

#else

  pthread_mutex_unlock(&p->mutex);

#endif

#ifdef SQLITE_DEBUG

  if( p->trace ){

    printf("leave mutex %p (%d) with nRef=%d\n", p, p->trace, p->nRef);

  }

#endif

}

sqlite3_mutex_methods *sqlite3DefaultMutex(void){

  static sqlite3_mutex_methods sMutex = {

    pthreadMutexInit,

    pthreadMutexEnd,

    pthreadMutexAlloc,

    pthreadMutexFree,

    pthreadMutexEnter,

    pthreadMutexTry,

    pthreadMutexLeave,

#ifdef SQLITE_DEBUG

    pthreadMutexHeld,

    pthreadMutexNotheld

#endif

  };

  return &sMutex;

}

#endif /* SQLITE_MUTEX_PTHREAD */