/*

 ** 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 */