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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

Artifact a898fa969823b100c0f5fdc57e54c9a1e419ab4d:


/*
** 2007 August 14
**
** 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 Win32.
*/
#include "sqliteInt.h"

#if SQLITE_OS_WIN
/*
** Include code that is common to all os_*.c files
*/
#include "os_common.h"

/*
** Include the header file for the Windows VFS.
*/
#include "os_win.h"
#endif

/*
** The code in this file is only used if we are compiling multithreaded
** on a Win32 system.
*/
#ifdef SQLITE_MUTEX_W32

/*
** Each recursive mutex is an instance of the following structure.
*/
struct sqlite3_mutex {
  CRITICAL_SECTION mutex;    /* Mutex controlling the lock */
  int id;                    /* Mutex type */
#ifdef SQLITE_DEBUG
  volatile int nRef;         /* Number of enterances */
  volatile DWORD owner;      /* Thread holding this mutex */
  volatile int trace;        /* True to trace changes */
#endif
};

/*
** These are the initializer values used when declaring a "static" mutex
** on Win32.  It should be noted that all mutexes require initialization
** on the Win32 platform.
*/
#define SQLITE_W32_MUTEX_INITIALIZER { 0 }

#ifdef SQLITE_DEBUG
#define SQLITE3_MUTEX_INITIALIZER { SQLITE_W32_MUTEX_INITIALIZER, 0, \
                                    0L, (DWORD)0, 0 }
#else
#define SQLITE3_MUTEX_INITIALIZER { SQLITE_W32_MUTEX_INITIALIZER, 0 }
#endif

#ifdef SQLITE_DEBUG
/*
** The sqlite3_mutex_held() and sqlite3_mutex_notheld() routine are
** intended for use only inside assert() statements.
*/
static int winMutexHeld(sqlite3_mutex *p){
  return p->nRef!=0 && p->owner==GetCurrentThreadId();
}

static int winMutexNotheld2(sqlite3_mutex *p, DWORD tid){
  return p->nRef==0 || p->owner!=tid;
}

static int winMutexNotheld(sqlite3_mutex *p){
  DWORD tid = GetCurrentThreadId();
  return winMutexNotheld2(p, tid);
}
#endif

/*
** Try to provide a memory barrier operation, needed for initialization
** and also for the xShmBarrier method of the VFS in cases when SQLite is
** compiled without mutexes (SQLITE_THREADSAFE=0).
*/
void sqlite3MemoryBarrier(void){
#if defined(SQLITE_MEMORY_BARRIER)
  SQLITE_MEMORY_BARRIER;
#elif defined(__GNUC__)
  __sync_synchronize();
#elif MSVC_VERSION>=1300
  _ReadWriteBarrier();
#elif defined(MemoryBarrier)
  MemoryBarrier();
#endif
}

/*
** Initialize and deinitialize the mutex subsystem.
*/
static sqlite3_mutex winMutex_staticMutexes[] = {
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER,
  SQLITE3_MUTEX_INITIALIZER
};

static int winMutex_isInit = 0;
static int winMutex_isNt = -1; /* <0 means "need to query" */

/* As the winMutexInit() and winMutexEnd() functions are called as part
** of the sqlite3_initialize() and sqlite3_shutdown() processing, the
** "interlocked" magic used here is probably not strictly necessary.
*/
static LONG SQLITE_WIN32_VOLATILE winMutex_lock = 0;

int sqlite3_win32_is_nt(void); /* os_win.c */
void sqlite3_win32_sleep(DWORD milliseconds); /* os_win.c */

static int winMutexInit(void){
  /* The first to increment to 1 does actual initialization */
  if( InterlockedCompareExchange(&winMutex_lock, 1, 0)==0 ){
    int i;
    for(i=0; i<ArraySize(winMutex_staticMutexes); i++){
#if SQLITE_OS_WINRT
      InitializeCriticalSectionEx(&winMutex_staticMutexes[i].mutex, 0, 0);
#else
      InitializeCriticalSection(&winMutex_staticMutexes[i].mutex);
#endif
    }
    winMutex_isInit = 1;
  }else{
    /* Another thread is (in the process of) initializing the static
    ** mutexes */
    while( !winMutex_isInit ){
      sqlite3_win32_sleep(1);
    }
  }
  return SQLITE_OK;
}

static int winMutexEnd(void){
  /* The first to decrement to 0 does actual shutdown
  ** (which should be the last to shutdown.) */
  if( InterlockedCompareExchange(&winMutex_lock, 0, 1)==1 ){
    if( winMutex_isInit==1 ){
      int i;
      for(i=0; i<ArraySize(winMutex_staticMutexes); i++){
        DeleteCriticalSection(&winMutex_staticMutexes[i].mutex);
      }
      winMutex_isInit = 0;
    }
  }
  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_OPEN
** <li>  SQLITE_MUTEX_STATIC_PRNG
** <li>  SQLITE_MUTEX_STATIC_LRU
** <li>  SQLITE_MUTEX_STATIC_PMEM
** <li>  SQLITE_MUTEX_STATIC_APP1
** <li>  SQLITE_MUTEX_STATIC_APP2
** <li>  SQLITE_MUTEX_STATIC_APP3
** <li>  SQLITE_MUTEX_STATIC_VFS1
** <li>  SQLITE_MUTEX_STATIC_VFS2
** <li>  SQLITE_MUTEX_STATIC_VFS3
** </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.  Six 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 *winMutexAlloc(int iType){
  sqlite3_mutex *p;

  switch( iType ){
    case SQLITE_MUTEX_FAST:
    case SQLITE_MUTEX_RECURSIVE: {
      p = sqlite3MallocZero( sizeof(*p) );
      if( p ){
        p->id = iType;
#ifdef SQLITE_DEBUG
#ifdef SQLITE_WIN32_MUTEX_TRACE_DYNAMIC
        p->trace = 1;
#endif
#endif
#if SQLITE_OS_WINRT
        InitializeCriticalSectionEx(&p->mutex, 0, 0);
#else
        InitializeCriticalSection(&p->mutex);
#endif
      }
      break;
    }
    default: {
#ifdef SQLITE_ENABLE_API_ARMOR
      if( iType-2<0 || iType-2>=ArraySize(winMutex_staticMutexes) ){
        (void)SQLITE_MISUSE_BKPT;
        return 0;
      }
#endif
      p = &winMutex_staticMutexes[iType-2];
      p->id = iType;
#ifdef SQLITE_DEBUG
#ifdef SQLITE_WIN32_MUTEX_TRACE_STATIC
      p->trace = 1;
#endif
#endif
      break;
    }
  }
  return p;
}


/*
** This routine deallocates a previously
** allocated mutex.  SQLite is careful to deallocate every
** mutex that it allocates.
*/
static void winMutexFree(sqlite3_mutex *p){
  assert( p );
  assert( p->nRef==0 && p->owner==0 );
  if( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE ){
    DeleteCriticalSection(&p->mutex);
    sqlite3_free(p);
  }else{
#ifdef SQLITE_ENABLE_API_ARMOR
    (void)SQLITE_MISUSE_BKPT;
#endif
  }
}

/*
** 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 winMutexEnter(sqlite3_mutex *p){
#if defined(SQLITE_DEBUG) || defined(SQLITE_TEST)
  DWORD tid = GetCurrentThreadId();
#endif
#ifdef SQLITE_DEBUG
  assert( p );
  assert( p->id==SQLITE_MUTEX_RECURSIVE || winMutexNotheld2(p, tid) );
#else
  assert( p );
#endif
  assert( winMutex_isInit==1 );
  EnterCriticalSection(&p->mutex);
#ifdef SQLITE_DEBUG
  assert( p->nRef>0 || p->owner==0 );
  p->owner = tid;
  p->nRef++;
  if( p->trace ){
    OSTRACE(("ENTER-MUTEX tid=%lu, mutex(%d)=%p (%d), nRef=%d\n",
             tid, p->id, p, p->trace, p->nRef));
  }
#endif
}

static int winMutexTry(sqlite3_mutex *p){
#if defined(SQLITE_DEBUG) || defined(SQLITE_TEST)
  DWORD tid = GetCurrentThreadId();
#endif
  int rc = SQLITE_BUSY;
  assert( p );
  assert( p->id==SQLITE_MUTEX_RECURSIVE || winMutexNotheld2(p, tid) );
  /*
  ** The sqlite3_mutex_try() routine is very rarely used, and when it
  ** is used it is merely an optimization.  So it is OK for it to always
  ** fail.
  **
  ** The TryEnterCriticalSection() interface is only available on WinNT.
  ** And some windows compilers complain if you try to use it without
  ** first doing some #defines that prevent SQLite from building on Win98.
  ** For that reason, we will omit this optimization for now.  See
  ** ticket #2685.
  */
#if defined(_WIN32_WINNT) && _WIN32_WINNT >= 0x0400
  assert( winMutex_isInit==1 );
  assert( winMutex_isNt>=-1 && winMutex_isNt<=1 );
  if( winMutex_isNt<0 ){
    winMutex_isNt = sqlite3_win32_is_nt();
  }
  assert( winMutex_isNt==0 || winMutex_isNt==1 );
  if( winMutex_isNt && TryEnterCriticalSection(&p->mutex) ){
#ifdef SQLITE_DEBUG
    p->owner = tid;
    p->nRef++;
#endif
    rc = SQLITE_OK;
  }
#else
  UNUSED_PARAMETER(p);
#endif
#ifdef SQLITE_DEBUG
  if( p->trace ){
    OSTRACE(("TRY-MUTEX tid=%lu, mutex(%d)=%p (%d), owner=%lu, nRef=%d, rc=%s\n",
             tid, p->id, p, p->trace, p->owner, p->nRef, sqlite3ErrName(rc)));
  }
#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 winMutexLeave(sqlite3_mutex *p){
#if defined(SQLITE_DEBUG) || defined(SQLITE_TEST)
  DWORD tid = GetCurrentThreadId();
#endif
  assert( p );
#ifdef SQLITE_DEBUG
  assert( p->nRef>0 );
  assert( p->owner==tid );
  p->nRef--;
  if( p->nRef==0 ) p->owner = 0;
  assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );
#endif
  assert( winMutex_isInit==1 );
  LeaveCriticalSection(&p->mutex);
#ifdef SQLITE_DEBUG
  if( p->trace ){
    OSTRACE(("LEAVE-MUTEX tid=%lu, mutex(%d)=%p (%d), nRef=%d\n",
             tid, p->id, p, p->trace, p->nRef));
  }
#endif
}

sqlite3_mutex_methods const *sqlite3DefaultMutex(void){
  static const sqlite3_mutex_methods sMutex = {
    winMutexInit,
    winMutexEnd,
    winMutexAlloc,
    winMutexFree,
    winMutexEnter,
    winMutexTry,
    winMutexLeave,
#ifdef SQLITE_DEBUG
    winMutexHeld,
    winMutexNotheld
#else
    0,
    0
#endif
  };
  return &sMutex;
}

#endif /* SQLITE_MUTEX_W32 */