SQLite

#define LOG_UNLOCK 0
#define LOG_RDLOCK 1
#define LOG_WRLOCK 2

static int logLockRegion(Log *pLog, u32 mRegion, int op){
  LogSummary *pSummary = pLog->pSummary;
  LogLock *p;                     /* Used to iterate through in-process locks */
  u32 mOther;                     /* Locks held by other connections */
  u32 mNew;                       /* New mask for pLog */



  assert( 
       /* Writer lock operations */
          (op==LOG_WRLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))

       /* Normal reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B))

       /* Region D reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_D))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_D))

       /* Checkpointer lock operations */
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B|LOG_REGION_C))
  );

  /* Assert that a connection never tries to go from an EXCLUSIVE to a 
  ** SHARED lock on a region. Moving from SHARED to EXCLUSIVE sometimes
  ** happens though (when a region D reader upgrades to a writer).
  */
  assert( op!=LOG_RDLOCK || 0==(pLog->lock.mLock & (mRegion<<8)) );

  sqlite3_mutex_enter(pSummary->mutex);

  /* Calculate a mask of logs held by all connections in this process apart
  ** from this one. The least significant byte of the mask contains a mask
  ** of the SHARED logs held. The next least significant byte of the mask
  ** indicates the EXCLUSIVE locks held. For example, to test if some other
  ** connection is holding a SHARED lock on region A, or an EXCLUSIVE lock
  ** on region C, do:
  **
  **   hasSharedOnA    = (mOther & (LOG_REGION_A<<0));
  **   hasExclusiveOnC = (mOther & (LOG_REGION_C<<8));
  **
  ** In all masks, if the bit in the EXCLUSIVE byte mask is set, so is the 
  ** corresponding bit in the SHARED mask.
  */
  mOther = 0;

  for(p=pSummary->pLock; p; p=p->pNext){
    assert( (p->mLock & (p->mLock<<8))==(p->mLock&0x0000FF00) );
    if( p!=&pLog->lock ){
      mOther |= p->mLock;


    }
  }

  /* If this call is to lock a region (not to unlock one), test if locks held
  ** by any other connection in this process prevent the new locks from
  ** begin granted. If so, exit the summary mutex and return SQLITE_BUSY.
  */
  if( op && (mOther & (mRegion << (op==LOG_RDLOCK ? 8 : 0))) ){
    sqlite3_mutex_leave(pSummary->mutex);
    return SQLITE_BUSY;
  }

  /* Figure out the new log mask for this connection. */
  switch( op ){
    case LOG_UNLOCK: 
      mNew = (pLog->lock.mLock & ~(mRegion|(mRegion<<8)));
      break;
    case LOG_RDLOCK:
      mNew = (pLog->lock.mLock | mRegion);
      break;
    default:
      assert( op==LOG_WRLOCK );
      mNew = (pLog->lock.mLock | (mRegion<<8) | mRegion);
      break;
  }

  /* Now modify the locks held on the log-summary file descriptor. This
  ** file descriptor is shared by all log connections in this process. 
  ** Therefore:

  **
  **   + If one or more log connections in this process hold a SHARED lock
  **     on a region, the file-descriptor should hold a SHARED lock on
  **     the file region.
  **
  **   + If a log connection in this process holds an EXCLUSIVE lock on a
  **     region, the file-descriptor should also hold an EXCLUSIVE lock on
  **     the region in question.
  **
  ** If this is an LOG_UNLOCK operation, only regions for which no other
  ** connection holds a lock should actually be unlocked. And if this
  ** is a LOG_RDLOCK operation and other connections already hold all
  ** the required SHARED locks, then no system call is required.
  */
  if( op==LOG_UNLOCK ){
    mRegion = (mRegion & ~mOther);
  }
  if( (op==LOG_WRLOCK)
   || (op==LOG_UNLOCK && mRegion) 
   || (op==LOG_RDLOCK && (mOther&mRegion)!=mRegion)
  ){
    struct LockMap {
      int iStart;                 /* Byte offset to start locking operation */
      int iLen;                   /* Length field for locking operation */
    } aMap[] = {
      /* 0000 */ {0, 0},    /* 0001 */ {4, 1}, 
      /* 0010 */ {3, 1},    /* 0011 */ {3, 2},
      /* 0100 */ {2, 1},    /* 0101 */ {0, 0}, 
      /* 0110 */ {2, 2},    /* 0111 */ {2, 3},
      /* 1000 */ {1, 1},    /* 1001 */ {0, 0}, 
      /* 1010 */ {0, 0},    /* 1011 */ {0, 0},
      /* 1100 */ {1, 2},    /* 1101 */ {0, 0}, 
      /* 1110 */ {1, 3},    /* 1111 */ {0, 0}
    };
    int rc;                       /* Return code of fcntl() */
    struct flock f;               /* Locking operation */

    assert( mRegion<ArraySize(aMap) && aMap[mRegion].iStart!=0 );




    memset(&f, 0, sizeof(f));
    f.l_type = (op==LOG_WRLOCK?F_WRLCK:(op==LOG_RDLOCK?F_RDLCK:F_UNLCK));
    f.l_whence = SEEK_SET;
    f.l_start = 32 + aMap[mRegion].iStart;




    f.l_len = aMap[mRegion].iLen;





    rc = fcntl(pSummary->fd, F_SETLK, &f);
    if( rc!=0 ){
      sqlite3_mutex_leave(pSummary->mutex);
      return SQLITE_BUSY;
    }
  }

  pLog->lock.mLock = mNew;
  sqlite3_mutex_leave(pSummary->mutex);
  return SQLITE_OK;
}

/*
** Try to read the log-summary header. Attempt to verify the header
** checksum. If the checksum can be verified, copy the log-summary

  if( op ){

    /* Obtain the writer lock */
    int rc = logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_WRLOCK);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* TODO: What if this is a region D reader? And after writing this
    ** transaction it continues to hold a read-lock on the db? Maybe we 
    ** need to switch it to a region A reader here so that unlocking C|D
    ** does not leave the connection with no lock at all.
    */
    assert( pLog->isLocked!=LOG_REGION_D );

    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){