SQLite

** SQLITE_MAX_WORKER_THREADS==0).  The other values are only used
** when there exists one or more separate worker threads.
*/
#define INCRINIT_NORMAL 0
#define INCRINIT_TASK   1
#define INCRINIT_ROOT   2

/* 
** Forward reference required as the vdbeIncrMergeInit() and
** vdbePmaReaderIncrInit() routines are called mutually recursively when
** building a merge tree.
*/
static int vdbePmaReaderIncrInit(PmaReader *pReadr, int eMode);

/*
** Initialize the MergeEngine object passed as the second argument. Once this
** function returns, the first key of merged data may be read from the 
** MergeEngine object in the usual fashion.
**
** If argument eMode is INCRINIT_ROOT, then it is assumed that any IncrMerge

      ** However, in the INCRINIT_ROOT case, if PmaReader aReadr[nTask-1] is
      ** in use it will block the vdbePmaReaderNext() call while it uses
      ** the main thread to fill its buffer. So calling PmaReaderNext()
      ** on this PmaReader before any of the multi-threaded PmaReaders takes
      ** better advantage of multi-processor hardware. */
      rc = vdbePmaReaderNext(&pMerger->aReadr[nTree-i-1]);
    }else{
      rc = vdbePmaReaderIncrInit(&pMerger->aReadr[i], INCRINIT_NORMAL);
    }
    if( rc!=SQLITE_OK ) return rc;
  }

  for(i=pMerger->nTree-1; i>0; i--){
    vdbeMergeEngineCompare(pMerger, i);
  }
  return pTask->pUnpacked->errCode;
}

/*


** The PmaReader passed as the first argument is guaranteed to be an
** incremental-reader (pReadr->pIncr!=0). This function serves to open
** and/or initialize the temp file related fields of the IncrMerge
** object at (pReadr->pIncr).
**
** If argument eMode is set to INCRINIT_NORMAL, then all PmaReaders
** in the sub-tree headed by pReadr are also initialized. Data is then 
** loaded into the buffers belonging to pReadr and it is set to point to 
** the first key in its range.
**
** If argument eMode is set to INCRINIT_TASK, then pReadr is guaranteed
** to be a multi-threaded PmaReader and this function is being called in a
** background thread. In this case all PmaReaders in the sub-tree are 
** initialized as for INCRINIT_NORMAL and the aFile[1] buffer belonging to
** pReadr is populated. However, pReadr itself is not set up to point
** to its first key. A call to vdbePmaReaderNext() is still required to do

** the current PmaReader set to point to the first key in its range.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int vdbePmaReaderIncrMergeInit(PmaReader *pReadr, int eMode){
  int rc = SQLITE_OK;
  IncrMerger *pIncr = pReadr->pIncr;
  SortSubtask *pTask = pIncr->pTask;
  sqlite3 *db = pTask->pSorter->db;

  /* eMode is always INCRINIT_NORMAL in single-threaded mode */
  assert( SQLITE_MAX_WORKER_THREADS>0 || eMode==INCRINIT_NORMAL );





  rc = vdbeMergeEngineInit(pTask, pIncr->pMerger, eMode);

  /* Set up the required files for pIncr. A multi-theaded IncrMerge object
  ** requires two temp files to itself, whereas a single-threaded object
  ** only requires a region of pTask->file2. */
  if( rc==SQLITE_OK ){
    int mxSz = pIncr->mxSz;
#if SQLITE_MAX_WORKER_THREADS>0
    if( pIncr->bUseThread ){
      rc = vdbeSorterOpenTempFile(db, mxSz, &pIncr->aFile[0].pFd);
      if( rc==SQLITE_OK ){
        rc = vdbeSorterOpenTempFile(db, mxSz, &pIncr->aFile[1].pFd);
      }
    }else
#endif
    /*if( !pIncr->bUseThread )*/{
      if( pTask->file2.pFd==0 ){
        assert( pTask->file2.iEof>0 );
        rc = vdbeSorterOpenTempFile(db, pTask->file2.iEof, &pTask->file2.pFd);
        pTask->file2.iEof = 0;
      }
      if( rc==SQLITE_OK ){
        pIncr->aFile[1].pFd = pTask->file2.pFd;
        pIncr->iStartOff = pTask->file2.iEof;
        pTask->file2.iEof += mxSz;
      }
    }
  }

#if SQLITE_MAX_WORKER_THREADS>0
  if( rc==SQLITE_OK && pIncr->bUseThread ){
    /* Use the current thread to populate aFile[1], even though this
    ** PmaReader is multi-threaded. If this is an INCRINIT_TASK object,
    ** then this function is already running in background thread 
    ** pIncr->pTask->thread. 
    **
    ** If this is the INCRINIT_ROOT object, then it is running in the 
    ** main VDBE thread. But that is Ok, as that thread cannot return
    ** control to the VDBE or proceed with anything useful until the 
    ** first results are ready from this merger object anyway.
    */
    assert( eMode==INCRINIT_ROOT || eMode==INCRINIT_TASK );
    rc = vdbeIncrPopulate(pIncr);
  }
#endif


  if( rc==SQLITE_OK && (SQLITE_MAX_WORKER_THREADS==0 || eMode!=INCRINIT_TASK) ){

    rc = vdbePmaReaderNext(pReadr);
  }

  return rc;
}

#if SQLITE_MAX_WORKER_THREADS>0
/*
** The main routine for vdbePmaReaderIncrMergeInit() operations run in 
** background threads.
*/
static void *vdbePmaReaderBgIncrInit(void *pCtx){
  PmaReader *pReader = (PmaReader*)pCtx;
  void *pRet = SQLITE_INT_TO_PTR(
                  vdbePmaReaderIncrMergeInit(pReader,INCRINIT_TASK)
               );
  pReader->pIncr->pTask->bDone = 1;
  return pRet;
}
#endif

/*

** If the PmaReader passed as the first argument is not an incremental-reader
** (if pReadr->pIncr==0), then this function is a no-op. Otherwise, it invokes
** the vdbePmaReaderIncrMergeInit() function with the parameters passed to
** this routine to initialize the incremental merge.
** 

** If the IncrMerger object is multi-threaded (IncrMerger.bUseThread==1), 
** then a background thread is launched to call vdbePmaReaderIncrMergeInit().
** Or, if the IncrMerger is single threaded, the same function is called
** using the current thread.
*/
static int vdbePmaReaderIncrInit(PmaReader *pReadr, int eMode){
  IncrMerger *pIncr = pReadr->pIncr;   /* Incremental merger */
  int rc = SQLITE_OK;                  /* Return code */
  if( pIncr ){
#if SQLITE_MAX_WORKER_THREADS>0
    assert( pIncr->bUseThread==0 || eMode==INCRINIT_TASK );
    if( pIncr->bUseThread ){
      void *pCtx = (void*)pReadr;
      rc = vdbeSorterCreateThread(pIncr->pTask, vdbePmaReaderBgIncrInit, pCtx);
    }else
#endif
    {
      rc = vdbePmaReaderIncrMergeInit(pReadr, eMode);
    }
  }
  return rc;
}

/*
** Allocate a new MergeEngine object to merge the contents of nPMA level-0
** PMAs from pTask->file. If no error occurs, set *ppOut to point to
** the new object and return SQLITE_OK. Or, if an error does occur, set *ppOut
** to NULL and return an SQLite error code.
**

            IncrMerger *pIncr;
            if( (pIncr = pMain->aReadr[iTask].pIncr) ){
              vdbeIncrMergerSetThreads(pIncr);
              assert( pIncr->pTask!=pLast );
            }
          }
          for(iTask=0; rc==SQLITE_OK && iTask<pSorter->nTask; iTask++){
            /* Check that:
            **   
            **   a) The incremental merge object is configured to use the
            **      right task, and
            **   b) If it is using task (nTask-1), it is configured to run
            **      in single-threaded mode. This is important, as the
            **      root merge (INCRINIT_ROOT) will be using the same task
            **      object.
            */
            PmaReader *p = &pMain->aReadr[iTask];
            assert( p->pIncr==0 || (
                (p->pIncr->pTask==&pSorter->aTask[iTask])             /* a */
             && (iTask!=pSorter->nTask-1 || p->pIncr->bUseThread==0)  /* b */


            ));
            rc = vdbePmaReaderIncrInit(p, INCRINIT_TASK);


          }
        }
        pMain = 0;
      }
      if( rc==SQLITE_OK ){
        rc = vdbePmaReaderIncrMergeInit(pReadr, INCRINIT_ROOT);
      }