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Comment:In vdbesort.c, change the names of PmaReader variables "pIter" to "pReadr". Other related comment changes. The generated object code should be the same.
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SHA1: 99efb235a08784020535a770d7e7d96d9ccee12d
User & Date: drh 2014-05-20 11:03:53
Context
2014-05-20
19:11
Improvements to the testability of the threads.c module. check-in: 386e0888 user: drh tags: threads
11:03
In vdbesort.c, change the names of PmaReader variables "pIter" to "pReadr". Other related comment changes. The generated object code should be the same. check-in: 99efb235 user: drh tags: threads
2014-05-19
23:17
Merge recent changes from trunk. check-in: 6eefdad9 user: drh tags: threads
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/*
** The MergeEngine object is used to combine two or more smaller PMAs into
** one big PMA using a merge operation.  Separate PMAs all need to be
** combined into one big PMA in order to be able to step through the sorted
** records in order.
**
** The aIter[] array contains a PmaReader object for each of the PMAs being
** merged.  An aIter[] object either points to a valid key or else is at EOF.
** For the purposes of the paragraphs below, we assume that the array is
** actually N elements in size, where N is the smallest power of 2 greater
** to or equal to the number of PMAs being merged. The extra aIter[] elements
** are treated as if they are empty (always at EOF).
**
** The aTree[] array is also N elements in size. The value of N is stored in
** the MergeEngine.nTree variable.
**
** The final (N/2) elements of aTree[] contain the results of comparing
** pairs of PMA keys together. Element i contains the result of 
** comparing aIter[2*i-N] and aIter[2*i-N+1]. Whichever key is smaller, the
** aTree element is set to the index of it. 
**
** For the purposes of this comparison, EOF is considered greater than any
** other key value. If the keys are equal (only possible with two EOF
** values), it doesn't matter which index is stored.
**
** The (N/4) elements of aTree[] that precede the final (N/2) described 
** above contains the index of the smallest of each block of 4 iterators.
** And so on. So that aTree[1] contains the index of the iterator that 
** currently points to the smallest key value. aTree[0] is unused.
**
** Example:
**
**     aIter[0] -> Banana
**     aIter[1] -> Feijoa
**     aIter[2] -> Elderberry
**     aIter[3] -> Currant
**     aIter[4] -> Grapefruit
**     aIter[5] -> Apple
**     aIter[6] -> Durian
**     aIter[7] -> EOF
**
**     aTree[] = { X, 5   0, 5    0, 3, 5, 6 }
**
** The current element is "Apple" (the value of the key indicated by 
** iterator 5). When the Next() operation is invoked, iterator 5 will
** be advanced to the next key in its segment. Say the next key is
** "Eggplant":
**
**     aIter[5] -> Eggplant
**
** The contents of aTree[] are updated first by comparing the new iterator
** 5 key to the current key of iterator 4 (still "Grapefruit"). The iterator
** 5 value is still smaller, so aTree[6] is set to 5. And so on up the tree.
** The value of iterator 6 - "Durian" - is now smaller than that of iterator
** 5, so aTree[3] is set to 6. Key 0 is smaller than key 6 (Banana<Durian),
** so the value written into element 1 of the array is 0. As follows:
**
**     aTree[] = { X, 0   0, 6    0, 3, 5, 6 }
**
** In other words, each time we advance to the next sorter element, log2(N)
** key comparison operations are required, where N is the number of segments
** being merged (rounded up to the next power of 2).
*/
struct MergeEngine {
  int nTree;                 /* Used size of aTree/aIter (power of 2) */
  int *aTree;                /* Current state of incremental merge */
  PmaReader *aIter;          /* Array of iterators to merge data from */
};

/*
** Exactly VdbeSorter.nTask instances of this object are allocated
** as part of each VdbeSorter object. Instances are never allocated any
** other way. VdbeSorter.nTask is set to the number of worker threads allowed
** (see SQLITE_CONFIG_WORKER_THREADS) plus one (the main thread).
................................................................................
**   largest record in the sorter.
*/
struct VdbeSorter {
  int mnPmaSize;                  /* Minimum PMA size, in bytes */
  int mxPmaSize;                  /* Maximum PMA size, in bytes.  0==no limit */
  int mxKeysize;                  /* Largest serialized key seen so far */
  int pgsz;                       /* Main database page size */
  PmaReader *pReader;             /* Read data from here after Rewind() */
  MergeEngine *pMerger;           /* Or here, if bUseThreads==0 */
  sqlite3 *db;                    /* Database connection */
  KeyInfo *pKeyInfo;              /* How to compare records */
  UnpackedRecord *pUnpacked;      /* Used by VdbeSorterCompare() */
  SorterList list;                /* List of in-memory records */
  int iMemory;                    /* Offset of free space in list.aMemory */
  int nMemory;                    /* Size of list.aMemory allocation in bytes */
................................................................................
/*
** An instance of the following object is used to read records out of a
** PMA, in sorted order.  The next key to be read is cached in nKey/aKey.
** pFile==0 at EOF.
*/
struct PmaReader {
  i64 iReadOff;                   /* Current read offset */
  i64 iEof;                       /* 1 byte past EOF for this iterator */
  int nAlloc;                     /* Bytes of space at aAlloc */
  int nKey;                       /* Number of bytes in key */
  sqlite3_file *pFile;            /* File iterator is reading from */
  u8 *aAlloc;                     /* Allocated space */
  u8 *aKey;                       /* Pointer to current key */
  u8 *aBuffer;                    /* Current read buffer */
  int nBuffer;                    /* Size of read buffer in bytes */
  u8 *aMap;                       /* Pointer to mapping of entire file */
  IncrMerger *pIncr;              /* Incremental merger */
};
................................................................................
static int vdbeIncrSwap(IncrMerger*);
static void vdbeIncrFree(IncrMerger *);

/*
** Free all memory belonging to the PmaReader object passed as the second
** argument. All structure fields are set to zero before returning.
*/
static void vdbePmaReaderClear(PmaReader *pIter){
  sqlite3_free(pIter->aAlloc);
  sqlite3_free(pIter->aBuffer);
  if( pIter->aMap ) sqlite3OsUnfetch(pIter->pFile, 0, pIter->aMap);
  vdbeIncrFree(pIter->pIncr);
  memset(pIter, 0, sizeof(PmaReader));
}

/*
** Read nByte bytes of data from the stream of data iterated by object p.
** If successful, set *ppOut to point to a buffer containing the data
** and return SQLITE_OK. Otherwise, if an error occurs, return an SQLite
** error code.
**
** The buffer indicated by *ppOut may only be considered valid until the
** next call to this function.
*/
static int vdbePmaReadBlob(
  PmaReader *p,                   /* Iterator */
  int nByte,                      /* Bytes of data to read */
  u8 **ppOut                      /* OUT: Pointer to buffer containing data */
){
  int iBuf;                       /* Offset within buffer to read from */
  int nAvail;                     /* Bytes of data available in buffer */

  if( p->aMap ){
................................................................................
    if( (p->iEof - p->iReadOff) > (i64)p->nBuffer ){
      nRead = p->nBuffer;
    }else{
      nRead = (int)(p->iEof - p->iReadOff);
    }
    assert( nRead>0 );

    /* Read data from the file. Return early if an error occurs. */
    rc = sqlite3OsRead(p->pFile, p->aBuffer, nRead, p->iReadOff);
    assert( rc!=SQLITE_IOERR_SHORT_READ );
    if( rc!=SQLITE_OK ) return rc;
  }
  nAvail = p->nBuffer - iBuf; 

  if( nByte<=nAvail ){
................................................................................
  if( pFile->iEof<=(i64)(pTask->pSorter->db->nMaxSorterMmap) ){
    rc = sqlite3OsFetch(pFile->pFd, 0, (int)pFile->iEof, (void**)pp);
  }
  return rc;
}

/*
** Seek iterator pIter to offset iOff within file pFile. Return SQLITE_OK 
** if successful, or an SQLite error code if an error occurs.
*/
static int vdbePmaReaderSeek(
  SortSubtask *pTask,             /* Task context */
  PmaReader *pIter,               /* Iterate to populate */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iOff                        /* Offset in pFile */
){
  int rc = SQLITE_OK;

  assert( pIter->pIncr==0 || pIter->pIncr->bEof==0 );

  if( pIter->aMap ){
    sqlite3OsUnfetch(pIter->pFile, 0, pIter->aMap);
    pIter->aMap = 0;
  }
  pIter->iReadOff = iOff;
  pIter->iEof = pFile->iEof;
  pIter->pFile = pFile->pFd;

  rc = vdbeSorterMapFile(pTask, pFile, &pIter->aMap);
  if( rc==SQLITE_OK && pIter->aMap==0 ){
    int pgsz = pTask->pSorter->pgsz;
    int iBuf = pIter->iReadOff % pgsz;
    if( pIter->aBuffer==0 ){
      pIter->aBuffer = (u8*)sqlite3Malloc(pgsz);
      if( pIter->aBuffer==0 ) rc = SQLITE_NOMEM;
      pIter->nBuffer = pgsz;
    }
    if( rc==SQLITE_OK && iBuf ){
      int nRead = pgsz - iBuf;
      if( (pIter->iReadOff + nRead) > pIter->iEof ){
        nRead = (int)(pIter->iEof - pIter->iReadOff);
      }
      rc = sqlite3OsRead(
          pIter->pFile, &pIter->aBuffer[iBuf], nRead, pIter->iReadOff
      );
      assert( rc!=SQLITE_IOERR_SHORT_READ );
    }
  }

  return rc;
}

/*
** Advance iterator pIter to the next key in its PMA. Return SQLITE_OK if
** no error occurs, or an SQLite error code if one does.
*/
static int vdbePmaReaderNext(PmaReader *pIter){
  int rc = SQLITE_OK;             /* Return Code */
  u64 nRec = 0;                   /* Size of record in bytes */


  if( pIter->iReadOff>=pIter->iEof ){
    IncrMerger *pIncr = pIter->pIncr;
    int bEof = 1;
    if( pIncr ){
      rc = vdbeIncrSwap(pIncr);
      if( rc==SQLITE_OK && pIncr->bEof==0 ){
        rc = vdbePmaReaderSeek(
            pIncr->pTask, pIter, &pIncr->aFile[0], pIncr->iStartOff
        );
        bEof = 0;
      }
    }

    if( bEof ){
      /* This is an EOF condition */
      vdbePmaReaderClear(pIter);
      return rc;
    }
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReadVarint(pIter, &nRec);
  }
  if( rc==SQLITE_OK ){
    pIter->nKey = (int)nRec;
    rc = vdbePmaReadBlob(pIter, (int)nRec, &pIter->aKey);
  }

  return rc;
}

/*
** Initialize iterator pIter to scan through the PMA stored in file pFile
** starting at offset iStart and ending at offset iEof-1. This function 
** leaves the iterator pointing to the first key in the PMA (or EOF if the 
** PMA is empty).
**
** If the pnByte parameter is NULL, then it is assumed that the file 
** contains a single PMA, and that that PMA omits the initial length varint.
*/
static int vdbePmaReaderInit(
  SortSubtask *pTask,             /* Task context */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iStart,                     /* Start offset in pFile */
  PmaReader *pIter,               /* Iterator to populate */
  i64 *pnByte                     /* IN/OUT: Increment this value by PMA size */
){
  int rc;

  assert( pFile->iEof>iStart );
  assert( pIter->aAlloc==0 && pIter->nAlloc==0 );
  assert( pIter->aBuffer==0 );
  assert( pIter->aMap==0 );

  rc = vdbePmaReaderSeek(pTask, pIter, pFile, iStart);
  if( rc==SQLITE_OK ){
    u64 nByte;                    /* Size of PMA in bytes */
    rc = vdbePmaReadVarint(pIter, &nByte);
    pIter->iEof = pIter->iReadOff + nByte;
    *pnByte += nByte;
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReaderNext(pIter);
  }
  return rc;
}


/*
** Compare key1 (buffer pKey1, size nKey1 bytes) with key2 (buffer pKey2, 
................................................................................
  if( pKey2 ){
    sqlite3VdbeRecordUnpack(pTask->pSorter->pKeyInfo, nKey2, pKey2, r2);
  }
  return sqlite3VdbeRecordCompare(nKey1, pKey1, r2, 0);
}

/*
** This function is called to compare two iterator keys when merging 
** multiple b-tree segments. Parameter iOut is the index of the aTree[] 
** value to recalculate.
*/
static int vdbeSorterDoCompare(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int iOut
................................................................................
    i1 = (iOut - pMerger->nTree/2) * 2;
    i2 = i1 + 1;
  }else{
    i1 = pMerger->aTree[iOut*2];
    i2 = pMerger->aTree[iOut*2+1];
  }

  p1 = &pMerger->aIter[i1];
  p2 = &pMerger->aIter[i2];

  if( p1->pFile==0 ){
    iRes = i2;
  }else if( p2->pFile==0 ){
    iRes = i1;
  }else{
    int res;
................................................................................
}
#else
# define vdbeSorterJoinAll(x,rcin) (rcin)
# define vdbeSorterJoinThread(pTask) SQLITE_OK
#endif

/*
** Allocate a new MergeEngine object with space for nIter iterators.
*/
static MergeEngine *vdbeMergeEngineNew(int nIter){
  int N = 2;                      /* Smallest power of two >= nIter */
  int nByte;                      /* Total bytes of space to allocate */
  MergeEngine *pNew;              /* Pointer to allocated object to return */

  assert( nIter<=SORTER_MAX_MERGE_COUNT );

  while( N<nIter ) N += N;
  nByte = sizeof(MergeEngine) + N * (sizeof(int) + sizeof(PmaReader));

  pNew = sqlite3FaultSim(100) ? 0 : (MergeEngine*)sqlite3MallocZero(nByte);
  if( pNew ){
    pNew->nTree = N;
    pNew->aIter = (PmaReader*)&pNew[1];
    pNew->aTree = (int*)&pNew->aIter[N];
  }
  return pNew;
}

/*
** Free the MergeEngine object passed as the only argument.
*/
static void vdbeMergeEngineFree(MergeEngine *pMerger){
  int i;
  if( pMerger ){
    for(i=0; i<pMerger->nTree; i++){
      vdbePmaReaderClear(&pMerger->aIter[i]);
    }
  }
  sqlite3_free(pMerger);
}

/*
** Free all resources associated with the IncrMerger object indicated by
................................................................................
  vdbeSorterWorkDebug(pTask, "exit");
  assert( rc!=SQLITE_OK || pList->pList==0 );
  assert( rc!=SQLITE_OK || pTask->file.iEof==iSz );
  return rc;
}

/*
** Advance the MergeEngine iterator passed as the second argument to
** the next entry. Set *pbEof to true if this means the iterator has 
** reached EOF.
**
** Return SQLITE_OK if successful or an error code if an error occurs.
*/
static int vdbeSorterNext(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int *pbEof
){
  int rc;
  int iPrev = pMerger->aTree[1];/* Index of iterator to advance */

  /* Advance the current iterator */
  rc = vdbePmaReaderNext(&pMerger->aIter[iPrev]);

  /* Update contents of aTree[] */
  if( rc==SQLITE_OK ){
    int i;                      /* Index of aTree[] to recalculate */
    PmaReader *pIter1;     /* First iterator to compare */
    PmaReader *pIter2;     /* Second iterator to compare */
    u8 *pKey2;                  /* To pIter2->aKey, or 0 if record cached */

    /* Find the first two iterators to compare. The one that was just
    ** advanced (iPrev) and the one next to it in the array.  */
    pIter1 = &pMerger->aIter[(iPrev & 0xFFFE)];
    pIter2 = &pMerger->aIter[(iPrev | 0x0001)];
    pKey2 = pIter2->aKey;

    for(i=(pMerger->nTree+iPrev)/2; i>0; i=i/2){
      /* Compare pIter1 and pIter2. Store the result in variable iRes. */
      int iRes;
      if( pIter1->pFile==0 ){
        iRes = +1;
      }else if( pIter2->pFile==0 ){
        iRes = -1;
      }else{
        iRes = vdbeSorterCompare(pTask, 
            pIter1->aKey, pIter1->nKey, pKey2, pIter2->nKey
        );
      }

      /* If pIter1 contained the smaller value, set aTree[i] to its index.
      ** Then set pIter2 to the next iterator to compare to pIter1. In this
      ** case there is no cache of pIter2 in pTask->pUnpacked, so set
      ** pKey2 to point to the record belonging to pIter2.
      **
      ** Alternatively, if pIter2 contains the smaller of the two values,
      ** set aTree[i] to its index and update pIter1. If vdbeSorterCompare()
      ** was actually called above, then pTask->pUnpacked now contains
      ** a value equivalent to pIter2. So set pKey2 to NULL to prevent
      ** vdbeSorterCompare() from decoding pIter2 again.
      **
      ** If the two values were equal, then the value from the oldest
      ** PMA should be considered smaller. The VdbeSorter.aIter[] array
      ** is sorted from oldest to newest, so pIter1 contains older values
      ** than pIter2 iff (pIter1<pIter2).  */
      if( iRes<0 || (iRes==0 && pIter1<pIter2) ){
        pMerger->aTree[i] = (int)(pIter1 - pMerger->aIter);
        pIter2 = &pMerger->aIter[ pMerger->aTree[i ^ 0x0001] ];
        pKey2 = pIter2->aKey;
      }else{
        if( pIter1->pFile ) pKey2 = 0;
        pMerger->aTree[i] = (int)(pIter2 - pMerger->aIter);
        pIter1 = &pMerger->aIter[ pMerger->aTree[i ^ 0x0001] ];
      }
    }
    *pbEof = (pMerger->aIter[pMerger->aTree[1]].pFile==0);
  }

  return (rc==SQLITE_OK ? pTask->pUnpacked->errCode : rc);
}

#if SQLITE_MAX_WORKER_THREADS>0
/*
................................................................................
  assert( pIncr->bEof==0 );

  vdbeSorterPopulateDebug(pTask, "enter");

  vdbePmaWriterInit(pOut->pFd, &writer, pTask->pSorter->pgsz, iStart);
  while( rc==SQLITE_OK ){
    int dummy;
    PmaReader *pReader = &pMerger->aIter[ pMerger->aTree[1] ];
    int nKey = pReader->nKey;
    i64 iEof = writer.iWriteOff + writer.iBufEnd;

    /* Check if the output file is full or if the input has been exhausted.
    ** In either case exit the loop. */
    if( pReader->pFile==0 ) break;
    if( (iEof + nKey + sqlite3VarintLen(nKey))>(iStart + pIncr->mxSz) ) break;
................................................................................
  return vdbeSorterCreateThread(pIncr->pTask, vdbeIncrPopulateThread, p);
}
#endif

/*
** This function is called when the PmaReader corresponding to pIncr has
** finished reading the contents of aFile[0]. Its purpose is to "refill"
** aFile[0] such that the iterator should start rereading it from the
** beginning.
**
** For single-threaded objects, this is accomplished by literally reading 
** keys from pIncr->pMerger and repopulating aFile[0]. 
**
** For multi-threaded objects, all that is required is to wait until the 
** background thread is finished (if it is not already) and then swap 
................................................................................
  pIncr->pTask->file2.iEof -= pIncr->mxSz;
}
#endif /* SQLITE_MAX_WORKER_THREADS>0 */

#define INCRINIT_NORMAL 0
#define INCRINIT_TASK   1
#define INCRINIT_ROOT   2
static int vdbePmaReaderIncrInit(PmaReader *pIter, int eMode);

/*
** Initialize the merger argument passed as the second argument. Once this
** function returns, the first key of merged data may be read from the merger
** object in the usual fashion.
**
** If argument eMode is INCRINIT_ROOT, then it is assumed that any IncrMerge
** objects attached to the PmaReader objects that the merger reads from have
** already been populated, but that they have not yet populated aFile[0] and
** set the PmaReader objects up to read from it. In this case all that is
** required is to call vdbePmaReaderNext() on each iterator to point it at
** its first key.
**
** Otherwise, if eMode is any value other than INCRINIT_ROOT, then use 
** vdbePmaReaderIncrInit() to initialize each PmaReader that feeds data 
** to pMerger.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
................................................................................
*/
static int vdbeIncrInitMerger(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int eMode                       /* One of the INCRINIT_XXX constants */
){
  int rc = SQLITE_OK;             /* Return code */
  int i;                          /* For iterating through PmaReader objects */
  int nTree = pMerger->nTree;

  for(i=0; rc==SQLITE_OK && i<nTree; i++){
    if( eMode==INCRINIT_ROOT ){
      /* Iterators should be normally initialized in order, as if they are
      ** reading from the same temp file this makes for more linear file IO.
      ** However, in the INCRINIT_ROOT case, if iterator aIter[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 iterator before any of the multi-threaded iterators takes
      ** better advantage of multi-processor hardware. */
      rc = vdbePmaReaderNext(&pMerger->aIter[nTree-i-1]);
    }else{
      rc = vdbePmaReaderIncrInit(&pMerger->aIter[i], INCRINIT_NORMAL);
    }
  }

  for(i=pMerger->nTree-1; rc==SQLITE_OK && i>0; i--){
    rc = vdbeSorterDoCompare(pTask, pMerger, i);
  }

  return (rc==SQLITE_OK ? pTask->pUnpacked->errCode : rc);
}

/*
** If the PmaReader passed as the first argument is not an incremental-reader
** (if pIter->pIncr==0), then this function is a no-op. Otherwise, it serves
** to open and/or initialize the temp file related fields of the IncrMerge
** object at (pIter->pIncr).
**
** If argument eMode is set to INCRINIT_NORMAL, then PmaReader iterators
** in the sub-tree headed by pIter are also initialized. Data is then loaded
** into the buffers belonging to this iterator, pIter, and it is set to
** point to the first key in its range.
**
** If argument eMode is set to INCRINIT_TASK, then PmaReader is guaranteed
** to be a multi-threaded iterator and this function is being called in a
** background thread. In this case all iterators in the sub-tree are 
** initialized as for INCRINIT_NORMAL and the aFile[1] buffer belonging to
** pIter is populated. However, the iterator itself is not set up to point
** to its first key. A call to vdbePmaReaderNext() is still required to do
** that. 
**
** The reason this function does not call vdbePmaReaderNext() immediately 
** in the INCRINIT_TASK case is that vdbePmaReaderNext() assumes that has
** to block on thread (pTask->thread) before accessing aFile[1]. But, since
** this entire function is being run by thread (pTask->thread), that will
** lead to the current background thread attempting to join itself.
**
** Finally, if argument eMode is set to INCRINIT_ROOT, it may be assumed
** that pIter->pIncr is a multi-threaded IncrMerge objects, and that all
** child-trees have already been initialized using IncrInit(INCRINIT_TASK).
** In this case vdbePmaReaderNext() is called on all child iterators and
** the current iterator set to point to the first key in its range.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int vdbePmaReaderIncrInit(PmaReader *pIter, int eMode){
  int rc = SQLITE_OK;
  IncrMerger *pIncr = pIter->pIncr;
  if( pIncr ){
    SortSubtask *pTask = pIncr->pTask;
    sqlite3 *db = pTask->pSorter->db;

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

    /* Set up the required files for pIncr. A multi-theaded IncrMerge object
................................................................................
        }
      }
    }

#if SQLITE_MAX_WORKER_THREADS>0
    if( rc==SQLITE_OK && pIncr->bUseThread ){
      /* Use the current thread to populate aFile[1], even though this
      ** iterator is multi-threaded. The reason being that this function
      ** is already running in background thread pIncr->pTask->thread. */
      assert( eMode==INCRINIT_ROOT || eMode==INCRINIT_TASK );
      rc = vdbeIncrPopulate(pIncr);
    }
#endif

    if( rc==SQLITE_OK && eMode!=INCRINIT_TASK ){
      rc = vdbePmaReaderNext(pIter);
    }
  }
  return rc;
}

#if SQLITE_MAX_WORKER_THREADS>0
/*
................................................................................
  return pRet;
}

/*
** Use a background thread to invoke vdbePmaReaderIncrInit(INCRINIT_TASK) 
** on the the PmaReader object passed as the first argument.
**
** This call will initialize the various fields of the pIter->pIncr 
** structure and, if it is a multi-threaded IncrMerger, launch a 
** background thread to populate aFile[1].
*/
static int vdbePmaReaderBgIncrInit(PmaReader *pIter){
  void *pCtx = (void*)pIter;
  return vdbeSorterCreateThread(pIter->pIncr->pTask, vdbePmaReaderBgInit, pCtx);
}
#endif

/*
** 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
................................................................................
** set to the offset immediately following the last byte of the last
** PMA before returning. If an error does occur, then the final value of
** *piOffset is undefined.
*/
static int vdbeMergeEngineLevel0(
  SortSubtask *pTask,             /* Sorter task to read from */
  int nPMA,                       /* Number of PMAs to read */
  i64 *piOffset,                  /* IN/OUT: Read offset in pTask->file */
  MergeEngine **ppOut             /* OUT: New merge-engine */
){
  MergeEngine *pNew;              /* Merge engine to return */
  i64 iOff = *piOffset;
  int i;
  int rc = SQLITE_OK;

  *ppOut = pNew = vdbeMergeEngineNew(nPMA);
  if( pNew==0 ) rc = SQLITE_NOMEM;

  for(i=0; i<nPMA && rc==SQLITE_OK; i++){
    i64 nDummy;
    PmaReader *pIter = &pNew->aIter[i];
    rc = vdbePmaReaderInit(pTask, &pTask->file, iOff, pIter, &nDummy);
    iOff = pIter->iEof;
  }

  if( rc!=SQLITE_OK ){
    vdbeMergeEngineFree(pNew);
    *ppOut = 0;
  }
  *piOffset = iOff;
................................................................................

  for(i=1; i<nDepth; i++){
    nDiv = nDiv * SORTER_MAX_MERGE_COUNT;
  }

  for(i=1; i<nDepth && rc==SQLITE_OK; i++){
    int iIter = (iSeq / nDiv) % SORTER_MAX_MERGE_COUNT;
    PmaReader *pIter = &p->aIter[iIter];

    if( pIter->pIncr==0 ){
      MergeEngine *pNew = vdbeMergeEngineNew(SORTER_MAX_MERGE_COUNT);
      if( pNew==0 ){
        rc = SQLITE_NOMEM;
      }else{
        rc = vdbeIncrNew(pTask, pNew, &pIter->pIncr);
      }
    }
    if( rc==SQLITE_OK ){
      p = pIter->pIncr->pMerger;
      nDiv = nDiv / SORTER_MAX_MERGE_COUNT;
    }
  }

  if( rc==SQLITE_OK ){
    p->aIter[iSeq % SORTER_MAX_MERGE_COUNT].pIncr = pIncr;
  }else{
    vdbeIncrFree(pIncr);
  }
  return rc;
}

/*
................................................................................
          }
        }
      }

      if( rc==SQLITE_OK ){
#if SQLITE_MAX_WORKER_THREADS>0
        if( pMain!=0 ){
          rc = vdbeIncrNew(pTask, pRoot, &pMain->aIter[iTask].pIncr);
        }else
#endif
        {
          assert( pMain==0 );
          pMain = pRoot;
        }
      }else{
................................................................................

  rc = vdbeSorterMergeTreeBuild(pSorter, &pMain);
  if( rc==SQLITE_OK ){
#if SQLITE_MAX_WORKER_THREADS
    assert( pSorter->bUseThreads==0 || pSorter->nTask>1 );
    if( pSorter->bUseThreads ){
      int iTask;
      PmaReader *pIter;
      SortSubtask *pLast = &pSorter->aTask[pSorter->nTask-1];
      rc = vdbeSortAllocUnpacked(pLast);
      if( rc==SQLITE_OK ){
        pIter = (PmaReader*)sqlite3DbMallocZero(db, sizeof(PmaReader));
        pSorter->pReader = pIter;
        if( pIter==0 ) rc = SQLITE_NOMEM;
      }
      if( rc==SQLITE_OK ){
        rc = vdbeIncrNew(pLast, pMain, &pIter->pIncr);
        if( rc==SQLITE_OK ){
          vdbeIncrSetThreads(pIter->pIncr);
          for(iTask=0; iTask<(pSorter->nTask-1); iTask++){
            IncrMerger *pIncr;
            if( (pIncr = pMain->aIter[iTask].pIncr) ){
              vdbeIncrSetThreads(pIncr);
              assert( pIncr->pTask!=pLast );
            }
          }
          for(iTask=0; rc==SQLITE_OK && iTask<pSorter->nTask; iTask++){
            PmaReader *p = &pMain->aIter[iTask];
            assert( p->pIncr==0 || p->pIncr->pTask==&pSorter->aTask[iTask] );
            if( p->pIncr ){ 
              if( iTask==pSorter->nTask-1 ){
                rc = vdbePmaReaderIncrInit(p, INCRINIT_TASK);
              }else{
                rc = vdbePmaReaderBgIncrInit(p);
              }
            }
          }
        }
        pMain = 0;
      }
      if( rc==SQLITE_OK ){
        rc = vdbePmaReaderIncrInit(pIter, INCRINIT_ROOT);
      }
    }else
#endif
    {
      rc = vdbeIncrInitMerger(pTask0, pMain, INCRINIT_NORMAL);
      pSorter->pMerger = pMain;
      pMain = 0;
................................................................................
    PmaReader *pReader;
#if SQLITE_MAX_WORKER_THREADS>0
    if( pSorter->bUseThreads ){
      pReader = pSorter->pReader;
    }else
#endif
    /*if( !pSorter->bUseThreads )*/{
      pReader = &pSorter->pMerger->aIter[pSorter->pMerger->aTree[1]];
    }
    *pnKey = pReader->nKey;
    pKey = pReader->aKey;
  }else{
    *pnKey = pSorter->list.pList->nVal;
    pKey = SRVAL(pSorter->list.pList);
  }







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/*
** The MergeEngine object is used to combine two or more smaller PMAs into
** one big PMA using a merge operation.  Separate PMAs all need to be
** combined into one big PMA in order to be able to step through the sorted
** records in order.
**
** The aReadr[] array contains a PmaReader object for each of the PMAs being
** merged.  An aReadr[] object either points to a valid key or else is at EOF.
** For the purposes of the paragraphs below, we assume that the array is
** actually N elements in size, where N is the smallest power of 2 greater
** to or equal to the number of PMAs being merged. The extra aReadr[] elements
** are treated as if they are empty (always at EOF).
**
** The aTree[] array is also N elements in size. The value of N is stored in
** the MergeEngine.nTree variable.
**
** The final (N/2) elements of aTree[] contain the results of comparing
** pairs of PMA keys together. Element i contains the result of 
** comparing aReadr[2*i-N] and aReadr[2*i-N+1]. Whichever key is smaller, the
** aTree element is set to the index of it. 
**
** For the purposes of this comparison, EOF is considered greater than any
** other key value. If the keys are equal (only possible with two EOF
** values), it doesn't matter which index is stored.
**
** The (N/4) elements of aTree[] that precede the final (N/2) described 
** above contains the index of the smallest of each block of 4 PmaReaders
** And so on. So that aTree[1] contains the index of the PmaReader that 
** currently points to the smallest key value. aTree[0] is unused.
**
** Example:
**
**     aReadr[0] -> Banana
**     aReadr[1] -> Feijoa
**     aReadr[2] -> Elderberry
**     aReadr[3] -> Currant
**     aReadr[4] -> Grapefruit
**     aReadr[5] -> Apple
**     aReadr[6] -> Durian
**     aReadr[7] -> EOF
**
**     aTree[] = { X, 5   0, 5    0, 3, 5, 6 }
**
** The current element is "Apple" (the value of the key indicated by 
** PmaReader 5). When the Next() operation is invoked, PmaReader 5 will
** be advanced to the next key in its segment. Say the next key is
** "Eggplant":
**
**     aReadr[5] -> Eggplant
**
** The contents of aTree[] are updated first by comparing the new PmaReader
** 5 key to the current key of PmaReader 4 (still "Grapefruit"). The PmaReader
** 5 value is still smaller, so aTree[6] is set to 5. And so on up the tree.
** The value of PmaReader 6 - "Durian" - is now smaller than that of PmaReader
** 5, so aTree[3] is set to 6. Key 0 is smaller than key 6 (Banana<Durian),
** so the value written into element 1 of the array is 0. As follows:
**
**     aTree[] = { X, 0   0, 6    0, 3, 5, 6 }
**
** In other words, each time we advance to the next sorter element, log2(N)
** key comparison operations are required, where N is the number of segments
** being merged (rounded up to the next power of 2).
*/
struct MergeEngine {
  int nTree;                 /* Used size of aTree/aReadr (power of 2) */
  int *aTree;                /* Current state of incremental merge */
  PmaReader *aReadr;         /* Array of PmaReaders to merge data from */
};

/*
** Exactly VdbeSorter.nTask instances of this object are allocated
** as part of each VdbeSorter object. Instances are never allocated any
** other way. VdbeSorter.nTask is set to the number of worker threads allowed
** (see SQLITE_CONFIG_WORKER_THREADS) plus one (the main thread).
................................................................................
**   largest record in the sorter.
*/
struct VdbeSorter {
  int mnPmaSize;                  /* Minimum PMA size, in bytes */
  int mxPmaSize;                  /* Maximum PMA size, in bytes.  0==no limit */
  int mxKeysize;                  /* Largest serialized key seen so far */
  int pgsz;                       /* Main database page size */
  PmaReader *pReader;             /* Readr data from here after Rewind() */
  MergeEngine *pMerger;           /* Or here, if bUseThreads==0 */
  sqlite3 *db;                    /* Database connection */
  KeyInfo *pKeyInfo;              /* How to compare records */
  UnpackedRecord *pUnpacked;      /* Used by VdbeSorterCompare() */
  SorterList list;                /* List of in-memory records */
  int iMemory;                    /* Offset of free space in list.aMemory */
  int nMemory;                    /* Size of list.aMemory allocation in bytes */
................................................................................
/*
** An instance of the following object is used to read records out of a
** PMA, in sorted order.  The next key to be read is cached in nKey/aKey.
** pFile==0 at EOF.
*/
struct PmaReader {
  i64 iReadOff;                   /* Current read offset */
  i64 iEof;                       /* 1 byte past EOF for this PmaReader */
  int nAlloc;                     /* Bytes of space at aAlloc */
  int nKey;                       /* Number of bytes in key */
  sqlite3_file *pFile;            /* File we are reading from */
  u8 *aAlloc;                     /* Allocated space */
  u8 *aKey;                       /* Pointer to current key */
  u8 *aBuffer;                    /* Current read buffer */
  int nBuffer;                    /* Size of read buffer in bytes */
  u8 *aMap;                       /* Pointer to mapping of entire file */
  IncrMerger *pIncr;              /* Incremental merger */
};
................................................................................
static int vdbeIncrSwap(IncrMerger*);
static void vdbeIncrFree(IncrMerger *);

/*
** Free all memory belonging to the PmaReader object passed as the second
** argument. All structure fields are set to zero before returning.
*/
static void vdbePmaReaderClear(PmaReader *pReadr){
  sqlite3_free(pReadr->aAlloc);
  sqlite3_free(pReadr->aBuffer);
  if( pReadr->aMap ) sqlite3OsUnfetch(pReadr->pFile, 0, pReadr->aMap);
  vdbeIncrFree(pReadr->pIncr);
  memset(pReadr, 0, sizeof(PmaReader));
}

/*
** Read nByte bytes of data from the stream of data iterated by object p.
** If successful, set *ppOut to point to a buffer containing the data
** and return SQLITE_OK. Otherwise, if an error occurs, return an SQLite
** error code.
**
** The buffer indicated by *ppOut may only be considered valid until the
** next call to this function.
*/
static int vdbePmaReadBlob(
  PmaReader *p,                   /* PmaReader from which to take the blob */
  int nByte,                      /* Bytes of data to read */
  u8 **ppOut                      /* OUT: Pointer to buffer containing data */
){
  int iBuf;                       /* Offset within buffer to read from */
  int nAvail;                     /* Bytes of data available in buffer */

  if( p->aMap ){
................................................................................
    if( (p->iEof - p->iReadOff) > (i64)p->nBuffer ){
      nRead = p->nBuffer;
    }else{
      nRead = (int)(p->iEof - p->iReadOff);
    }
    assert( nRead>0 );

    /* Readr data from the file. Return early if an error occurs. */
    rc = sqlite3OsRead(p->pFile, p->aBuffer, nRead, p->iReadOff);
    assert( rc!=SQLITE_IOERR_SHORT_READ );
    if( rc!=SQLITE_OK ) return rc;
  }
  nAvail = p->nBuffer - iBuf; 

  if( nByte<=nAvail ){
................................................................................
  if( pFile->iEof<=(i64)(pTask->pSorter->db->nMaxSorterMmap) ){
    rc = sqlite3OsFetch(pFile->pFd, 0, (int)pFile->iEof, (void**)pp);
  }
  return rc;
}

/*
** Seek PmaReader pReadr to offset iOff within file pFile. Return SQLITE_OK 
** if successful, or an SQLite error code if an error occurs.
*/
static int vdbePmaReaderSeek(
  SortSubtask *pTask,             /* Task context */
  PmaReader *pReadr,              /* Iterate to populate */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iOff                        /* Offset in pFile */
){
  int rc = SQLITE_OK;

  assert( pReadr->pIncr==0 || pReadr->pIncr->bEof==0 );

  if( pReadr->aMap ){
    sqlite3OsUnfetch(pReadr->pFile, 0, pReadr->aMap);
    pReadr->aMap = 0;
  }
  pReadr->iReadOff = iOff;
  pReadr->iEof = pFile->iEof;
  pReadr->pFile = pFile->pFd;

  rc = vdbeSorterMapFile(pTask, pFile, &pReadr->aMap);
  if( rc==SQLITE_OK && pReadr->aMap==0 ){
    int pgsz = pTask->pSorter->pgsz;
    int iBuf = pReadr->iReadOff % pgsz;
    if( pReadr->aBuffer==0 ){
      pReadr->aBuffer = (u8*)sqlite3Malloc(pgsz);
      if( pReadr->aBuffer==0 ) rc = SQLITE_NOMEM;
      pReadr->nBuffer = pgsz;
    }
    if( rc==SQLITE_OK && iBuf ){
      int nRead = pgsz - iBuf;
      if( (pReadr->iReadOff + nRead) > pReadr->iEof ){
        nRead = (int)(pReadr->iEof - pReadr->iReadOff);
      }
      rc = sqlite3OsRead(
          pReadr->pFile, &pReadr->aBuffer[iBuf], nRead, pReadr->iReadOff
      );
      assert( rc!=SQLITE_IOERR_SHORT_READ );
    }
  }

  return rc;
}

/*
** Advance PmaReader pReadr to the next key in its PMA. Return SQLITE_OK if
** no error occurs, or an SQLite error code if one does.
*/
static int vdbePmaReaderNext(PmaReader *pReadr){
  int rc = SQLITE_OK;             /* Return Code */
  u64 nRec = 0;                   /* Size of record in bytes */


  if( pReadr->iReadOff>=pReadr->iEof ){
    IncrMerger *pIncr = pReadr->pIncr;
    int bEof = 1;
    if( pIncr ){
      rc = vdbeIncrSwap(pIncr);
      if( rc==SQLITE_OK && pIncr->bEof==0 ){
        rc = vdbePmaReaderSeek(
            pIncr->pTask, pReadr, &pIncr->aFile[0], pIncr->iStartOff
        );
        bEof = 0;
      }
    }

    if( bEof ){
      /* This is an EOF condition */
      vdbePmaReaderClear(pReadr);
      return rc;
    }
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReadVarint(pReadr, &nRec);
  }
  if( rc==SQLITE_OK ){
    pReadr->nKey = (int)nRec;
    rc = vdbePmaReadBlob(pReadr, (int)nRec, &pReadr->aKey);
  }

  return rc;
}

/*
** Initialize PmaReader pReadr to scan through the PMA stored in file pFile
** starting at offset iStart and ending at offset iEof-1. This function 
** leaves the PmaReader pointing to the first key in the PMA (or EOF if the 
** PMA is empty).
**
** If the pnByte parameter is NULL, then it is assumed that the file 
** contains a single PMA, and that that PMA omits the initial length varint.
*/
static int vdbePmaReaderInit(
  SortSubtask *pTask,             /* Task context */
  SorterFile *pFile,              /* Sorter file to read from */
  i64 iStart,                     /* Start offset in pFile */
  PmaReader *pReadr,              /* PmaReader to populate */
  i64 *pnByte                     /* IN/OUT: Increment this value by PMA size */
){
  int rc;

  assert( pFile->iEof>iStart );
  assert( pReadr->aAlloc==0 && pReadr->nAlloc==0 );
  assert( pReadr->aBuffer==0 );
  assert( pReadr->aMap==0 );

  rc = vdbePmaReaderSeek(pTask, pReadr, pFile, iStart);
  if( rc==SQLITE_OK ){
    u64 nByte;                    /* Size of PMA in bytes */
    rc = vdbePmaReadVarint(pReadr, &nByte);
    pReadr->iEof = pReadr->iReadOff + nByte;
    *pnByte += nByte;
  }

  if( rc==SQLITE_OK ){
    rc = vdbePmaReaderNext(pReadr);
  }
  return rc;
}


/*
** Compare key1 (buffer pKey1, size nKey1 bytes) with key2 (buffer pKey2, 
................................................................................
  if( pKey2 ){
    sqlite3VdbeRecordUnpack(pTask->pSorter->pKeyInfo, nKey2, pKey2, r2);
  }
  return sqlite3VdbeRecordCompare(nKey1, pKey1, r2, 0);
}

/*
** This function is called to compare two PmaReader keys when merging 
** multiple b-tree segments. Parameter iOut is the index of the aTree[] 
** value to recalculate.
*/
static int vdbeSorterDoCompare(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int iOut
................................................................................
    i1 = (iOut - pMerger->nTree/2) * 2;
    i2 = i1 + 1;
  }else{
    i1 = pMerger->aTree[iOut*2];
    i2 = pMerger->aTree[iOut*2+1];
  }

  p1 = &pMerger->aReadr[i1];
  p2 = &pMerger->aReadr[i2];

  if( p1->pFile==0 ){
    iRes = i2;
  }else if( p2->pFile==0 ){
    iRes = i1;
  }else{
    int res;
................................................................................
}
#else
# define vdbeSorterJoinAll(x,rcin) (rcin)
# define vdbeSorterJoinThread(pTask) SQLITE_OK
#endif

/*
** Allocate a new MergeEngine object with space for nReader PmaReaders.
*/
static MergeEngine *vdbeMergeEngineNew(int nReader){
  int N = 2;                      /* Smallest power of two >= nReader */
  int nByte;                      /* Total bytes of space to allocate */
  MergeEngine *pNew;              /* Pointer to allocated object to return */

  assert( nReader<=SORTER_MAX_MERGE_COUNT );

  while( N<nReader ) N += N;
  nByte = sizeof(MergeEngine) + N * (sizeof(int) + sizeof(PmaReader));

  pNew = sqlite3FaultSim(100) ? 0 : (MergeEngine*)sqlite3MallocZero(nByte);
  if( pNew ){
    pNew->nTree = N;
    pNew->aReadr = (PmaReader*)&pNew[1];
    pNew->aTree = (int*)&pNew->aReadr[N];
  }
  return pNew;
}

/*
** Free the MergeEngine object passed as the only argument.
*/
static void vdbeMergeEngineFree(MergeEngine *pMerger){
  int i;
  if( pMerger ){
    for(i=0; i<pMerger->nTree; i++){
      vdbePmaReaderClear(&pMerger->aReadr[i]);
    }
  }
  sqlite3_free(pMerger);
}

/*
** Free all resources associated with the IncrMerger object indicated by
................................................................................
  vdbeSorterWorkDebug(pTask, "exit");
  assert( rc!=SQLITE_OK || pList->pList==0 );
  assert( rc!=SQLITE_OK || pTask->file.iEof==iSz );
  return rc;
}

/*
** Advance the MergeEngine PmaReader passed as the second argument to
** the next entry. Set *pbEof to true if this means the PmaReader has 
** reached EOF.
**
** Return SQLITE_OK if successful or an error code if an error occurs.
*/
static int vdbeSorterNext(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int *pbEof
){
  int rc;
  int iPrev = pMerger->aTree[1];/* Index of PmaReader to advance */

  /* Advance the current PmaReader */
  rc = vdbePmaReaderNext(&pMerger->aReadr[iPrev]);

  /* Update contents of aTree[] */
  if( rc==SQLITE_OK ){
    int i;                      /* Index of aTree[] to recalculate */
    PmaReader *pReadr1;         /* First PmaReader to compare */
    PmaReader *pReadr2;         /* Second PmaReader to compare */
    u8 *pKey2;                  /* To pReadr2->aKey, or 0 if record cached */

    /* Find the first two PmaReaders to compare. The one that was just
    ** advanced (iPrev) and the one next to it in the array.  */
    pReadr1 = &pMerger->aReadr[(iPrev & 0xFFFE)];
    pReadr2 = &pMerger->aReadr[(iPrev | 0x0001)];
    pKey2 = pReadr2->aKey;

    for(i=(pMerger->nTree+iPrev)/2; i>0; i=i/2){
      /* Compare pReadr1 and pReadr2. Store the result in variable iRes. */
      int iRes;
      if( pReadr1->pFile==0 ){
        iRes = +1;
      }else if( pReadr2->pFile==0 ){
        iRes = -1;
      }else{
        iRes = vdbeSorterCompare(pTask, 
            pReadr1->aKey, pReadr1->nKey, pKey2, pReadr2->nKey
        );
      }

      /* If pReadr1 contained the smaller value, set aTree[i] to its index.
      ** Then set pReadr2 to the next PmaReader to compare to pReadr1. In this
      ** case there is no cache of pReadr2 in pTask->pUnpacked, so set
      ** pKey2 to point to the record belonging to pReadr2.
      **
      ** Alternatively, if pReadr2 contains the smaller of the two values,
      ** set aTree[i] to its index and update pReadr1. If vdbeSorterCompare()
      ** was actually called above, then pTask->pUnpacked now contains
      ** a value equivalent to pReadr2. So set pKey2 to NULL to prevent
      ** vdbeSorterCompare() from decoding pReadr2 again.
      **
      ** If the two values were equal, then the value from the oldest
      ** PMA should be considered smaller. The VdbeSorter.aReadr[] array
      ** is sorted from oldest to newest, so pReadr1 contains older values
      ** than pReadr2 iff (pReadr1<pReadr2).  */
      if( iRes<0 || (iRes==0 && pReadr1<pReadr2) ){
        pMerger->aTree[i] = (int)(pReadr1 - pMerger->aReadr);
        pReadr2 = &pMerger->aReadr[ pMerger->aTree[i ^ 0x0001] ];
        pKey2 = pReadr2->aKey;
      }else{
        if( pReadr1->pFile ) pKey2 = 0;
        pMerger->aTree[i] = (int)(pReadr2 - pMerger->aReadr);
        pReadr1 = &pMerger->aReadr[ pMerger->aTree[i ^ 0x0001] ];
      }
    }
    *pbEof = (pMerger->aReadr[pMerger->aTree[1]].pFile==0);
  }

  return (rc==SQLITE_OK ? pTask->pUnpacked->errCode : rc);
}

#if SQLITE_MAX_WORKER_THREADS>0
/*
................................................................................
  assert( pIncr->bEof==0 );

  vdbeSorterPopulateDebug(pTask, "enter");

  vdbePmaWriterInit(pOut->pFd, &writer, pTask->pSorter->pgsz, iStart);
  while( rc==SQLITE_OK ){
    int dummy;
    PmaReader *pReader = &pMerger->aReadr[ pMerger->aTree[1] ];
    int nKey = pReader->nKey;
    i64 iEof = writer.iWriteOff + writer.iBufEnd;

    /* Check if the output file is full or if the input has been exhausted.
    ** In either case exit the loop. */
    if( pReader->pFile==0 ) break;
    if( (iEof + nKey + sqlite3VarintLen(nKey))>(iStart + pIncr->mxSz) ) break;
................................................................................
  return vdbeSorterCreateThread(pIncr->pTask, vdbeIncrPopulateThread, p);
}
#endif

/*
** This function is called when the PmaReader corresponding to pIncr has
** finished reading the contents of aFile[0]. Its purpose is to "refill"
** aFile[0] such that the PmaReader should start rereading it from the
** beginning.
**
** For single-threaded objects, this is accomplished by literally reading 
** keys from pIncr->pMerger and repopulating aFile[0]. 
**
** For multi-threaded objects, all that is required is to wait until the 
** background thread is finished (if it is not already) and then swap 
................................................................................
  pIncr->pTask->file2.iEof -= pIncr->mxSz;
}
#endif /* SQLITE_MAX_WORKER_THREADS>0 */

#define INCRINIT_NORMAL 0
#define INCRINIT_TASK   1
#define INCRINIT_ROOT   2
static int vdbePmaReaderIncrInit(PmaReader *pReadr, int eMode);

/*
** Initialize the merger argument passed as the second argument. Once this
** function returns, the first key of merged data may be read from the merger
** object in the usual fashion.
**
** If argument eMode is INCRINIT_ROOT, then it is assumed that any IncrMerge
** objects attached to the PmaReader objects that the merger reads from have
** already been populated, but that they have not yet populated aFile[0] and
** set the PmaReader objects up to read from it. In this case all that is
** required is to call vdbePmaReaderNext() on each PmaReader to point it at
** its first key.
**
** Otherwise, if eMode is any value other than INCRINIT_ROOT, then use 
** vdbePmaReaderIncrInit() to initialize each PmaReader that feeds data 
** to pMerger.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
................................................................................
*/
static int vdbeIncrInitMerger(
  SortSubtask *pTask, 
  MergeEngine *pMerger, 
  int eMode                       /* One of the INCRINIT_XXX constants */
){
  int rc = SQLITE_OK;             /* Return code */
  int i;                          /* For looping over PmaReader objects */
  int nTree = pMerger->nTree;

  for(i=0; rc==SQLITE_OK && i<nTree; i++){
    if( eMode==INCRINIT_ROOT ){
      /* PmaReaders should be normally initialized in order, as if they are
      ** reading from the same temp file this makes for more linear file IO.
      ** 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);
    }
  }

  for(i=pMerger->nTree-1; rc==SQLITE_OK && i>0; i--){
    rc = vdbeSorterDoCompare(pTask, pMerger, i);
  }

  return (rc==SQLITE_OK ? pTask->pUnpacked->errCode : rc);
}

/*
** 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 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 PmaReader PmaReaders
** in the sub-tree headed by pReadr are also initialized. Data is then loaded
** into the buffers belonging to this PmaReader, pReadr, and it is set to
** point to the first key in its range.
**
** If argument eMode is set to INCRINIT_TASK, then PmaReader 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, the PmaReader itself is not set up to point
** to its first key. A call to vdbePmaReaderNext() is still required to do
** that. 
**
** The reason this function does not call vdbePmaReaderNext() immediately 
** in the INCRINIT_TASK case is that vdbePmaReaderNext() assumes that has
** to block on thread (pTask->thread) before accessing aFile[1]. But, since
** this entire function is being run by thread (pTask->thread), that will
** lead to the current background thread attempting to join itself.
**
** Finally, if argument eMode is set to INCRINIT_ROOT, it may be assumed
** that pReadr->pIncr is a multi-threaded IncrMerge objects, and that all
** child-trees have already been initialized using IncrInit(INCRINIT_TASK).
** In this case vdbePmaReaderNext() is called on all child PmaReaders and
** 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 vdbePmaReaderIncrInit(PmaReader *pReadr, int eMode){
  int rc = SQLITE_OK;
  IncrMerger *pIncr = pReadr->pIncr;
  if( pIncr ){
    SortSubtask *pTask = pIncr->pTask;
    sqlite3 *db = pTask->pSorter->db;

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

    /* Set up the required files for pIncr. A multi-theaded IncrMerge object
................................................................................
        }
      }
    }

#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. The reason being that this function
      ** is already running in background thread pIncr->pTask->thread. */
      assert( eMode==INCRINIT_ROOT || eMode==INCRINIT_TASK );
      rc = vdbeIncrPopulate(pIncr);
    }
#endif

    if( rc==SQLITE_OK && eMode!=INCRINIT_TASK ){
      rc = vdbePmaReaderNext(pReadr);
    }
  }
  return rc;
}

#if SQLITE_MAX_WORKER_THREADS>0
/*
................................................................................
  return pRet;
}

/*
** Use a background thread to invoke vdbePmaReaderIncrInit(INCRINIT_TASK) 
** on the the PmaReader object passed as the first argument.
**
** This call will initialize the various fields of the pReadr->pIncr 
** structure and, if it is a multi-threaded IncrMerger, launch a 
** background thread to populate aFile[1].
*/
static int vdbePmaReaderBgIncrInit(PmaReader *pReadr){
  void *pCtx = (void*)pReadr;
  return vdbeSorterCreateThread(pReadr->pIncr->pTask, vdbePmaReaderBgInit, pCtx);
}
#endif

/*
** 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
................................................................................
** set to the offset immediately following the last byte of the last
** PMA before returning. If an error does occur, then the final value of
** *piOffset is undefined.
*/
static int vdbeMergeEngineLevel0(
  SortSubtask *pTask,             /* Sorter task to read from */
  int nPMA,                       /* Number of PMAs to read */
  i64 *piOffset,                  /* IN/OUT: Readr offset in pTask->file */
  MergeEngine **ppOut             /* OUT: New merge-engine */
){
  MergeEngine *pNew;              /* Merge engine to return */
  i64 iOff = *piOffset;
  int i;
  int rc = SQLITE_OK;

  *ppOut = pNew = vdbeMergeEngineNew(nPMA);
  if( pNew==0 ) rc = SQLITE_NOMEM;

  for(i=0; i<nPMA && rc==SQLITE_OK; i++){
    i64 nDummy;
    PmaReader *pReadr = &pNew->aReadr[i];
    rc = vdbePmaReaderInit(pTask, &pTask->file, iOff, pReadr, &nDummy);
    iOff = pReadr->iEof;
  }

  if( rc!=SQLITE_OK ){
    vdbeMergeEngineFree(pNew);
    *ppOut = 0;
  }
  *piOffset = iOff;
................................................................................

  for(i=1; i<nDepth; i++){
    nDiv = nDiv * SORTER_MAX_MERGE_COUNT;
  }

  for(i=1; i<nDepth && rc==SQLITE_OK; i++){
    int iIter = (iSeq / nDiv) % SORTER_MAX_MERGE_COUNT;
    PmaReader *pReadr = &p->aReadr[iIter];

    if( pReadr->pIncr==0 ){
      MergeEngine *pNew = vdbeMergeEngineNew(SORTER_MAX_MERGE_COUNT);
      if( pNew==0 ){
        rc = SQLITE_NOMEM;
      }else{
        rc = vdbeIncrNew(pTask, pNew, &pReadr->pIncr);
      }
    }
    if( rc==SQLITE_OK ){
      p = pReadr->pIncr->pMerger;
      nDiv = nDiv / SORTER_MAX_MERGE_COUNT;
    }
  }

  if( rc==SQLITE_OK ){
    p->aReadr[iSeq % SORTER_MAX_MERGE_COUNT].pIncr = pIncr;
  }else{
    vdbeIncrFree(pIncr);
  }
  return rc;
}

/*
................................................................................
          }
        }
      }

      if( rc==SQLITE_OK ){
#if SQLITE_MAX_WORKER_THREADS>0
        if( pMain!=0 ){
          rc = vdbeIncrNew(pTask, pRoot, &pMain->aReadr[iTask].pIncr);
        }else
#endif
        {
          assert( pMain==0 );
          pMain = pRoot;
        }
      }else{
................................................................................

  rc = vdbeSorterMergeTreeBuild(pSorter, &pMain);
  if( rc==SQLITE_OK ){
#if SQLITE_MAX_WORKER_THREADS
    assert( pSorter->bUseThreads==0 || pSorter->nTask>1 );
    if( pSorter->bUseThreads ){
      int iTask;
      PmaReader *pReadr;
      SortSubtask *pLast = &pSorter->aTask[pSorter->nTask-1];
      rc = vdbeSortAllocUnpacked(pLast);
      if( rc==SQLITE_OK ){
        pReadr = (PmaReader*)sqlite3DbMallocZero(db, sizeof(PmaReader));
        pSorter->pReader = pReadr;
        if( pReadr==0 ) rc = SQLITE_NOMEM;
      }
      if( rc==SQLITE_OK ){
        rc = vdbeIncrNew(pLast, pMain, &pReadr->pIncr);
        if( rc==SQLITE_OK ){
          vdbeIncrSetThreads(pReadr->pIncr);
          for(iTask=0; iTask<(pSorter->nTask-1); iTask++){
            IncrMerger *pIncr;
            if( (pIncr = pMain->aReadr[iTask].pIncr) ){
              vdbeIncrSetThreads(pIncr);
              assert( pIncr->pTask!=pLast );
            }
          }
          for(iTask=0; rc==SQLITE_OK && iTask<pSorter->nTask; iTask++){
            PmaReader *p = &pMain->aReadr[iTask];
            assert( p->pIncr==0 || p->pIncr->pTask==&pSorter->aTask[iTask] );
            if( p->pIncr ){ 
              if( iTask==pSorter->nTask-1 ){
                rc = vdbePmaReaderIncrInit(p, INCRINIT_TASK);
              }else{
                rc = vdbePmaReaderBgIncrInit(p);
              }
            }
          }
        }
        pMain = 0;
      }
      if( rc==SQLITE_OK ){
        rc = vdbePmaReaderIncrInit(pReadr, INCRINIT_ROOT);
      }
    }else
#endif
    {
      rc = vdbeIncrInitMerger(pTask0, pMain, INCRINIT_NORMAL);
      pSorter->pMerger = pMain;
      pMain = 0;
................................................................................
    PmaReader *pReader;
#if SQLITE_MAX_WORKER_THREADS>0
    if( pSorter->bUseThreads ){
      pReader = pSorter->pReader;
    }else
#endif
    /*if( !pSorter->bUseThreads )*/{
      pReader = &pSorter->pMerger->aReadr[pSorter->pMerger->aTree[1]];
    }
    *pnKey = pReader->nKey;
    pKey = pReader->aKey;
  }else{
    *pnKey = pSorter->list.pList->nVal;
    pKey = SRVAL(pSorter->list.pList);
  }