Bug regarding function TBufferedFileStream.ReadPageBeforeWrite

by 'lagprogramming'.

packages/fcl-base/src/bufstream.pp has the following function:

    function TBufferedFileStream.ReadPageBeforeWrite: Boolean;
    var
      j: integer;
      pCache: PStreamCacheEntry=nil;
      lStreamPosition: int64;
      lExpectedBytesToRead: integer;
      lEffectiveRead: integer;
    begin
      // Find free page entry
      for j := 0 to Pred(FStreamCachePageMaxCount) do begin
        if not Assigned(FCachePages[j]^.Buffer) then begin
          pCache:=FCachePages[j];
          FCacheLastUsedPage:=j;
          break;
        end;
      end;
      if not Assigned(pCache) then begin
        // Free last used page
        pCache:=FreeOlderInUsePage(false);
      end;
      if not Assigned(pCache^.Buffer) then begin
        Getmem(pCache^.Buffer,FStreamCachePageSize);
      end;
      lStreamPosition:=(FCacheStreamPosition div FStreamCachePageSize)*FStreamCachePageSize;
      inherited Seek(lStreamPosition,soBeginning);
      if (lStreamPosition+FStreamCachePageSize) > FCacheStreamSize then begin
        lExpectedBytesToRead:=FCacheStreamSize-lStreamPosition;
      end else begin
        lExpectedBytesToRead:=FStreamCachePageSize;
      end;
      pCache^.PageBegin:=lStreamPosition;
      pCache^.PageRealSize:=inherited Read(pCache^.Buffer^,FStreamCachePageSize);
      if pCache^.PageRealSize<>lExpectedBytesToRead then begin
        lEffectiveRead:=pCache^.PageRealSize;
        pCache^.IsDirty:=false;
        pCache^.LastTick:=0;
        pCache^.PageBegin:=0;
        pCache^.PageRealSize:=0;
        Freemem(pCache^.Buffer);
        pCache^.Buffer:=nil;
        Raise EStreamError.CreateFmt(SErrCacheUnableToReadExpected,[lExpectedBytesToRead,lEffectiveRead]);
      end;
      pCache^.LastTick:=GetOpCounter;
      Result:=true;
    end;

The function never returns false. Either it returns true, either it raises an exception. ReadPageBeforeWrite is called by function TBufferedFileStream.DoCacheWrite, which expects boolean results, not exceptions.

This is how DoCacheWrite tries to use the result:

    if ReadPageBeforeWrite then begin
      Result:=DoCacheWrite(Buffer,Count);
    end else begin
      Result:=0;
    end;