Issue with setBuffer

[ykempf]

When setBuffer() is not being called, outputBuffer is a NULL pointer as initialised in the constructor. https://github.com/fmihpc/vlsv/blob/master/vlsv_writer.cpp#L53 It seems that this situation leads to malloc probblems/OOM crashes in Vlasiator in some cases when it tries to empty the buffer before writing at https://github.com/fmihpc/vlsv/blob/master/vlsv_writer.cpp#L549.

When setBuffer() is called with an argument of 0, which should behave similarly as the default behaviour when the function is not called when looking at the rest of the code, what happens is that outputBuffer is set as a well-behaving zero-length char array https://github.com/fmihpc/vlsv/blob/master/vlsv_writer.cpp#L830. This does not lead to the crashes otherwise detected.

Now looking at the top of setBuffer(), if the user calls setBuffer() a second time and there is data left in the buffer, it will be silently deallocated and a new one will be allocated. Should this call emptyBuffer() first? Or should the user simply be prevented from calling setBuffer() multiple times, and then the delete is not needed any more?

Until this is solved we recommend to call setBuffer(0) or with the desired buffer size once and only once before any writing sequence.

[ykempf]

Sorry I now saw that the default in Vlasiator is not 0. It actually crashes also with a buffer size manually set to 0. So the behaviour for zero buffer size is ill-defined and should be fixed, as it seemed to me that the assumed default in the code was to do the standard default writing if the buffer size is 0, which would also be a sensible expectation from the user side. Probably then the emptyBuffer() call at line 830 (and similar if there is) should be bypassed if the buffer size is set to 0.