Ensure the buffer provided to MPAS_io_get_var_generic is large enough.

[jim-p-w]

This PR fixes a potential buffer overrun when reading string variables from a netcdf file.

A fixed size array is provided as an output buffer when reading a 0d-char character variable.
Call MPAS_io_inq_var prior to the read to get the size of the variable, and only proceed with the read if the size of the variable will fit in the provided array.
Return an error code if the variable value is larger than the provided output buffer.

A unit test is included to verify:

1. An attempt to read a string variable into a buffer which is too small to hold the string value is detected
2. When a buffer which is too small is detected, the read won't occur and an error code is returned
3. When the test is run via valgrind, valgrind detects no memory errors or corruption
4. If the code which detects the too small buffer is commented out the test crashes and valgrind reports memory corruption

Fixes issue #1350

Note

When building with PIO, if the charArray (or the charArray1d) value exceeds the size of the tempchar buffer provided to the call to PIO_get_var, the value will be truncated to the size of the provided tempchar buffer (lines 2018, 2023, and 2042 in src/framework/mpas_io.F).

[mgduda]

In case of an MPAS_IO_ERR_INSUFFICIENT_BUF error, this will be the last statement executed in this subroutine, and there may be an opportunity to minimize the changes to the code below this block. Rather than logic that is structured like

      if (local_ierr == MPAS_IO_ERR_INSUFFICIENT_ARG) then
         ... code ...
      else
#ifdef MPAS_PIO_SUPPORT
        if (pio_ierr /= PIO_noerr) then
           ... code ...
           return
        end if
#endif

#ifdef MPAS_SMIOL_SUPPORT
        if (local_ierr /= SMIOL_SUCCESS) then
           ... code ...
           return
        end if
#endif
      end if

what if we used something like this:

      if (local_ierr == MPAS_IO_ERR_INSUFFICIENT_ARG) then
         ... code ...
         return
      end if

#ifdef MPAS_PIO_SUPPORT
      if (pio_ierr /= PIO_noerr) then
         ... code ...
         return
      end if
#endif

#ifdef MPAS_SMIOL_SUPPORT
      if (local_ierr /= SMIOL_SUCCESS) then
         ... code ...
         return
      end if
#endif

The idea here is that everything from the #ifdef MPAS_PIO_SUPPORT line downward is completely unchanged from the base code.

[jim-p-w]

Nice observation. Done.

[mgduda]

So that we don't need to make up an unused argument in cases where we don't have any arguments to the mpas_log_write call, for example:

IO_ERROR_WRITE('Here is a message', intArgs=[0])

we could instead define this macro as:

#define IO_ERROR_WRITE(M) call mpas_log_write(M, messageType=MPAS_LOG_ERR)

In order to allow for the passing of other arguments to mpas_log_write, we can also define the macro COMMA as:

#define COMMA ,

Then, an error message with arguments would look something like:

IO_ERROR_WRITE('The answer is $i' COMMA intArgs=[42])

[jim-p-w]

Clever! I had wondered about the definition of COMMA in other files. Done.

[mgduda]

It might make this message more helpful to users if we used something like:

            message = 'Length of string variable "'//trim(fieldname)//'" in file "'//trim(handle % filename)//'"'
            IO_ERROR_WRITE(message, intArgs=(/0/))
            message = '    exceeds buffer size: len('//trim(fieldname)//')=$i, len(buffer)=$i'
            IO_ERROR_WRITE(message, intArgs=(/dimsizes(1), len(charVal)/)) 

This would generate an error log entry something like:

ERROR: Length of string variable "xtime" in file "x1.10242.init.nc"
ERROR:     exceeds buffer size: len(xtime)=128, len(buffer)=64

[jim-p-w]

Good idea. I love printing out the filename which contains the variable. Done.