netCDF output fails for WCS in 7.4.1

[constantinius]

netCDF output fails for the below map-file (referencing a vrt of 3 tiffs) with the following exception message:

<?xml version="1.0" encoding="UTF-8"?>
<ows:ExceptionReport xmlns:ows="http://www.opengis.net/ows/2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.0.1" xml:lang="en-US" xsi:schemaLocation="http://www.opengis.net/ows/2.0 http://schemas.opengis.net/ows/2.0/owsExceptionReport.xsd">
  <ows:Exception exceptionCode="NoApplicableCode" locator="mapserv">
    <ows:ExceptionText>msWCSWriteFile20(): General error message. msSaveImage() failed
msSaveImageGDAL(): General error message. Failed to create output netCDF file.
Unable to create netCDF file /vsimem/wcsout/5e1848d0_1b8_0./mosaic_MER_FRS_1P_reduced_RGB_20200110090535.nc (Error code 2): No such file or directory .</ows:ExceptionText>
  </ows:Exception>
</ows:ExceptionReport>

I test it using this command: mapserv -nh QUERY_STRING="map=test.map&version=2.0.1&service=WCS&request=GetCoverage&coverageid=mosaic_MER_FRS_1P_reduced_RGB&format=application/x-netcdf"

The same error (or very similar, at least) happens for WCS 1.0, so this has nothing to do with WCS 2.0:

# mapserv -nh QUERY_STRING="map=test.map&version=1.0.0&service=WCS&request=GetCoverage&coverage=mosaic_MER_FRS_1P_reduced_RGB&format=application/x-netcdf&crs=EPSG:4326&bbox=-3.75,32.19025,28.263455,46.268645&width=100&height=100"
<?xml version='1.0' encoding="UTF-8" ?>
<ServiceExceptionReport version="1.2.0"
xmlns="http://www.opengis.net/ogc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/ogc http://schemas.opengis.net/wcs/1.0.0/OGC-exception.xsd">
  <ServiceException>msSaveImageGDAL(): General error message. Failed to create output netCDF file.
Unable to create netCDF file /vsimem/msout/5e1872a0_1c8_0.nc (Error code 2): No such file or directory .
  </ServiceException>
</ServiceExceptionReport>

This is my setup:

# mapserv -v
MapServer version 7.4.1 OUTPUT=PNG OUTPUT=JPEG OUTPUT=KML SUPPORTS=PROJ SUPPORTS=AGG SUPPORTS=FREETYPE SUPPORTS=CAIRO SUPPORTS=SVG_SYMBOLS SUPPORTS=RSVG SUPPORTS=ICONV SUPPORTS=FRIBIDI SUPPORTS=WMS_SERVER SUPPORTS=WMS_CLIENT SUPPORTS=WFS_SERVER SUPPORTS=WFS_CLIENT SUPPORTS=WCS_SERVER SUPPORTS=SOS_SERVER SUPPORTS=FASTCGI SUPPORTS=THREADS SUPPORTS=GEOS SUPPORTS=PBF INPUT=JPEG INPUT=POSTGIS INPUT=OGR INPUT=GDAL INPUT=SHAPEFILE
# gdalinfo --version
GDAL 2.4.2, released 2019/06/28

On 7.0.7 (and GDAL 2.2.3, released 2017/11/20) the same request works and produces an output file.

MAP
  IMAGETYPE "png"
  NAME "MS"
  SIZE -1 -1
  STATUS ON
  UNITS METERS

  OUTPUTFORMAT
    NAME "netCDF"
    MIMETYPE "application/x-netcdf"
    DRIVER "GDAL/netCDF"
    EXTENSION ".nc"
    IMAGEMODE BYTE
    TRANSPARENT FALSE
    FORMATOPTION "FILENAME=mosaic_MER_FRS_1P_reduced_RGB_20200110090535.nc"
  END # OUTPUTFORMAT

  LEGEND
    KEYSIZE 20 10
    KEYSPACING 5 5
    LABEL
      SIZE 10
      OFFSET 0 0
      SHADOWSIZE 1 1
    END # LABEL
    STATUS OFF
  END # LEGEND

  QUERYMAP
    SIZE -1 -1
    STATUS OFF
    STYLE HILITE
  END # QUERYMAP

  SCALEBAR
    INTERVALS 4
    LABEL
      SIZE 10
      OFFSET 0 0
      SHADOWSIZE 1 1
    END # LABEL
    SIZE 200 3
    STATUS OFF
    UNITS MILES
  END # SCALEBAR

  WEB
    METADATA
      "ows_updateSequence"	"20131219T132000Z"
      "ows_onlineresource"	"*"
      "ows_enable_request"	"*"
    END # METADATA
  END # WEB

  LAYER
    DATA "test.vrt"
    EXTENT -3.75 32.19025 28.263455 46.268645
    METADATA
      "wcs_label"	"mosaic_MER_FRS_1P_reduced_RGB"
      "wcs_nilvalues_reasons"	"http://www.opengis.net/def/nil/OGC/0/unknown"
      "wcs_bandcount"	"3"
      "wcs_size"	"1021 449"
      "red_band_description"	"Red Channel"
      "wcs_formats"	"image/tiff image/jp2 application/x-netcdf application/x-hdf image/png"
      "wcs_green_interval"	"0 255"
      "wcs_srs"	"EPSG:4326 EPSG:3857 EPSG:3035"
      "wcs_band_names"	"red green blue"
      "wcs_imagemode"	"BYTE"
      "wcs_resolution"	"0.031355 -0.031355"
      "red_band_uom"	"W.m-2.Sr-1"
      "wcs_nilvalues"	"0"
      "blue_band_uom"	"W.m-2.Sr-1"
      "wcs_rangeset_name"	"RGB"
      "wcs_interval"	"0.000000 255.000000"
      "blue_band_definition"	"http://www.opengis.net/def/property/OGC/0/Radiance"
      "ows_enable_request"	"*"
      "wcs_green_label"	"green"
      "wcs_rangeset_label"	"RGB"
      "wcs_blue_label"	"blue"
      "blue_band_description"	"Blue Channel"
      "red_band_definition"	"http://www.opengis.net/def/property/OGC/0/Radiance"
      "ows_srs"	"EPSG:4326 EPSG:3857 EPSG:3035"
      "green_band_uom"	"W.m-2.Sr-1"
      "ows_title"	"mosaic_MER_FRS_1P_reduced_RGB"
      "wcs_red_interval"	"0 255"
      "wcs_extent"	"-3.75 32.19025 28.263455 46.268645"
      "green_band_definition"	"http://www.opengis.net/def/property/OGC/0/Radiance"
      "wcs_red_label"	"red"
      "wcs_significant_figures"	"3"
      "wcs_blue_interval"	"0 255"
      "green_band_description"	"Green Channel"
    END # METADATA
    NAME "mosaic_MER_FRS_1P_reduced_RGB"
    PROJECTION
      "proj=longlat"
      "datum=WGS84"
      "no_defs"
    END # PROJECTION
    STATUS OFF
    TILEITEM "location"
    TYPE RASTER
    UNITS METERS
  END # LAYER

END # MAP

[rouault]

Yes, this must be related to the fact the netCDF driver in recent versions of GDAL reports DCAP_VIRTUALIO support, but this is actually just for reading, not writing. MapServer shoud likely be patched to ignore DCAP_VIRTUALIO when the driver is netCDF

[constantinius]

Ah, that makes sense, so it was basically since version 2.4 of GDAL, right? (OSGeo/gdal#786)

[rouault]

so it was basically since version 2.4 of GDAL

yes

[constantinius]

How would you determine if we are dealing with a netCDF driver? Just check the drivers short name? We may need to check the GDAL version as-well, right?
I think these are the lines in question: https://github.com/mapserver/mapserver/blob/master/mapgdal.c#L185-L189

[rouault]

Just check the drivers short name?

yes

We may need to check the GDAL version as-well, right?

Seems more complicated than needed

I think these are the lines in question: https://github.com/mapserver/mapserver/blob/master/mapgdal.c#L185-L189

I also think so

[constantinius]

Thank you very much, Even!