Inconsistent Checksum behavior in DAP4

[WardF]

As reported by @lesserwhirls, we are seeing inconsistent behavior when accessing dap4 data without specifying checksum=true or checksum=false.

Consistent behavior

$ ncdump "dap4://test.opendap.org:8080/opendap/data/nc/fnoc1.nc?dap4.checksum=true"
$ ncdump "dap4://test.opendap.org:8080/opendap/data/nc/fnoc1.nc?dap4.checksum=false"

Sometimes works, sometimes fails.

$ ncdump "dap4://test.opendap.org:8080/opendap/data/nc/fnoc1.nc"

Supposition

Sean supposes that there may be an uninitialized variable being read that may or may not contain garbage data and may result in this inconsistent behavior. I'm going to check on this.

[lesserwhirls]

Tagging @ndp-opendap @Mikejmnez

[ndp-opendap]

When we run those test commands using cURL instead:

curl -o fnoc1_chksm_false.dap http://test.opendap.org:8080/opendap/data/nc/fnoc1.nc.dap?dap4.checksum=false
curl -o fnoc1_chksm_true.dap http://test.opendap.org:8080/opendap/data/nc/fnoc1.nc.dap?dap4.checksum=true
curl -o fnoc1_no_chksm.dap http://test.opendap.org:8080/opendap/data/nc/fnoc1.nc.dap

The returned binaries are consistent, and the no_chksum responses also match the chksm_false responses.
BUT....
We have been seeing inconsistent responses from the server for some larger datasets, so this issue may be the Hyrax instance as well as the netcdf client code.

[DennisHeimbigner]

I changed netcdf-c to support the the change described here:

• Issue: Amend specification so that dap4 data responses with checksums are identifiable as such OPENDAP/dap4-specification#32
• PR: Draft Checksums Change OPENDAP/dap4-specification#34

However, I need to propagate this change into netcdf-java and tds repositories.
Unfortunately, this has led to me having to wrestle with IntelliJ community edition, which has taken some time.
I have been working on PRs for this change, but am now struggling to test it (mostly because I have forgotten how to do things like standing up a test version of TDS).
Bottom line: PRs for this are coming.
Also, I cannot guarantee that making these changes will fix the problem you are seeing.
I would have thought that adding "?dap4.checksum=false" would have worked since that is the currrent default.