Chem: Prevent seg fault due to numgas==0

[lpilz]

TYPE: bug fix

KEYWORDS: chem, drydep, wesely

SOURCE: Lukas Pilz (Heidelberg University)

DESCRIPTION OF CHANGES:
Problem:
A namelist option incompatibility between chem_opt (16) and gas_drydep_opt (1) leads to trying to access a variable
that is not available in memory. This leads to unpredictable behaviors. When choosing a tracer-only chem_opt, numgas
is 0. If gas_drydep_opt is 1 (the default), in the Wesely scheme initialization (dep_init), the field dvj is initialized with
size numgas (here 0). Accessing the variable dvj is an error.

Solution:
A fatal error was added for when the Wesely scheme initialization is called with numgas = 0.

This fix is in chem/chemics_init.F and not share/module_check_a_mundo.F because checking the namelist options
would require some tricky-to-maintain hardcoding.

LIST OF MODIFIED FILES:
chem/chemics_init.F

TESTS CONDUCTED:

1. Run with chem_opt = 16 and gas_drydep_opt = 1 fails appropriately
2. Run with chem_opt = 16 and gas_drydep_opt = 0 runs as it should
3. The jenkins testing is OK.

RELEASE NOTE: For WRF-Chem, a namelist option incompatibility between chem_opt (16) and gas_drydep_opt (1) leads to trying to access a variable that is not available in memory. This leads to unpredictable behaviors. A fatal error was added for when the Wesely scheme initialization is called with numgas = 0.

[davegill]

@lpilz
Lukas,

1. Can we add a better title? It needs to refer to WRF Chem. Also, is only Wesely dry dep a problem with numgas=0? If so, let's include that focused information in the PR title. Users will want to know if they need this update. The PR title should let them know if this important for their research.

2. Previously, when the wrong namelist options were set, what happened? Did the model fail? Did the model run to completion, but with bad results?

[jordanschnell]

@davegill I can review/approve this, but what is happening with the jenkins test?

[davegill]

@jordanschnell @dudhia @weiwangncar @kkeene44 @smileMchen @powersjg @liujake

I can review/approve this, but what is happening with the jenkins test?

Jordan,
The automated jenkins testing is broken. Please hold off approving or merging. The testing has been broken for 2+ weeks. I know that this is frustrating.

[lpilz]

Hi @davegill

1. How about "WRF-Chem: Prevent stack smashing due to numgas==0"
2. I checked around a bit and found instanciations with numgas in wesely_driver, rc, and HL_init. wesely_driver is called in the case we prevent, dep_init is, rc is only called inside of wesely_driver and as far as I see, HL_init is never called. I'll check some more but for now, I think this covers it.
3. There were random segfaults in the execution, which is why I started debugging. Sometimes the model even ran through...

[lpilz]

@davegill @jordanschnell

What's the latest update on this PR? Is it ready to be merged?

[jordanschnell]

@lpilz We were waiting on the automated tests to be back up but it seems the check has passed. @davegill, OK to approve and merge?

[davegill]

@lpilz @jordanschnell
Folks,
Yes, we are all good with this PR.

[jordanschnell]

Approved by chem