GMT_PAD_DEFAULT doesn't exist in libgmt: Possible upstream GMT_GET_Enum API Change

[akshmakov]

Feedback or description of feature requested

After upgrading from gmt 6.0.0_r20005 to 6.0.0_r20469 of upstream trunk gmt library, and every commit since, gmt-python stopped functioning (import fails with CLIB Error). I understand these are development commits, I am maintaining some images with cutting edge builds of gmt for test purposes. I have done due diligence to verify it isn't other incompatabilities, or build errors. This is a change in behavior from previous versions of GMT which I have tested with development versions of gmt-python

It appears an API change may been introduced in r20437. An API Pointer has been added to the function signature, this may contribute to the error. GMT_PAD_DEFAULT was moved from a pre processor constant to enum in Revision 18648 and gmt-python was updated to accommodate this change at some point

Before and after docker images are available. for verification nc5ng/gmt:6.0.0_r20005-python (Functioning) and nc5ng/gmt:6.0.0_r20469-python (Broken). docker hub link

note: these have slightly different versions of gmt-python, but I have downgraded/upgraded to verify and it is not due to gmt-python.

Are you willing to help implement and maintain this bug? Yes if guidance is provided

Full code that generated the error

import gmt

Full error message

/usr/local/lib/python3.5/dist-packages/gmt/__init__.py in <module>()
     26 
     27 # Start our global modern mode session
---> 28 _begin()
     29 # Tell Python to run _end when shutting down
     30 _atexit.register(_end)

/usr/local/lib/python3.5/dist-packages/gmt/session_management.py in begin()
     15     """
     16     prefix = 'gmt-python-session'
---> 17     with LibGMT() as lib:
     18         lib.call_module('begin', prefix)
     19 

/usr/local/lib/python3.5/dist-packages/gmt/clib/core.py in __enter__(self)
    128         Start the GMT session and keep the session argument.
    129         """
--> 130         self.current_session = self.create_session('gmt-python-session')
    131         return self
    132 

/usr/local/lib/python3.5/dist-packages/gmt/clib/core.py in create_session(self, session_name)
    170         # print function.
    171         print_func = None
--> 172         padding = self.get_constant('GMT_PAD_DEFAULT')
    173         session_type = self.get_constant('GMT_SESSION_EXTERNAL')
    174         session = c_create_session(session_name.encode(), padding,

/usr/local/lib/python3.5/dist-packages/gmt/clib/core.py in get_constant(self, name)
    237         if value is None or value == -99999:
    238             raise GMTCLibError(
--> 239                 "Constant '{}' doesn't exits in libgmt.".format(name))
    240 
    241         return value
   GMTCLibError: Constant 'GMT_PAD_DEFAULT' doesn't exits in libgmt.

System information

• Operating system: Ubuntu 16.04.5 (Docker Image)
• Python installation (Anaconda, system, ETS): python3 from ubuntu repository
• Version of GMT: Tested 6.0.0_r20005 (works) and 6.0.0_r20471 (broken) and 6.0.0_r20469 (broken)
• Version of Python: 3.5.1-3
• Version of this package: 0.1a3, 0.1a3+112.gefabb47 and `0.1a3+79.gf979f02'

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[akshmakov]

Confirmed by making a quick fix to session.py#L216-L218

c_get_enum = self.get_libgmt_func(
            "GMT_Get_Enum", argtypes=[ctp.c_char_p], restype=ctp.c_int
        )

value = c_get_enum(name.encode())

changed to

c_get_enum = self.get_libgmt_func(
            "GMT_Get_Enum", argtypes=[ctp.c_void_p, ctp.c_char_p], restype=ctp.c_int
        )

value = c_get_enum(None, name.encode())

This works because API change accepts a null pointer for new argument

Commit on my fork here: nc5ng@c539b62

Verified on the gmt 6.0.0_r20471 and gmt-python 0.1a3+113.gc539b62

[akshmakov]

Possibly related to #215

[leouieda]

Hi @akshmakov, thanks for the detailed report and investigating the issue! I really appreciate it!

This was a recent change to the API. GMT_Get_Enum was one of the only functions that didn't take an API pointer as the first argument. This was recently changed to make the API more uniform and allow for possible extensions in the future. Your change should probably work (great investigating, BTW).

Would you like to open a pull request with that fix? Let me know if you need any guidance for this.

Your build will break for now because of #221 but once I have that fixed everything should be fine.

Again, big thanks for investigating this! 🥇

[akshmakov]

@leouieda no problem I will reword the commit to reference gmt commit and submit a pr today or tomorrow

A few questions

1. This is a distinct break with previous interface, so will require gmt 6.0.0 after the specific revision with this change. Would some kind of test or check for gmt version be preferable as an intermediate step to support both versions?

2. Are you handling API pointers in other calls that I should copy? Is passing a null void pointer sufficient?

This does build on my machine with pip install directly from repo, is #221 related to your release build system?