Check area slices for correct step

[pnuu]

This PR makes sure that the slice(start, stop, step) has correct sign for the step argument when start > stop when slicing the area.

• Closes segmentation fault or AssertionError when resampling #274 Resampling to "met09globeFull" area definition fails satpy#930
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.04%) to 91.991% when pulling 5db5ae8 on pnuu:bugfix-get_area_slices into f661cf9 on pytroll:master.

[codecov]

Codecov Report

Merging #275 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   91.97%   92.01%   +0.04%     
==========================================
  Files          42       42              
  Lines        8208     8228      +20     
==========================================
+ Hits         7549     7571      +22     
+ Misses        659      657       -2     

Impacted Files	Coverage Δ
pyresample/geometry.py	82.65% <100.00%> (+0.12%)	⬆️
pyresample/test/test_utils.py	97.50% <100.00%> (+0.13%)	⬆️
pyresample/utils/__init__.py	83.90% <100.00%> (+1.19%)	⬆️

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[gerritholl]

I still get a segfault when trying to use this with pytroll/satpy#1188 when loading the area definition from the yaml file, but not when defining it inline. I'm trying to figure out what's going on.

[gerritholl]

My previous comment was a lie. It does also fail when defining it inline; I still had the reduce_data=False flag active.

Specifically, this still segfaults:

from satpy import Scene
from satpy.utils import debug_on
from pyresample import create_area_def
debug_on()
from glob import glob
area = create_area_def("fci_0deg_2km",
        {"proj": "geos", "lon_0": 0, "h": 35786400, "x_0": 0, "y_0": 0, "ellps": "WGS84"},
        units="m",
        shape=(5568, 5568),
        area_extent=(-5567999.994206558, -5567999.994200589,
            5567999.994206558, 5567999.994200589))
sc = Scene(glob("/media/nas/x21308/2020_04_MTG_unofficial_Testdata/20130804_RC72/W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--CHK-BODY--L2P-NC4E_C_EU*.nc"), reader="fci_l1c_fdhsi")                                                                                                                           
sc.load(["ir_123"])
nsc = sc.resample(area)
nsc.save_dataset("ir_123", "/tmp/ir_123.tif")

Meanwhile, this completes successfully (previously AssertionError):

from satpy import Scene
from satpy.utils import debug_on
from pyresample import create_area_def
debug_on()
from glob import glob
area = create_area_def("fci_0deg_2km",
        {"proj": "geos", "lon_0": 0, "h": 35786400, "x_0": 0, "y_0": 0, "ellps": "WGS84"},
        units="m",
        shape=(5568//10, 5568//10),
        area_extent=(-5567999.994206558/10, -5567999.994200589/10,
            5567999.994206558/10, 5567999.994200589/10))
sc = Scene(glob("/media/nas/x21308/2020_04_MTG_unofficial_Testdata/20130804_RC72/W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--CHK-BODY--L2P-NC4E_C_EU*.nc"), reader="fci_l1c_fdhsi")                                                                                                                           
sc.load(["ir_123"])
nsc = sc.resample(area)
nsc.save_dataset("ir_123", "/tmp/ir_123.tif")

[gerritholl]

After the segmentation fault, the last line in dmesg is:

[9527227.302060] python[13518]: segfault at 564456e99e40 ip 00007fcaba50b033 sp 00007fcaa6ffc098 error 4 in kdtree.cpython-38-x86_64-linux-gnu.so[7fcaba4fa000+13000]

[mraspaud]

LGTM. An explanation of what problem was solved in the description of the PR would be nice :)

[pnuu]

Added explanation, and merging.