wrong start_time with BackgroundCompositor

[gerritholl]

Describe the bug

Loading a composite that uses the BackgroundCompositor, such as natural_color_with_night_ir, leads to a bad start_time attribute in the resulting metadata.

To Reproduce

import os
from satpy import Scene
from glob import glob
from pathlib import Path

fn = "/media/nas/x21308/scratch/data/MSG4-SEVI-MSG15-0100-NA-20220216002743.011000000Z-NA.nat"

sc = Scene(filenames=[fn], reader=["seviri_l1b_native"])
sc.load(["natural_color", "natural_color_with_night_ir"])
ls = sc.resample("eurol")
print(ls["natural_color"].attrs["start_time"])
print(ls["natural_color_with_night_ir"].attrs["start_time"])

Expected behavior

2022-02-16 00:15:00
2022-02-16 00:15:00

Actual results

2022-02-16 00:15:00
2022-07-23 15:40:51.982830

Environment Info:

• Satpy Version: main

Additional context

This means that {start_time} when passed to save_datasets is also wrong.

[gerritholl]

Related: #2447 #2446 #2427 #1174

[djhoese]

Any fixes for this will require some user testing most likely as a None start_time (or non-existent?) would work fine for all compositor related stuff but then fail in the writers or something similar. In some cases we may be OK with that (saving a file to a filename scheme that includes {start_time:%Y%m%d_%H%M%S} when none is available), but others it may not be expected.

[gerritholl]

The problem would also be fixed if combine_metadata defaulted to .min() rather than .mean() when combining different start times, although only because the fictional start_time in the BackgroundCompositor is set to utcnow(), which should be always newer than any other dataset.

[yukaribbba]

I did a little trick to get off this annoying issue.

scn.load(['night_ir_alpha_with_background'])

# this will actually load 'night_ir_alpha' which is the foreground of this composite
scn.load([scn['night_ir_alpha_with_background'].attrs['prerequisites'][0]['name']])

# set the start_time of 'night_ir_alpha_with_background' to the start_time of 'night_ir_alpha'
scn['night_ir_alpha_with_background'].attrs['start_time'] = scn[scn['night_ir_alpha_with_background'].attrs['prerequisites'][0]['name']].attrs['start_time']