Check duplicate depends_on of Plugin

[yuema137]

Solve error #846.

Explanation

Basically what happened was:

• In plugin.py, the chunks are fetched for each self.depends_on and stored in a dictionary:

  strax/strax/plugins/plugin.py

  Lines 458 to 493 in 2218e78

  	inputs = dict()
  	# Fetch other inputs (when needed)
  	for d in self.depends_on:
  	if d != pacemaker:
  	while (
  	self.input_buffer[d] is None
  	or self.input_buffer[d].end < this_chunk_end
  	):
  	self._fetch_chunk(d, iters, check_end_not_before=this_chunk_end)
  	inputs[d], self.input_buffer[d] = self.input_buffer[d].split(
  	t=this_chunk_end, allow_early_split=True
  	)
  	# If any of the inputs were trimmed due to early splits,
  	# trim the others too.
  	# In very hairy cases this can take multiple passes.
  	# can we optimize this, or code it more elegantly?
  	max_passes_left = 10
  	while max_passes_left > 0:
  	this_chunk_end = min([x.end for x in inputs.values()] + [this_chunk_end])
  	if len(set([x.end for x in inputs.values()])) <= 1:
  	break
  	for d in self.depends_on:
  	inputs[d], back_to_buffer = inputs[d].split(
  	t=this_chunk_end, allow_early_split=True
  	)
  	self.input_buffer[d] = strax.Chunk.concatenate(
  	[back_to_buffer, self.input_buffer[d]],
  	self.allow_hyperrun,
  	)
  	max_passes_left -= 1
  	else:
  	raise RuntimeError(
  	f"{self} was unable to get time-consistent "
  	f"inputs after ten passess. Inputs: \n{inputs}\n"
  	f"Input buffer:\n{self.input_buffer}"
  	)

• Therefore, when there are duplicate items in self.depends_on , the later one overwrites the previous one
• The later one cannot get anything, because the fetch is done in this way:

  strax/strax/plugins/plugin.py

  Lines 467 to 468 in 2218e78

  	inputs[d], self.input_buffer[d] = self.input_buffer[d].split(
  	t=this_chunk_end, allow_early_split=True

  As the data is already read and split by the previous fetch, the later one can only get an empty result
• So finally both ones are empty

Solution

The fix is simple, which is to remove the duplicate items in self.depends_on of the Plugin class.

[coveralls]

coverage: 90.437% (+0.007%) from 90.43%
when pulling 94248d4 on fix_zero_length_repeated
into 529886d on master.

[yuema137]

@dachengx In order to pass the pre-commit check I had to format it with black & flake. I think it happens now and then when we make PR for the files that were not checked previously. Should we do a through cleaning for strax?

[dachengx]

This PR was initially aimed to solve #846, but replaced by #860.

But this PR reminds us that we need to make sure that the depends_on is unique. I will change this PR to a safe guard of Plugin.