snakemake temp() causing unnecessary rerun of rules - snakemake

Question

I'm using snakemake v 5.4.0, and I'm running into a problem with temp(). In a hypothetical scenario:
Rule A --> Rule B1 --> Rule C1
|
--> Rule B2 --> Rule C2
where Rule A generates temp() files used by both pathways 1 (B1 + C1) and 2 (B2 + C2).
If I run the pipeline, the temp() files generated by RuleA are deleted by after they are used in both pathways, which is what I expect. However, if I then want to re-run Pathway 2, the temp() files for RuleA must be recreated which triggers the re-run of the entire pipeline, not just Pathway2. This becomes very computationally expensive for long pipelines. Is there a good way to prevent this besides not using temp(), which in my case would require many TB of extra hard drive space?

Answer 1

You could create the list of input files to rule all, or whatever the first rule is called, dynamically depending on whether the output of Pathway 2 already exists (and satisfies some sanity checks).
output= ['P1.out']
if not os.path.exists('P2.out'): # Some more conditions here...
output.append('P2.out')
rule all:
input:
output
rule make_tmp:
output:
temp('a.out')
shell:
r"""
touch {output}
"""
rule make_P1:
input:
'a.out'
output:
'P1.out'
shell:
r"""
touch {output}
"""
rule make_P2:
input:
'a.out'
output:
'P2.out'
shell:
r"""
touch {output}
"""
However, this somewhat defeats the point of using snakemake. If the input of Pathway 1 has to be recreated, how can you be sure that its output is still up-to-date?