Determine nonstandard cache location from within make()

[brendanf]

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Question

Is there a way to determine from inside target code what the location of the drake cache used in the current make invocation is?

library(drake)
cache2 <- new_cache(".cache2")
plan <- drake_plan(x = 1, y = readd(x))

make(plan, memory_strategy = "none", cache = cache2)
## target x
## target y
## fail y
## Error: Target `y` failed. Call `diagnose(y)` for details. Error message:
##  key 'x' ('objects') not found

Of course it can be passed explicitly from outside, but it would be nice to be able to specify "readd from whatever cache make is currently using".

[wlandau]

I would actually prefer to limit automatic cache detection. Searching for the default .drake/ folder from readd() is usually fine because it is clear what drake is doing, but as soon as people start creating their own caches, we need to worry about situations where people create multiple caches side by side, to say nothing of alternative storr backends like storr_dbi().

Even with the default .drake folder, it is dangerous to recurse upwards in make(). I regret this feature of make(), which is why I plan to remove it next time I have an excuse for a round of breaking changes, re #919. Related:

drake/R/make.R

Lines 570 to 582 in 8004f4c

	"Running make() in a subdirectory of your project. \n",
	"This could cause problems if your ",
	"file_in()/file_out()/knitr_in() files ",
	"are relative paths.\n",
	"Please either\n",
	" (1) run make() from your drake project root, or\n",
	" (2) create a cache in your working ",
	"directory with new_cache('path_name'), or\n",
	" (3) supply a cache of your own (e.g. make(cache = your_cache))\n",
	" whose folder name is not '.drake'.\n",
	" running make() from: ", wd, "\n",
	" drake project root: ", dir, "\n",
	" cache directory: ", config$cache$path,

[brendanf]

If I understand correctly, then the only solution when using make(memory_strategy = "none", cache = my_cache) is to explicitly code it into every readd in the plan?

library(drake)
plan <- drake_plan(x = 1, y = readd(x, cache = ignore(my_cache)))

my_cache <- new_cache(".my_cache")
make(plan, memory_strategy = "none", cache = my_cache)

[wlandau]

Or wrap readd() in your own function.

my_cache <- new_cache(".my_cache")
rdd <- function(...) readd(..., cache = my_cache)

NB the latest drake should now automatically ignore() caches.