Restart execution enhancement

[anthonysena]

Scenario: you execute a Strategus analysis specification (with several modules) and find there is a problem with a module that causes it to fail. Ideally, we'd like to update the module reference and pass the same analysisSpecifications to the execute function with restart = TRUE and have the targets script only run the updated targets.

Currently, when Strategus' execute function is invoked, we write out the analysisSpecifications parameter to the file system as shown here:

Strategus/R/Execution.R

Lines 92 to 102 in a12dee1

	fileName <- file.path(executionScriptFolder, "script.R")
	if (restart) {
	return(fileName)
	}
	# Store settings objects in the temp folder so they are available in targets
	analysisSpecificationsFileName <- gsub("\\\\", "/", file.path(executionScriptFolder, "analysisSpecifications.rds"))
	saveRDS(analysisSpecifications, analysisSpecificationsFileName)
	executionSettingsFileName <- gsub("\\\\", "/", file.path(executionScriptFolder, "executionSettings.rds"))
	saveRDS(executionSettings, executionSettingsFileName)
	keyringSettingsFileName <- gsub("\\\\", "/", file.path(executionScriptFolder, "keyringSettings.rds"))
	saveRDS(list(keyringName = keyringName), keyringSettingsFileName)

The challenge here (I believe) is that in the internal targets folder where targets tracks dependencies, the analysisSpecifications object it manages internally remains unchanged and so targets continues to use the older specification in place of the newer one. It may be possible to restructure the way in which we create the targets so that we have an analysisSpecifications per module such that if a module's specification is updated, targets can restart and only run updated targets.

[anthonysena]

This feature of the targets package may be useful here when restarting: https://docs.ropensci.org/targets/reference/tar_cue.html

[anthonysena]

Closing this as this scenario is better handled by an approach like the one described in #128 and would add more complexity to the existing v0.x line.