build: add check to enforce max allowed terraform version

[jbw976]

Description of your changes

This PR adds a new check to the Makefile that enforces the TERRAFORM_VERSION cannot be set to something higher than 1.5.x. Starting with v1.6, Terraform started being licensed under the BSL, which is not a permitted license for the Crossplane project.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

I have run various make commands, such as make reviewable and make, with various test values and observed the behavior in the table below. Versions below 1.6 are built successfully and versions greater than or equal to 1.6 will fail and stop the build.

TERRAFORM_VERSION	command	result
1.5.7	make	OK
1.5.7	make reviewable	OK
1.6.0	make	Makefile:111: *** invalid TERRAFORM_VERSION 1.6.0, must be less than 1.6. Stop.
1.8.4	make reviewable	Makefile:111: *** invalid TERRAFORM_VERSION 1.8.4, must be less than 1.6. Stop.

[ulucinar]

Thank you @jbw976, lgtm. Left some further points for us to discuss but I'm completely fine merging this as is.

[ulucinar]

nit:
Not critical as these are already expected to be version strings but to prevent globbing & word splitting, we may consider using double quotes:

Suggested change

	TERRAFORM_VERSION_VALID := $(shell [ $(TERRAFORM_VERSION) = `echo "$(TERRAFORM_VERSION)\n$(TERRAFORM_VERSION_CEILING)" | sort -V | head -n1` ] && echo 1 || echo 0)
	TERRAFORM_VERSION_VALID := $(shell [ "$(TERRAFORM_VERSION)" = "`echo "$(TERRAFORM_VERSION)\n$(TERRAFORM_VERSION_CEILING)" | sort -V | head -n1`" ] && echo 1 || echo 0)

[ulucinar]

nit: I think it makes sense not to allow overriding the ceiling from the environment:

Suggested change

	export TERRAFORM_VERSION_CEILING ?= 1.6
	export TERRAFORM_VERSION_CEILING := 1.6

At least, this will further prevent something like TERRAFORM_VERSION_CEILING=1.7 TERRAFORM_VERSION=1.6.0 make build.

If you like this idea and consider this as a valid concern, I will even suggest not defining the ceiling in a variable and instead hardcoding the string literal 1.6 below as we will have also covered make command-line variables, i.e., the case make TERRAFORM_VERSION_CEILING=1.7 build.

[jbw976]

great idea @ulucinar - I removed this from being a variable at all and have hardcoded this value into the makefile.

[ulucinar]

nit: Should we mention about the licensing concerns here?

Suggested change

	$(error invalid TERRAFORM_VERSION $(TERRAFORM_VERSION), must be less than $(TERRAFORM_VERSION_CEILING))
	$(error invalid TERRAFORM_VERSION $(TERRAFORM_VERSION), must be less than $(TERRAFORM_VERSION_CEILING) as versions starting with 1.6.0 use BSL)

[jbw976]

I have expanded the message here to make it visibly clear why this version check exists when this error is shown.

[jbw976]

also a bit worried about the portability of using control chars (\n) with echo - that isn't allowed as per https://www.shellcheck.net/

^-- [SC2028](https://www.shellcheck.net/wiki/SC2028) (info): echo may not expand escape sequences. Use printf.

Will look into using printf for this check instead 😇

[jbw976]

i have incorporated printf now instead of using echo and have tested the same scenarios are still working.

[jbw976]

Thanks for the feedback @ulucinar! I have incorporated all of your feedback 🙇‍♂️

[jbw976]

great idea @ulucinar - I removed this from being a variable at all and have hardcoded this value into the makefile.

[jbw976]

i have incorporated printf now instead of using echo and have tested the same scenarios are still working.

[jbw976]

I have expanded the message here to make it visibly clear why this version check exists when this error is shown.

[jbw976]

Let me know if there's anything else needed before merging @ulucinar @jeanduplessis 🎉