Fixes unnaturally high test coverage by excluding test utilities (#406)

Before, our test coverage was accidentally 0.5% higher than it should be
due to inclusion of packages not in the binary (notably test utilities).

This fixes it in native make, which helps us maintain what is main or
not in one place. We use make-derived source lists in order to taint
targets properly.

Signed-off-by: Adrian Cole <[email protected]>

Makefile

@@ -5,6 +5,11 @@
 # Please see GNU make's documentation if unfamiliar: https://www.gnu.org/software/make/manual/html_node/
 .PHONY: test build e2e dist clean format lint check site
 
+# Make functions strip spaces and use commas to separate parameters. The below variables escape these characters.
+comma := ,
+space :=
+space +=
+
 # Include versions of tools we build on-demand
 include Tools.mk
 
@@ -77,9 +82,14 @@ test: ## Run all unit tests
 	@$(go) test . ./internal/...
 	@printf "$(ansi_format_bright)" test "ok"
 
+# coverage_packages is a lazy inclusion of source packages that comprise the binary.
+# To do this in make, we collect the unique main source directories (sort will dedupe).
+# Paths need to all start with ./, so we do that manually vs foreach which strips it.
+main_packages = $(sort $(foreach f,$(dir $(main_sources)),$(if $(findstring ./,$(f)),./,./$(f))))
+coverpkg      = $(subst $(space),$(comma),$(main_packages))
 coverage: ## Generate test coverage
 	@printf "$(ansi_format_dark)" coverage "running unit tests with coverage"
-	@$(go) test -coverprofile=coverage.txt -covermode=atomic --coverpkg=.,./internal/... . ./internal/...
+	@$(go) test -coverprofile=coverage.txt -covermode=atomic --coverpkg=$(coverpkg) $(main_packages)
 	@$(go) tool cover -func coverage.txt
 	@printf "$(ansi_format_bright)" coverage "ok"
 
@@ -99,6 +109,7 @@ windows_platforms     := windows_amd64
 # Make 3.81 doesn't support '**' globbing: Set explicitly instead of recursion.
 all_patterns := *.go */*.go */*/*.go */*/*/*.go */*/*/*.go */*/*/*/*.go
 all_sources  := $(wildcard $(all_patterns))
+# main_sources compose the binary, so exclude tests, test utilities and linters
 main_sources := $(wildcard $(subst *,*[!_test],$(all_patterns)))
 
 build/func-e_%/func-e: $(main_sources)