Merge pull request #11664 from cladmi/pr/make/variables_lazy_evaluation

makefiles/utils/variables: add functions to help managing variables

Makefile.include

@@ -80,6 +80,9 @@ MAKEOVERRIDES += $(foreach v,$(__DIRECTORY_VARIABLES),$(v)=$($(v)))
 # trailing '/' is important when RIOTPROJECT == CURDIR
 BUILDRELPATH ?= $(patsubst $(RIOTPROJECT)/%,%,$(CURDIR)/)
 
+# include makefiles utils tools
+include $(RIOTMAKE)/utils/variables.mk
+
 # get host operating system
 OS := $(shell uname)
 

makefiles/utils/test-variables.mk

@@ -0,0 +1,38 @@
+include variables.mk
+
+# Timestamp in nanoseconds (to be an integer)
+# OSx 'date' does not support 'date +%s%N' so rely on python instead
+# It could be OSx specific but we do not have 'OS' defined here to differentiate
+date_nanoseconds = $(shell python -c 'import time; print(int(time.time() * 1000000000))')
+
+EXPORTED_VARIABLES = MY_VARIABLE CURRENT_TIME
+MY_VARIABLE = my_variable
+# Defered evaluation to the test
+CURRENT_TIME = $(call date_nanoseconds)
+
+$(call target-export-variables,test-exported-variables,$(EXPORTED_VARIABLES))
+test-exported-variables:
+	$(Q)bash -c 'test "$(MY_VARIABLE)" = "$${MY_VARIABLE}" || { echo ERROR: "$(MY_VARIABLE)" != "$${MY_VARIABLE}"; exit 1; }'
+	$(Q)bash -c 'test $(PARSE_TIME) -lt $${CURRENT_TIME} || { echo ERROR: $(PARSE_TIME) \>= $${CURRENT_TIME} >&2; exit 1; }'
+
+
+MEMOIZED_CURRENT_TIME = $(call memoized,MEMOIZED_CURRENT_TIME,$(call date_nanoseconds))
+MEMOIZED_CURRENT_TIME_2 = $(call memoized,MEMOIZED_CURRENT_TIME_2,$(call date_nanoseconds))
+
+PRE_MEMOIZED_TIME := $(call date_nanoseconds)
+# Two separate evaluations
+REF_CURRENT_TIME_1 := $(MEMOIZED_CURRENT_TIME)
+# Strip to detect added whitespaces by function
+REF_CURRENT_TIME_2 := $(strip $(MEMOIZED_CURRENT_TIME))
+
+test-memoized-variables:
+	@# The value was only evaluated on first use
+	$(Q)test $(PRE_MEMOIZED_TIME) -lt $(REF_CURRENT_TIME_1) || { echo ERROR: $(PRE_MEMOIZED_TIME) \>= $(REF_CURRENT_TIME_1) >&2; exit 1; }
+	@# Both evaluation return the same time and without added whitespace
+	$(Q)test "$(REF_CURRENT_TIME_1)" = "$(REF_CURRENT_TIME_2)" || { echo ERROR: "$(REF_CURRENT_TIME_1)" != "$(REF_CURRENT_TIME_2)" >&2; exit 1; }
+	@# The second memoized value was only evaluated when calling the target
+	$(Q)test $(PARSE_TIME) -lt $(MEMOIZED_CURRENT_TIME_2) || { echo ERROR: $(PARSE_TIME) \>= $(MEMOIZED_CURRENT_TIME_2) >&2; exit 1; }
+
+
+# Immediate evaluation for comparing
+PARSE_TIME := $(call date_nanoseconds)

makefiles/utils/variables.mk

@@ -0,0 +1,33 @@
+# Utilities to set variables and environment for targets
+# These functions should help replacing immediate evaluation and global 'export'
+
+
+# Evaluate a deferred variable only once on its first usage
+# Uses after that will be as if it was an immediate evaluation
+# This can replace using `:=` by default
+#
+# The goal is to use it for `shell` commands
+#
+# variable = $(call memoized,<variable>,<value>)
+#
+# Parameters
+#  variable: name of the variable you set
+#  value: value that should be set when evaluated
+memoized = $2$(eval $1:=$2)
+
+
+# Target specific export the variables for that target
+#
+# target-export-variables <target> <variables>
+#
+# Parameters
+#   target: name of target
+#   variables: the variables to export
+#
+# The variable will only be evaluated when executing the target as when
+# doing export
+target-export-variables = $(foreach var,$(2),$(call _target-export-variable,$1,$(var)))
+
+# '$1: export $2' cannot be used alone
+# By using '?=' the variable is evaluated at runtime only
+_target-export-variable = $(eval $1: export $2?=)

tests/build_system_utils/Makefile

@@ -16,7 +16,9 @@ endef
 
 MAKEFILES_UTILS = $(RIOTMAKE)/utils
 
-COMPILE_TESTS = test-ensure_value test-ensure_value-negative
+COMPILE_TESTS += test-ensure_value test-ensure_value-negative
+COMPILE_TESTS += test-exported-variables
+COMPILE_TESTS += test-memoized-variables
 
 # Tests will be run both in the host machine and in `docker`
 all: build-system-utils-tests
@@ -31,3 +33,9 @@ test-ensure_value:
 
 test-ensure_value-negative:
 	$(Q)$(call command_should_fail,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-checks.mk test-ensure_value-negative)
+
+test-exported-variables:
+	$(Q)$(call command_should_succeed,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-variables.mk test-exported-variables)
+
+test-memoized-variables:
+	$(Q)$(call command_should_succeed,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-variables.mk test-memoized-variables)