diff options
Diffstat (limited to 'unit-tests/var-class-cmdline.mk')
| -rw-r--r-- | unit-tests/var-class-cmdline.mk | 80 |
1 files changed, 76 insertions, 4 deletions
diff --git a/unit-tests/var-class-cmdline.mk b/unit-tests/var-class-cmdline.mk index c43b5351c329..679e051bb242 100644 --- a/unit-tests/var-class-cmdline.mk +++ b/unit-tests/var-class-cmdline.mk @@ -1,8 +1,80 @@ -# $NetBSD: var-class-cmdline.mk,v 1.2 2020/08/16 14:25:16 rillig Exp $ +# $NetBSD: var-class-cmdline.mk,v 1.5 2021/02/23 21:59:31 rillig Exp $ # # Tests for variables specified on the command line. +# +# Variables that are specified on the command line override those from the +# global scope. +# +# For performance reasons, the actual implementation is more complex than the +# above single-sentence rule, in order to avoid unnecessary lookups in scopes, +# which before var.c 1.586 from 2020-10-25 calculated the hash value of the +# variable name once for each lookup. Instead, when looking up the value of +# a variable, the search often starts in the global scope since that is where +# most of the variables are stored. This conflicts with the statement that +# variables from the cmdline scope override global variables, since after the +# common case of finding a variable in the global scope, another lookup would +# be needed in the cmdline scope to ensure that there is no overriding +# variable there. +# +# Instead of this costly lookup scheme, make implements it in a different +# way: +# +# Whenever a global variable is created, this creation is ignored if +# there is a cmdline variable of the same name. +# +# Whenever a cmdline variable is created, any global variable of the +# same name is deleted. +# +# Whenever a global variable is deleted, nothing special happens. +# +# Deleting a cmdline variable is not possible. +# +# These 4 rules provide the guarantee that whenever a global variable exists, +# there cannot be a cmdline variable of the same name. Therefore, after +# finding a variable in the global scope, no additional lookup is needed in +# the cmdline scope. +# +# The above ruleset provides the same guarantees as the simple rule "cmdline +# overrides global". Due to an implementation mistake, the actual behavior +# was not entirely equivalent to the simple rule though. The mistake was +# that when a cmdline variable with '$$' in its name was added, a global +# variable was deleted, but not with the exact same name as the cmdline +# variable. Instead, the name of the global variable was expanded one more +# time than the name of the cmdline variable. For variable names that didn't +# have a '$$' in their name, it was implemented correctly all the time. +# +# The bug was added in var.c 1.183 on 2013-07-16, when Var_Set called +# Var_Delete to delete the global variable. Just two months earlier, in var.c +# 1.174 from 2013-05-18, Var_Delete had started to expand the variable name. +# Together, these two changes made the variable name be expanded twice in a +# row. This bug was fixed in var.c 1.835 from 2021-02-22. +# +# Another bug was the wrong assumption that "deleting a cmdline variable is +# not possible". Deleting such a variable has been possible since var.c 1.204 +# from 2016-02-19, when the variable modifier ':@' started to delete the +# temporary loop variable after finishing the loop. It was probably not +# intended back then that a side effect of this seemingly simple change was +# that both global and cmdline variables could now be undefined at will as a +# side effect of evaluating a variable expression. As of 2021-02-23, this is +# still possible. +# +# Most cmdline variables are set at the very beginning, when parsing the +# command line arguments. Using the special target '.MAKEFLAGS', it is +# possible to set cmdline variables at any later time. + +# A normal global variable, without any cmdline variable nearby. +VAR= global +.info ${VAR} -# TODO: Implementation +# The global variable is "overridden" by simply deleting it and then +# installing the cmdline variable instead. Since there is no obvious way to +# undefine a cmdline variable, there is no need to remember the old value +# of the global variable could become visible again. +# +# See varmod-loop.mk for a non-obvious way to undefine a cmdline variable. +.MAKEFLAGS: VAR=makeflags +.info ${VAR} -all: - @:; +# If Var_SetWithFlags should ever forget to delete the global variable, +# the below line would print "global" instead of the current "makeflags". +.MAKEFLAGS: -V VAR |