[Automake-NG] [PATCH] [ng] utils: new function 'am.vars.is-undef'

[Stefano Lattarini]

Sometimes, in our makefiles, we want to assign a default value to a
variable that might not have been assigned yet.  One might think that
using the GNU make assignment '?=' is enough:

    VAR ?= DEFAULT-VALUE

But alas, such an usage allow environment overrides; i.e., if an
environment variable named 'VAR' is present there and defined to,
say, BAD-VALUE, the construct above will result in the $(VAR) make
variable being defined to BAD-VALUE, not to DEFAULT-VALUE.

To help overcoming this situation, we define a new make function
'am.vars.is-undef', that tells whether the given make variable is
not "safely" defined, i.e., either undefined, or defined to a value
that is inherited from the environment.

With such a tool, we can safely declare default values for variables
that avoids environmental interferences, as follow:

    ifeq ($(call am.vars.is-undef,VAR),yes)
      VAR = DEFAULT-VALUE
    endif

* lib/am/header-vars.am (am.vars.is-undef): New.
* t/internals.tap: Test it, and add few sanity checks.

Signed-off-by: Stefano Lattarini <address@hidden>
---
 lib/am/header-vars.am | 28 +++++++++++++++++++++++++++
 t/internals.tap       | 53 +++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/lib/am/header-vars.am b/lib/am/header-vars.am
index 472d0b2..545dea9 100644
--- a/lib/am/header-vars.am
+++ b/lib/am/header-vars.am
@@ -45,6 +45,34 @@ define am.error
 $(warning $1)$(eval am.error.seen := yes)
 endef
 
+#
+# Sometimes, in our makefiles, we want to assign a default value to a
+# variable that might not have been assigned yet.  One might think that
+# using the GNU make assignment '?=' is enough:
+#
+#    VAR ?= DEFAULT-VALUE
+#
+# But alas, such an usage allow interferences from the environment; i.e.,
+# if an environment variable named 'VAR' is present there and defined to,
+# say, BAD-VALUE, the construct above will result in the $(VAR) make
+# variable being defined to BAD-VALUE, not to DEFAULT-VALUE.
+#
+# To help overcoming this situation, we define a private make function
+# 'am.vars.is-undef', that tells whether the given make variable is not
+# "safely" defined, i.e., either undefined, or defined to a value that
+# is inherited from the environment.
+#
+# With such a tool, we can safely declare default values for variables
+# that avoids environmental interferences, as follow:
+#
+#     ifeq ($(call am.vars.is-undef,VAR),yes)
+#       VAR = DEFAULT-VALUE
+#     endif
+#
+define am.vars.is-undef
+$(if $(filter undefined environment,$(origin $(strip $1))),yes)
+endef
+
 # Some problematic characters (especially when used in arguments
 # to make functions, or for syntax highlighting).
 am.chars.bslash := \\
diff --git a/t/internals.tap b/t/internals.tap
index ac53396..3bb9642 100755
--- a/t/internals.tap
+++ b/t/internals.tap
@@ -19,7 +19,7 @@
 am_create_testdir=empty
 . ./defs || exit 1
 
-plan_ 10
+plan_ 12
 
 # Filter out Automake comments.
 grep -v '^##' "$am_amdir"/header-vars.am > defn.mk \
@@ -42,10 +42,18 @@ T ()
     echo 'digits = 0123456789'
   } > Makefile
   cat >> Makefile
-  command_ok_ "$*" $MAKE test
+  test_name=$1; shift
+  command_ok_ "$test_name" $MAKE ${1+"$@"} test
   cd ..
 }
 
+T 'sanity check' FOO=ok <<'END'
+FOO = ko
+test:
+       test $(FOO) = ok
+       touch sane
+END
+test -f T1.d/sane || fatal_ "function 'T' is buggy"
 
 T 'am.util.strip-first-word' <<'END'
 test:
@@ -174,6 +182,47 @@ test:
        test '$(call am.util.canon,x$(comma)@$(bslash))' = 'address@hidden'
 END
 
+FOO_ENVIRON=ko; export FOO_ENVIRON
+FOO_UNDEF=; unset FOO_UNDEF
+FOO_INDIR=; unset FOO_INDIR
+T 'am.vars.is-undef' FOO_CMDLINE=ok FOO_OVERRIDE=ko <<'END'
+
+FOO_MAKEFILE_1 = ok
+FOO_MAKEFILE_2 := ok
+override FOO_OVERRIDE_1 = ok
+override FOO_OVERRIDE_2 := ok
+FOO_EMPTY_1 =
+FOO_EMPTY_2 :=
+
+# Help avoiding typos.
+X = $(call am.vars.is-undef,$1)
+
+test:
+       @echo UNDEF:
+       test '$(call X,FOO_UNDEF)' = yes
+       @echo ENVIRON:
+       test '$(call X,FOO_ENVIRON)' = yes
+       @echo MAKEFILE 1:
+       test -z '$(call X,FOO_MAKEFILE_1)'
+       @echo MAKEFILE 2:
+       test -z '$(call X,FOO_MAKEFILE_2)'
+       @echo CMDLINE:
+       test -z '$(call X,FOO_CMDLINE)'
+       @echo OVERRIDE 1:
+       test -z '$(call X,FOO_OVERRIDE_1)'
+       @echo OVERRIDE 2:
+       test -z '$(call X,FOO_OVERRIDE_2)'
+       @echo EMPTY 1:
+       test -z '$(call X,FOO_EMPTY_1)'
+       @echo EMPTY 2:
+       test -z '$(call X,FOO_EMPTY_2)'
+       @echo DEFAULT:
+       test -z '$(call X,CC)'
+       @echo EXTRA SPACES:
+       test '$(call X,  FOO_UNDEF      )' = yes
+END
+unset FOO_ENVIRON
+
 T 'am.chars.newline (1)' <<'END'
 test:
        @echo OK > fo1$(am.chars.newline)@touch ba1$(am.chars.newline)-false > 
qu1
-- 
1.7.12.rc0