Fix make race conditions reharding lang.h

The bug is due to a stupid make misfeature. The article [1] contains much more
information but in a nutshell, the following code:

a b: c
	bla

is equivalent to:

a: c
	bla
b: c
	bla

This is bad because in parallel runs (make -j typically), "bla" can be run
TWICE and even worse, twice in PARALLEL. Obviously the result will be
completely unexpected. This is a real bummer because on the other hand,
the following code:

%.c %.h: %:in
	bla

actually expresses the fact that bla produces two files. For some reasons,
pattern rules work differently from implicit rules.

This commit attempts to fix the problem with lang.h by rewriting (simplified):

lang.c lang.h: lang.in
	genlang

as

lang.h: lang.in
	genlang
lang.c: lang.h

This works (it correctly expresses the dependency chain and ensures genlang runs
once) but as one drawback: if one manually removes lang.c, then genlang will not
re-run since the second rule does nothing. This is minor drawback since no one
ever removes lang.c manually and "clean" removes lang.h which triggers a rebuild.

[1]: https://www.gnu.org/software/automake/manual/html_node/Multiple-Outputs.html

Change-Id: Ic0bf7c7c203dc599b00fde457946d2316c70474e

1 files changed, 9 insertions, 1 deletions

diff --git a/apps/lang/lang.make b/apps/lang/lang.make
index 6465b426b7..cee2456b5d 100644
--- a/apps/lang/lang.make
+++ b/apps/lang/lang.make
@@ -31,11 +31,19 @@ $(BUILDDIR)/lang/max_language_size.h: $(LANGOBJ) $(BUILDDIR)/apps/lang/voicestri
 $(BUILDDIR)/lang/lang_core.o: $(BUILDDIR)/lang/lang.h $(BUILDDIR)/lang/lang_core.c
         $(call PRINTS,CC lang_core.c)$(CC) $(CFLAGS) -c $(BUILDDIR)/lang/lang_core.c -o $@
 
-$(BUILDDIR)/lang/lang.h $(BUILDDIR)/lang/lang_core.c: $(APPSDIR)/lang/$(LANGUAGE).lang $(BUILDDIR)/apps/features
+# genlang creates *both* lang.c and lang.h but in Make there is no wat to express this rule
+# (multiple target rules DO NOT express that, they are a simple shortcut for multiple rules)
+# instead we pretend that genlang create lang_core.c and that lang.c depends from lang.h
+# it will work fine as long as one never manually removes lang.c and not lang.h, and it will avoid
+# race conditions such as running genlang twice or worse in parallel with other things!
+$(BUILDDIR)/lang/lang.h: $(APPSDIR)/lang/$(LANGUAGE).lang $(BUILDDIR)/apps/features
         $(call PRINTS,GEN lang.h)
         $(SILENT)for f in `cat $(BUILDDIR)/apps/features`; do feat="$$feat:$$f" ; done; \
                 perl -s $(TOOLSDIR)/genlang -p=$(BUILDDIR)/lang -t=$(MODELNAME)$$feat $<
+$(BUILDDIR)/lang/lang_core.c: $(BUILDDIR)/lang/lang.h
 
+# NOTE: for some weird reasons in GNU make, multi targets rules WITH patterns actually express
+# the fact that the two files are created as the result of one invocation of the rule
 $(BUILDDIR)/%.lng $(BUILDDIR)/%.vstrings: $(ROOTDIR)/%.lang $(BUILDDIR)/apps/genlang-features
         $(call PRINTS,GENLANG $(subst $(ROOTDIR)/,,$<))
         $(SILENT)mkdir -p $(dir $@)