bug#10852: VPATH builds cannot recover from missing parser header

[Akim Demaille]

I am having problems in Bison (current master) to recover
from a lost parse-gram.h, generated from parse-gram.y
with regular Automake (1.11.3) handling:

> AM_YFLAGS = -d -v --warnings=all,error --report=all
> 
> src_bison_SOURCES =                             \
>   ...
>   src/output.h                                  \
>   src/parse-gram.y                              \
>   src/print-xml.c                               \
>   ...


Makefile.in:

> src/parse-gram.h: src/parse-gram.c
>       @if test ! -f $@; then rm -f src/parse-gram.c; else :; fi
>       @if test ! -f $@; then $(MAKE) $(AM_MAKEFLAGS) src/parse-gram.c; else 
> :; fi

The problem is that src/parse-gram.c is in $(srcdir), so
its occurrences in the commands should be prefixed by
$(srcdir)/.  Actually, I don't understand why we don't
simply use $< (lemme guess: it's not portable for non
generic recipes, ISTR some horrors in this area that my
brains decided to quickly forget :).  Also, why two "if"?
In case some concurrent execution of Make would already have
provided $@ in the meanwhile?

The following patch extends a test which is aimed at checking
this, but does it in a non-vpath build :)

I have a question though.  I don't understand how come
Make realizes it must provide $@ in srcdir too.

The Makefile.in features:

> .y.c:
>       $(AM_V_YACC)$(am__skipyacc) $(SHELL) $(YLWRAP) $< y.tab.c $@ y.tab.h 
> $*.h y.output $*.output -- $(YACCCOMPILE)

and at runtime I have:

> /bin/sh ../../build-aux/ylwrap ../../src/parse-gram.y y.tab.c 
> ../../src/parse-gram.c y.tab.h ../../src/parse-gram.h y.output 
> ../../src/parse-gram.output -- ./tests/bison -y -d -v --warnings=all,error 
> --report=all 
> updating ../../src/parse-gram.h
> ../../src/parse-gram.output is unchanged

which is what is expected, but I don't understand why
it works: that $< is prefixed with $(srcdir), this I
understand, but why does it appear on address@hidden  I
have to specify $(srcdir)/ by hand on similar patterns,
why does it work here?

Thanks!

0001-tests-check-yacc-parser-header-recovery-in-vpath-bui.patch
Description: Binary data