Re: [patch] Remove superfluous /dev/null on grep line

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Apr 06, 2016 at 09:50:48AM +0100, Jonathan Wakely wrote:
> On 06/04/16 09:39 +0200, Eric Botcazou wrote:
> >we recently ran into build failures on Windows systems using a somewhat old
> >grep, coming from a syntax error in the libstdc++-symbols.ver version file:
> >
> ># Symbol versioning for shared libraries.
> >if ENABLE_SYMVERS
> >libstdc++-symbols.ver:  ${glibcxx_srcdir}/$(SYMVER_FILE) \
> >		$(port_specific_symbol_files)
> >	cp ${glibcxx_srcdir}/$(SYMVER_FILE) $@.tmp
> >	chmod +w $@.tmp
> >	if test "x$(port_specific_symbol_files)" != x; then \
> >	  if grep '^# Appended to version file.' \
> >	       $(port_specific_symbol_files) /dev/null > /dev/null 2>&1; then
> >\
> >	    cat $(port_specific_symbol_files) >> $@.tmp; \
> >	  else \
> >	    sed -n '1,/DO NOT DELETE/p' $@.tmp > tmp.top; \
> >	    sed -n '/DO NOT DELETE/,$$p' $@.tmp > tmp.bottom; \
> >	    cat tmp.top $(port_specific_symbol_files) tmp.bottom > $@.tmp; \
> >	    rm tmp.top tmp.bottom; \
> >	  fi; \
> >	fi
> >
> >Note the double /dev/null on the grep command line.  The first one causes the
> >grep to fail when the command is invoked on these systems.  That's old code,
> >but it is now invoked for config/abi/pre/float128.ver on the mainline and 5
> >branch and this breaks the build on these systems (4.9 builds fine).
> >
> >This first /dev/null doesn't serve any useful purpose and seems to be a typo,
> 
> Doesn't it mean that if $port_specific_symbol_files contains only
> whitespace we don't hang waiting for input from stdin? The 'if' above
> it will be true when "x$port_specific_symbol_files" = "x " or similar.
> 
> I don't see any way for that to happen in the FSF tree, so it should
> be safe. I'm a bit concerned about making that change this late in
> stage 4 though. There isn't much time to find out if it breaks an
> obscure target.

As it is a make variable, can't make be used to test this?
So perhaps
	chmod +w $@.tmp
ifneq ($(port_specific_symbol_files),)
	  if grep '^# Appended to version file.' \
	       $(port_specific_symbol_files) /dev/null > /dev/null 2>&1; then \
	    cat $(port_specific_symbol_files) >> $@.tmp; \
	  else \
	    sed -n '1,/DO NOT DELETE/p' $@.tmp > tmp.top; \
	    sed -n '/DO NOT DELETE/,$$p' $@.tmp > tmp.bottom; \
	    cat tmp.top $(port_specific_symbol_files) tmp.bottom > $@.tmp; \
	    rm tmp.top tmp.bottom; \
	  fi;
endif
?  Though, I think the initial and trailing whitespace is removed during
expansion (or already parsing of the vars), so even the
test "x$(port_specific_symbol_files)" != x
check should work right.

	Jakub