Subject: bug#7864: closed (Re: bug#7864: odd configure bug with dependency checking and conditional C++ support)

From: [email-masked] (GNU bug Tracking System)
Date: Wed, 19 Jan 2011 19:03:02 +0000

Your bug report

#7864: odd configure bug with dependency checking and conditional C++ support

which was filed against the automake package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
7864: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7864
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#7864: odd configure bug with dependency checking and conditional C++ support
From: Ralf Wildenhues <[email-masked]>
To: Eric Blake <[email-masked]>, [email-masked]
Date: Wed, 19 Jan 2011 20:10:05 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

Fixed by v1.11-293-gd652a1c.

Subject: odd configure bug with dependency checking and conditional C++ support
From: Eric Blake <[email-masked]>
To: [email-masked], bug-gnulib <[email-masked]>
Date: Tue, 18 Jan 2011 21:52:08 -0700
List-Id: <debbugs-submit.debbugs.gnu.org>

http://lists.gnu.org/archive/html/bug-m4/2010-12/msg00001.html
reported that on Haiku, builds of m4 fail with a strange error:

checking dependency style of :... mkdir: cannot create directory
`conftest.dir': File or Directory already exists
cp: accessing `conftest.dir': Bad data
./configure: line 28036: cd: conftest.dir: Not a directory
./configure: line 28047: ./depcomp: No such file or directory
none

I traced it to the following: M4 is using gnulib's ansi-c++-opt module
to enable support for optional C++ compilation support, and has this
line in configure.ac to disable C++ by default:

AC_DEFUN([gl_CXX_CHOICE_DEFAULT_NO])

Then, when gnulib finally gets around to invoking gl_PROG_ANSI_CXX, $CXX
is : (the default for not using C++), and the macro does this (rather
than directly calling AC_PROG_CXX which unconditionally requires a
working C++ compiler:

 dnl This macro invocation resolves an automake error:
  dnl /usr/local/share/automake-1.11/am/depend2.am: am__fastdepCXX does
not appear in AM_CONDITIONAL
  dnl /usr/local/share/automake-1.11/am/depend2.am:   The usual way to
define `am__fastdepCXX' is to add `AC_PROG_CXX'
  dnl /usr/local/share/automake-1.11/am/depend2.am:   to `configure.ac'
and run `aclocal' and `autoconf' again.
  _AM_DEPENDENCIES([CXX])

Yes, Bruno is exploiting undocumented automake internals.  Normally,
_AM_DEPENDENCIES is called so early in the build process that
conftest.dir does not exist, but in m4's case, there is a leftover
conftest.dir from a prior test because the C++ compiler isn't checked
until about halfway through configure.  Also, _AM_DEPENDENCIES normally
assumes that $CXX is a valid compiler (since AC_PROG_CXX aborts
configure if the required compiler is not available), but obviously,
with Bruno's attempt to conditionalize things, CXX of : will flat out
fail the dependency check.

The end result is that halfway through configure, automake's attempt to
check for C++ dependency mode even though C++ compilation was not
requested ends up changing the working directory, so that the remainder
of the configure script falls flat because all sorts of assumptions
about running directly in $builddir have been violated.

First - should autoconf's m4/depend.m4 be changed to use mkdir -p
instead of mkdir, when creating conftest.dir?  Should it be taught that
if CXX is :, that the dependency check should be skipped?  Also, should
it be made more robust to cd failure, so that it does not leave the rest
of the configure running in an unknown temporary sub-directory?

Second - when gnulib is avoiding the C++ compiler, why is it still
trying to determine the dependency mode?  I'm applying this patch to
gnulib to avoid the issue in the first place when CXX=: (but it still
doesn't solve the automake issue of dependency checking not being robust
to a pre-existing conftest.dir when C++ compilation is requested via
./configure --enable-cxx).

From 8a5019c9db43ed95ad763683761a0023bf536efb Mon Sep 17 00:00:00 2001
From: Eric Blake <[email-masked]>
Date: Tue, 18 Jan 2011 21:49:45 -0700
Subject: [PATCH] ansi-c++-opt: skip C++ dependency style if C++ is unused

* m4/ansi-c++.m4 (gl_PROG_ANSI_CXX): Avoid full-blown dependency
tests when we know C++ compilation is not desired.
Reported by Scott McCreary.

Signed-off-by: Eric Blake <[email-masked]>
---
 ChangeLog      |    7 +++++++
 m4/ansi-c++.m4 |   16 ++++++++++------
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 00f9ebe..263f1a0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-01-18  Eric Blake  <[email-masked]>
+
+	ansi-c++-opt: skip C++ dependency style if C++ is unused
+	* m4/ansi-c++.m4 (gl_PROG_ANSI_CXX): Avoid full-blown dependency
+	tests when we know C++ compilation is not desired.
+	Reported by Scott McCreary.
+
 2011-01-18  Bruno Haible  <[email-masked]>

 	*printf-posix: Avoid test failures. Make tests work on MacOS X, Cygwin.
diff --git a/m4/ansi-c++.m4 b/m4/ansi-c++.m4
index b25cf56..d6352ee 100644
--- a/m4/ansi-c++.m4
+++ b/m4/ansi-c++.m4
@@ -1,4 +1,4 @@
-# ansi-c++.m4 serial 6
+# ansi-c++.m4 serial 7
 dnl Copyright (C) 2002-2003, 2005, 2010-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -116,9 +116,13 @@ EOF

   AM_CONDITIONAL([$2], [test "$$1" != ":"])

-  dnl This macro invocation resolves an automake error:
-  dnl /usr/local/share/automake-1.11/am/depend2.am: am__fastdepCXX does
not appear in AM_CONDITIONAL
-  dnl /usr/local/share/automake-1.11/am/depend2.am:   The usual way to
define `am__fastdepCXX' is to add `AC_PROG_CXX'
-  dnl /usr/local/share/automake-1.11/am/depend2.am:   to `configure.ac'
and run `aclocal' and `autoconf' again.
-  _AM_DEPENDENCIES([CXX])
+  if test "$$1" != ":"; then
+    dnl This macro invocation resolves an automake error:
+    dnl /usr/local/share/automake-1.11/am/depend2.am: am__fastdepCXX
does not appear in AM_CONDITIONAL
+    dnl /usr/local/share/automake-1.11/am/depend2.am:   The usual way
to define `am__fastdepCXX' is to add `AC_PROG_CXX'
+    dnl /usr/local/share/automake-1.11/am/depend2.am:   to
`configure.ac' and run `aclocal' and `autoconf' again.
+    _AM_DEPENDENCIES([CXX])
+  else
+    AM_CONDITIONAL([am__fastdepCXX], [false])
+  fi
 ])
-- 
1.7.3.4

-- 
Eric Blake   [email-masked]    +1-801-349-2682
Libvirt virtualization library http://libvirt.org