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Re: [PATCH] configure: warn if not using a separate build directory

From: Eric Blake
Subject: Re: [PATCH] configure: warn if not using a separate build directory
Date: Tue, 31 Mar 2020 09:44:37 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 3/31/20 5:37 AM, Daniel P. Berrangé wrote:
Running configure directly from the source directory is a build
configuration that will go away in future. It is also not currently
covered by any automated testing. Display a deprecation warning if
the user attempts to use an in-srcdir build setup, so that they are
aware that they're building QEMU in an undesirable manner.

Signed-off-by: Daniel P. Berrangé <address@hidden>
  configure | 30 ++++++++++++++++++++++++++++++
  1 file changed, 30 insertions(+)

It sounds like you already have a v2 coming up to address Phillipe's comments, but I'd be happy to see this concept make it into v5.0.

diff --git a/configure b/configure
index e225a1e3ff..1ab7492ab5 100755
--- a/configure
+++ b/configure
@@ -3,6 +3,19 @@
  # qemu configure script (c) 2003 Fabrice Bellard

Why fork out to 'pwd', when we can rely on $PWD for the same answer? Note that both $(pwd) and $PWD $BUILDDIR always give an absolute (but no necessarily canonical) path, which means...

+SRCDIR=$(dirname "$0")

...calling this ABS_BUILDDIR is a misnomer (it was already absolute). What it is really doing is chasing through symlinks to result in a canonical name, particularly since unless you have absolute names, using only string comparison to see if two filenames are equivalent will have false negatives (/tmp vs. /tmp/., for example).

+ABS_SRCDIR=$(realpath $SRCDIR)

Not robust if there are spaces in the directory names.  Safer would be
ABS_BUILDDIR=$(realpath -- "$BUILDDIR")
and similarly for ABS_SRCDIR.

+if [ "$ABS_SRCDIR" == "$ABS_BUILDDIR" ]

Bashism. You MUST spell this '=', not '==', since configure is run under /bin/sh which might be dash which does not understand ==.

+    in_srcdir=yes
  # Unset some variables known to interfere with behavior of common tools,
  # just as autoconf does.
@@ -6799,6 +6812,23 @@ if test "$supported_os" = "no"; then
      echo "us upstream at address@hidden."
+if test "$in_srcdir" = "yes"; then
+    echo
+    echo
+    echo "Support for running the 'configure' script directly from the"
+    echo "source directory is deprecated and will go away in a future"
+    echo "release. In source dir builds are not covered by automated"
+    echo "testing and are liable to break without warning. Users are"
+    echo "strongly recommended to switch to a separate build directory:"
+    echo
+    echo "  $ mkdir build"
+    echo "  $ cd build"
+    echo "  $ ../configure"
+    echo "  $ make"
+    echo

You know, it WOULD be possible to further enhance this to actually create 'build' as well as a shim GNUmakefile that would auto-forward on to building directly in build, so that you maintain the illusion of an in-place build (other than all the build artifacts now living in a different location), and muscle memory for in-tree 'make' still works; I've posted elsewhere and will repeat here the contents of my GNUmakefile:

# Hack for redirecting while reminding myself to use distinct builddir
recurse: all
%: force
        @echo 'changing directory to build...'
        @sleep 2
        @$(MAKE) -C build -f Makefile $(MAKECMDGOALS)
force: ;
.PHONY: force
GNUmakefile: ;

But we could do that as a patch on top of yours.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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