[PATCH 1/2] add warning about default --format changing

[Mike Frysinger]

Have the build & NEWS files warn that the --format value is changing.
This has been documented in the manual for almost 20 years, but not
everyone reads the manual.

* NEWS: Update
* README: Document DEFAULT_ARCHIVE_FORMAT will be changing.
* configure.ac: Add warning if DEFAULT_ARCHIVE_FORMAT is not set.
---
 NEWS         | 5 +++++
 README       | 6 +++---
 configure.ac | 5 +++++
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/NEWS b/NEWS
index 6ad1c377e4b0..84a6aea2ea76 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,11 @@
 GNU tar NEWS - User visible changes. 2021-02-13
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
+version 1.35
+
+* Add notice about the default format changing to POSIX.  This has been
+  documented in the manual for the last 19 years.
+
 version 1.34 - Sergey Poznyakoff, 2021-02-13
 
 * Fix extraction over pipe (savannah bug #60002)
diff --git a/README b/README
index b071c61cbcc7..86bec6c21f22 100644
--- a/README
+++ b/README
@@ -33,9 +33,9 @@ Besides those configure options documented in files 'INSTALL' 
and
 
 ** Selecting the default archive format.
 
-The default archive format is GNU, this can be overridden by
-presetting DEFAULT_ARCHIVE_FORMAT while configuring. The allowed
-values are GNU, V7, OLDGNU, USTAR and POSIX.
+The default archive format is GNU, but will be changing in the next release.
+It can be overridden by presetting DEFAULT_ARCHIVE_FORMAT while configuring.
+The allowed values are GNU, V7, OLDGNU, USTAR and POSIX.
 
 ** Selecting the default archive device
 
diff --git a/configure.ac b/configure.ac
index 0d31d1eeaf1a..dc45a7072207 100644
--- a/configure.ac
+++ b/configure.ac
@@ -283,8 +283,10 @@ AC_MSG_CHECKING(for default archive format)
 AC_ARG_VAR([DEFAULT_ARCHIVE_FORMAT],
            [Set the default archive format. Allowed values are: V7, OLDGNU, 
USTAR, POSIX, GNU. Default is GNU])
 
+configure_default_archive_format_selected=false
 if test -z "$DEFAULT_ARCHIVE_FORMAT"; then
   DEFAULT_ARCHIVE_FORMAT="GNU"
+  configure_default_archive_format_selected=true
 fi
 case $DEFAULT_ARCHIVE_FORMAT in
   V7|OLDGNU|USTAR|POSIX|GNU) ;;
@@ -293,6 +295,9 @@ esac
 AC_DEFINE_UNQUOTED(DEFAULT_ARCHIVE_FORMAT, ${DEFAULT_ARCHIVE_FORMAT}_FORMAT,
   [By default produce archives of this format])
 AC_MSG_RESULT($DEFAULT_ARCHIVE_FORMAT)
+if $configure_default_archive_format_selected; then
+  AC_MSG_WARN([The default format is changing to POSIX in a future release; 
see the README file for details])
+fi
 
 AC_MSG_CHECKING(for default archive)
 
-- 
2.33.0