Document new indentation policy.

* HACKING: Document indentation policy.
* .x-sc_prohibit_tab_based_indentation: New file.
* cfg.mk (sc_prohibit_tab_based_indentation): New rule.
* Makefile.am (syntax_check_exceptions): Ship exception.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit a8cef71593e876b027733fdb0137ed1655cae584)

5 files changed, 48 insertions, 4 deletions

diff --git a/.x-sc_prohibit_tab_based_indentation b/.x-sc_prohibit_tab_based_indentation
new file mode 100644
index 00000000..bd9127ed
--- /dev/null
+++ b/.x-sc_prohibit_tab_based_indentation
@@ -0,0 +1,5 @@
+^GNUMakefile$
+Makefile\.am$
+\.mk$
+^HACKING$
+ChangeLog.*
diff --git a/ChangeLog b/ChangeLog
index f9fc18e8..6c7c9b68 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2010-01-05  Eric Blake  <ebb9@byu.net>
 
+	Document new indentation policy.
+	* HACKING: Document indentation policy.
+	* .x-sc_prohibit_tab_based_indentation: New file.
+	* cfg.mk (sc_prohibit_tab_based_indentation): New rule.
+	* Makefile.am (syntax_check_exceptions): Ship exception.
+
 	Switch to indentation by space, not tab.
 	* .gitmodules: Convert leading tabs to spaces.
 	* TODO: Likewise.
diff --git a/HACKING b/HACKING
index 3b9c0458..bf23427a 100644
--- a/HACKING
+++ b/HACKING
@@ -217,8 +217,34 @@ yyyy-mm-dd  Name of Author  <email@address>  (tiny change)
 * See the GNU Coding Standards document for more details on ChangeLog
   formatting.
 
+6. Formatting
+=============
+
+* Use space-only indentation in nearly all files (Makefile and
+  ChangeLogs being the exception).
+
+  If you use Emacs and your m4 working directory name matches,
+  this code in your ~/.emacs enables the right mode:
+
+  ;; In m4, indent with spaces everywhere (not TABs).
+  ;; Exceptions: Makefile and ChangeLog modes.
+  (add-hook 'find-file-hook '(lambda ()
+    (if (and buffer-file-name
+             (string-match "/m4\\>" (buffer-file-name))
+             (not (string-equal mode-name "Change Log"))
+             (not (string-equal mode-name "Makefile")))
+        (setq indent-tabs-mode nil))))
+
+* Since the source code was massively converted from tabs in December
+  2009, you may find it helpful to use 'git diff -w' and 'git blame -w'
+  helpful for overlooking the whitespace changes.
+
+* Avoid #ifdefs inside function bodies, whenever possible.  If you
+  encounter a portability issue, it is better to propose a gnulib module
+  that works around it, and have m4 use that module.
+
 
-6. Release Procedure
+7. Release Procedure
 ====================
 
 * If you are an m4 maintainer, but have not yet registered your
@@ -230,8 +256,8 @@ yyyy-mm-dd  Name of Author  <email@address>  (tiny change)
         preferred email address.
 
     (b) an ASCII armored copy of your GnuPG key, as an attachment.
-	("gpg --export -a YOUR_KEY_ID > mykey.asc" should give you
-	this.)
+        ("gpg --export -a YOUR_KEY_ID > mykey.asc" should give you
+        this.)
 
   When you have received acknowledgement of your message, the proper GPG
   keys will be registered on ftp-upload.gnu.org and only then will you be
@@ -312,7 +338,7 @@ yyyy-mm-dd  Name of Author  <email@address>  (tiny change)
   content that needs to be updated.
 
 -- 
-Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Free Software
+Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software
 Foundation, Inc.
 
 The canonical source of this file is maintained with the
diff --git a/Makefile.am b/Makefile.am
index f073c1b5..89eb7f03 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -53,6 +53,7 @@ include ltdl/Makefile.inc
 # Additional configuration.  Version management comes from suggestions
 # given in build-aux/git-version-gen.
 syntax_check_exceptions = \
+	.x-sc_prohibit_tab_based_indentation \
 	.x-sc_require_config_h \
 	.x-sc_require_config_h_first \
 	.x-update-copyright
diff --git a/cfg.mk b/cfg.mk
index 336e8b31..dd710f83 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -28,3 +28,9 @@ old_NEWS_hash = b63892a79436f9f3cd05e10c3c4657ef
 update-copyright-env = \
   UPDATE_COPYRIGHT_USE_INTERVALS=0 \
   UPDATE_COPYRIGHT_MAX_LINE_LENGTH=72
+
+# Indent only with spaces.
+sc_prohibit_tab_based_indentation:
+ @re='^ *    '                                               \
+ msg='TAB in indentation; use only spaces'                   \
+   $(_prohibit_regexp)