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): New macro.
(EXTRA_DIST): Distribute exception files, and .prev-version.

Signed-off-by: Eric Blake <ebb9@byu.net>

5 files changed, 53 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 b709552e..ba963023 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2009-12-29  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): New macro.
+	(EXTRA_DIST): Distribute exception files, and .prev-version.
+
 	Switch to indentation by space, not tab.
 	* .gitmodules: Convert leading tabs to spaces.
 	* TODO: Likewise.
diff --git a/HACKING b/HACKING
index 8f5088f0..2d287879 100644
--- a/HACKING
+++ b/HACKING
@@ -188,8 +188,34 @@ 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
@@ -201,8 +227,8 @@ 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
diff --git a/Makefile.am b/Makefile.am
index 875750f3..1d505923 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,8 +21,13 @@
 ## Written by Gary V. Vaughan <gary@gnu.org>
 
 SUBDIRS = . examples lib src doc checks tests
+
+syntax_check_exceptions =		\
+  .x-sc_prohibit_tab_based_indentation	\
+  .x-update-copyright
+
 EXTRA_DIST = bootstrap c-boxes.el cfg.mk maint.mk \
-	.version m4/gnulib-cache.m4
+	.prev-version .version m4/gnulib-cache.m4 $(syntax_check_exceptions)
 DISTCLEANFILES = stamp-h
 ## maintainer-clean should remove as much as possible that ./bootstrap can
 ## recreate.  In the m4 directory, keep only gnulib-cache.m4.
diff --git a/cfg.mk b/cfg.mk
index 00a9e569..10fdcc52 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -33,3 +33,9 @@ config_h_header = "m4\.h"
 
 # Hash of NEWS contents, to ensure we don't add entries to wrong section.
 old_NEWS_hash = 0330971054cd4fb4e94b85fe367980f2
+
+# Indent only with spaces.
+sc_prohibit_tab_based_indentation:
+ @re='^ *    '                                               \
+ msg='TAB in indentation; use only spaces'                   \
+   $(_prohibit_regexp)