FYI: fix race in parallel lisp builds

[Alexandre Duret-Lutz]

There is a minor race condition in the current lisp rules when
run in parallel:

ELCFILES = one.elc three.elc two.elc
all-am: ... elc-stamp $(ELCFILES)

elc-stamp: $(am__ELFILES)
        @echo 'WARNING: Warnings can be ignored. :-)'
        if test "$(EMACS)" != no; then \
          set x; \
          list='$(am__ELFILES)'; for p in $$list; do \
            if test -f "$$p"; then d=; else d="$(srcdir)/"; fi; \
            set x "$$@" "$$d$$p"; shift; \
          done; \
          shift; \
          EMACS="$(EMACS)" $(SHELL) $(elisp_comp) "$$@" || exit 1; \
        else : ; fi
        touch $@

.el.elc:
        @if test ! -f $@; then \
          rm -f elc-stamp; \
          $(MAKE) $(AM_MAKEFLAGS) elc-stamp; \
        else : ; fi

Because the ELCFILES do not depend on elc-stamp, they could be
built in parallel.  With very bad luck, one could imagine that
the entire elc-stamp rule is executed between the `test ! -f $@'
and `rm -f elc-stamp' statements of the .el.elc command.

The idea is to replace `.el.elc:' by `$(ELCFILES): elc-stamp'.
Removing that weird inference rule looks cleaner anyway.

I'm installing this on HEAD and branch-1-8.

2004-02-01  Alexandre Duret-Lutz  <address@hidden>

        * automake.in (handle_emacs_lisp): Define $(ELCFILES) as
        $(am__ELCFILES), and always push it on @all.  Do not mention
        elc-stamp.
        * lib/am/lisp.am (.el.elc): Rewrite as ...
        ($(am__ELCFILES)): ... this, and depend on elc-stamp.
        (elc-stamp): Make sure elc-stamp is older that all .elc files, as
        explained in the manual entry below.

Index: NEWS
===================================================================
RCS file: /cvs/automake/automake/NEWS,v
retrieving revision 1.256.2.24
diff -u -r1.256.2.24 NEWS
--- NEWS        1 Feb 2004 09:31:07 -0000       1.256.2.24
+++ NEWS        1 Feb 2004 12:13:18 -0000
@@ -17,6 +17,9 @@
   - Do not AC_SUBST(LIBOBJS) in AM_WITH_REGEX.  This macro was unusable
     since Autoconf 2.54, which defines LIBOBJS itself.
 
+  - Fix a potential (but unlikely) race condition in parallel elisp
+    builds.  (Introduced in 1.7.3.)
+
 * New sections in manual:
 
   - Third-Party Makefiles: how to interface third party Makefiles.
Index: automake.in
===================================================================
RCS file: /cvs/automake/automake/automake.in,v
retrieving revision 1.1526.2.6
diff -u -r1.1526.2.6 automake.in
--- automake.in 10 Jan 2004 13:41:52 -0000      1.1526.2.6
+++ automake.in 1 Feb 2004 12:13:22 -0000
@@ -4257,24 +4257,14 @@
 
   return if ! @elfiles;
 
-  # Generate .elc files.
-  my @elcfiles = map { $_->[1] . 'c' } @elfiles;
-
-  define_pretty_variable ('ELCFILES', TRUE, INTERNAL, @elcfiles);
   define_pretty_variable ('am__ELFILES', TRUE, INTERNAL,
                          map { $_->[1] } @elfiles);
+  define_pretty_variable ('am__ELCFILES', TRUE, INTERNAL,
+                         '$(am__ELFILES:.el=.elc)');
+  # This one can be overridden by users.
+  define_pretty_variable ('ELCFILES', TRUE, INTERNAL, '$(am__ELCFILES)');
 
-  # Do not depend on the build rules if ELCFILES is empty.
-  # This is necessary because overriding ELCFILES= is a documented
-  # idiom to disable byte-compilation.
-  if (variable_value ('ELCFILES'))
-    {
-      # It's important that all depends on elc-stamp so that
-      # all .elc files get recompiled whenever a .el changes.
-      # It's important that all depends on $(ELCFILES) so that
-      # we can recover if any of them is deleted.
-      push (@all, 'elc-stamp', '$(ELCFILES)');
-    }
+  push @all, '$(ELCFILES)';
 
   require_variables ($elfiles[0][0], "Emacs Lisp sources seen", TRUE,
                     'EMACS', 'lispdir');
Index: lib/am/lisp.am
===================================================================
RCS file: /cvs/automake/automake/lib/am/lisp.am,v
retrieving revision 1.39.2.3
diff -u -r1.39.2.3 lisp.am
--- lib/am/lisp.am      28 Jan 2004 20:50:59 -0000      1.39.2.3
+++ lib/am/lisp.am      1 Feb 2004 12:13:24 -0000
@@ -24,6 +24,7 @@
 
 elc-stamp: $(am__ELFILES)
        @echo 'WARNING: Warnings can be ignored. :-)'
+       @rm -f elc-temp && touch elc-temp
        if test "$(EMACS)" != no; then \
 ## Make sure "$@" isn't empty initially.
          set x; \
@@ -37,14 +38,18 @@
          shift; \
          EMACS="$(EMACS)" $(SHELL) $(elisp_comp) "$$@" || exit 1; \
        else : ; fi
-       touch $@
+       @mv -f elc-temp $@
 
-.el.elc:
+## Do not use $(ELCFILES) as target, because it may have been emptied
+## by the user (to disable byte-compilation), and POSIX does not allow
+## an empty target.
+$(am__ELCFILES): elc-stamp
 ## Recover from the removal of $@
        @if test ! -f $@; then \
          rm -f elc-stamp; \
          $(MAKE) $(AM_MAKEFLAGS) elc-stamp; \
        else : ; fi
+
 
 ## ------------ ##
 ## Installing.  ##
--
Alexandre Duret-Lutz