autom4te: always update the output file, even if it hasn’t changed

Automake generates a Makefile rule for regenerating the configure
script, that relies on an invocation of ‘autoconf’ always bumping the
timestamp on the configure script, even if it hasn’t changed.
The patch to make autom4te update the output file atomically
(1725c947144d9bebfe7817c2c5f0d53d884b1297) broke this.

Fixes several failures in automake’s test suite.

 * bin/autom4te.in (handle_output): Always call update_file with force=1.
 * tests/tools.at (autoconf: timestamp changes): New test.

2 files changed, 49 insertions, 1 deletions

diff --git a/bin/autom4te.in b/bin/autom4te.in
index 4c2b905c..64653a44 100644
--- a/bin/autom4te.in
+++ b/bin/autom4te.in
@@ -609,7 +609,9 @@ sub handle_output ($$)
     }
 
   $out->close();
-  update_file ($scratchfile, $output, $force)
+  # Always update the file, even if it didn't change;
+  # Automake relies on this.
+  update_file ($scratchfile, $output, 1)
     if defined $scratchfile;
 
   # If no forbidden words, we're done.
diff --git a/tests/tools.at b/tests/tools.at
index 49710202..89b8b63b 100644
--- a/tests/tools.at
+++ b/tests/tools.at
@@ -662,6 +662,52 @@ arbitrary $text
 AT_CLEANUP
 
 
+# autoconf: timestamp changes
+# ---------------------------
+# Automake needs autoconf to update the timestamp on the configure script
+# whenever configure.ac or aclocal.m4 changes, even if the contents of the
+# configure script have not changed.
+AT_SETUP([autoconf: timestamp changes])
+
+AT_DATA([aclocal.m4],
+[[AC_DEFUN([local_KILROY], [# kilroy was here
+])
+]])
+
+AT_DATA_AUTOCONF([configure.ac],
+[[AC_INIT
+AC_PROG_CC
+local_KILROY
+AC_OUTPUT
+]])
+
+AT_CHECK_AUTOCONF
+
+cp configure configure.1
+sleep 1
+printf '%s\n' 'AC_LIBSOURCES([foo.c])dn@&t@l' >> configure.ac
+
+# This step must not use --force.
+# We don't need to check shell syntax, because if all goes well,
+# the script will be unchanged from what it was in the first autoconf
+# pass, and that was already checked.
+AT_CHECK_M4([autoconf])
+AT_CHECK([cmp configure configure.1])
+AT_CHECK([test configure -nt configure.1])
+
+cp configure configure.2
+sleep 1
+printf '%s\n' \
+  'AC_DEFUN([unused_MACRO], [# bob was there too' \
+  '])' >> aclocal.m4
+
+# Same as above.
+AT_CHECK_M4([autoconf])
+AT_CHECK([cmp configure configure.2])
+AT_CHECK([test configure -nt configure.2])
+
+AT_CLEANUP
+
 
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