configure: more nuanced time_t treatment.

I'm taking the position that on systems where time_t is 32
bits by default, but can be switched to 64 with some option
that we positively detect, configure will refuse to run unless
the user explicitly chooses what to do using either --big-time
or --no-big-time. We neither want to ignore this situation
(because Y2038 is a problem, and we don't want to contribute
to it) nor do we want to force 64 bit time_t, which could be
problematic in distributions where other applications and
components are not being configured that way for whatever
reason (like it being a system with a projected life span that
is not expected to go past Y2038).

* configure (big_time, big_time_given): New variables.
(help): Document big-time. New logic in the test to validate
the detected situation versus whether or not the big-time
variable has been specified, and which way. Error out in
several cases.

1 files changed, 47 insertions, 1 deletions

diff --git a/configure b/configure
index 353dd844..b140e72d 100755
--- a/configure
+++ b/configure
@@ -185,6 +185,8 @@ txr_dbg_opts=--gc-debug
 valgrind=
 extra_debugging=
 debug_support=y
+big_time=
+big_time_given=
 gen_gc=y
 small_mem=
 full_repl=y
@@ -533,6 +535,23 @@ extra-debugging [$extra_debugging]
   Use --extra_debugging to configure some additional debugging features,
   which incur a run-time penalty.
 
+big-time [$big_time]
+
+  Systems whose time_t type is 32 bits wide have a Y2038 problem.
+  On some of these systems, it is possible to specify some compiler option
+  to make time_t 64 bits wide. If this configure script detects this
+  situation: that time is 32 bits, but may be switched to 64, it
+  refuses to run. You must explicitly choose what will happen. Use
+  --big-time to choose the 64 bit time, or else --no-big-time.
+
+  If time_t is 64 bits by default then specifying --no-big-time is
+  erroneous. Downgrading from a 64 bit default is not supported by
+  this script, even if it is possible; if that is necessary, use external
+  options vial EXTRA_FLAGS or CFLAGS.
+
+  Specifying --big-time is erroneous if time_t is 32 bits, and there is no way
+  to enable 64.
+
 gen-gc [$gen_gc]
 
   Use --no-gen-gc to disable the generational garbage collector which
@@ -3457,9 +3476,36 @@ struct sizes {
     fi
   fi
 
-  [ $file_offset_define = none -o $time_bits_define = none ] || break
+  [ "$file_offset_define" = none -o "$time_bits_define" = none ] || break
 done
 
+case "$time_bits_define" in
+none )
+  if [ "$big_time" ] ; then
+    printf "\n$0: --big-time specified, yet no way to enable 64 bit time_t found\n"
+    exit 1
+  fi
+  ;;
+"" )
+  if [ $big_time_given ] && [ -z "$big_time" ] ; then
+    printf "\n$0: --no-big-time specified, yet time_t is 64 bits in this system\n"
+    exit 1
+  fi
+  ;;
+* )
+  if [ -n "$big_time_given" ] ; then
+    printf "\n$0: this system has a 32 bit time_t that can be overriden to 64\n"
+    printf "$0: specify --big-time to do this, or --no-big-time not to do it\n"
+    exit 1
+  else
+    if [ -z "$big_time" ]; then
+      printf " (%s disabled by --no-big-time)" "$time_bits_define"
+      time_bits_define=
+    fi
+  fi
+  ;;
+esac
+
 if [ "$file_offset_define" = none ] ; then
   printf " (no 64 bit off_t)"
 elif [ -n "$file_offset_define" ] ; then