Re: No need for object file removal before compilation

[Ralf Wildenhues]

OK, this issue is much more intricate than it looked like at first.

First, the existing code had some bugs:
- it removed files too often.  After the first compile, we *know* that
  there are no files to stumble over; if the compiler doesn't know -c
  -o, then we have moved away $output_obj ourselves.

- if the compiler doesn't grok '-c -o', it removed files when it may not
  remove them: the lockfile and the $output_obj may only be removed with
  a trap once we've obtained the lock.  Further, if $obj = $output_obj,
  then of course the same holds for $obj.  Yuck.
  OTOH, once we've obtained the lock, we should install that trap ASAP,
  so that we don't leave a lockfile when interrupted.

Then, while I have some doubts about the practical applicability of
Bob's scenario, I left the code so that it would be supported: all
files we cause to be created are unlinked beforehand, except for the
eventual lock file of course.

I found another very tiny race:  At the very end of func_mode_compile,
we remove the eventual lockfile before exiting.  If the user happens to
interrupt libtool after that, but before exiting, our trap will remove,
among others, $output_obj.  But that file may already be used by another
libtool instance, it could've grabbed the lock already.  Fixed by
updating $removlist before removing the lockfile.

> I would have been inclined to ok this one.

OK for the updated one?  It makes for 5% faster
  libtool --mode=compile gcc -c a.c

Thanks,
Ralf

2008-04-12  Ralf Wildenhues  <address@hidden>

        * libltdl/config/ltmain.m4sh (func_mode_compile): Avoid
        redundant removal of old output files before compilation.
        Do not remove $obj until we have successfully grabbed the
        lockfile (in case the compiler doesn't grok `-c -o'), because
        it might be identical to $output_obj.
        At the end of the function, before we remove the lockfile,
        update $removelist so that if the trap hits after the lockfile
        has been removed, we do not accidentally remove $output_obj that
        does not belong to us.
        (func_write_libtool_object): Use $MV instead of mv.

Index: libltdl/config/ltmain.m4sh
===================================================================
RCS file: /cvsroot/libtool/libtool/libltdl/config/ltmain.m4sh,v
retrieving revision 1.106
diff -u -r1.106 ltmain.m4sh
--- libltdl/config/ltmain.m4sh  11 Apr 2008 17:21:55 -0000      1.106
+++ libltdl/config/ltmain.m4sh  12 Apr 2008 06:55:46 -0000
@@ -650,7 +650,7 @@
 non_pic_object=$write_oldobj
 
 EOF
-      mv -f "${write_libobj}T" "${write_libobj}"
+      $MV "${write_libobj}T" "${write_libobj}"
     }
 }
 
@@ -831,9 +831,6 @@
       removelist="$lobj $libobj ${libobj}T"
     fi
 
-    $opt_dry_run || $RM $removelist
-    trap "$opt_dry_run || $RM $removelist; exit $EXIT_FAILURE" 1 2 15
-
     # On Cygwin there's no "real" PIC flag so we must build both object types
     case $host_os in
     cygwin* | mingw* | pw32* | os2*)
@@ -850,8 +847,6 @@
     if test "$compiler_c_o" = no; then
       output_obj=`$ECHO "X$srcfile" | $Xsed -e 's%^.*/%%' -e 
's%\.[^.]*$%%'`.${objext}
       lockfile="$output_obj.lock"
-      removelist="$removelist $output_obj $lockfile"
-      trap "$opt_dry_run || $RM $removelist; exit $EXIT_FAILURE" 1 2 15
     else
       output_obj=
       need_locks=no
@@ -881,17 +876,20 @@
        $opt_dry_run || $RM $removelist
        exit $EXIT_FAILURE
       fi
+      removelist="$removelist $output_obj"
       $ECHO "$srcfile" > "$lockfile"
     fi
 
+    $opt_dry_run || $RM $removelist
+    removelist="$removelist $lockfile"
+    trap '$opt_dry_run || $RM $removelist; exit $EXIT_FAILURE' 1 2 15
+
     if test -n "$fix_srcfile_path"; then
       eval srcfile=\"$fix_srcfile_path\"
     fi
     func_quote_for_eval "$srcfile"
     qsrcfile=$func_quote_for_eval_result
 
-    $opt_dry_run || $RM "$libobj" "${libobj}T"
-
     # Only build a PIC object if we are building libtool libraries.
     if test "$build_libtool_libs" = yes; then
       # Without this assignment, base_compile gets emptied.
@@ -911,8 +909,6 @@
        command="$command -o $lobj"
       fi
 
-      $opt_dry_run || $RM "$lobj" "$output_obj"
-
       func_show_eval_locale "$command" \
           'test -n "$output_obj" && $RM $removelist; exit $EXIT_FAILURE'
 
@@ -962,7 +958,6 @@
 
       # Suppress compiler output if we already did a PIC compilation.
       command="$command$suppress_output"
-      $opt_dry_run || $RM "$obj" "$output_obj"
       func_show_eval_locale "$command" \
         '$opt_dry_run || $RM $removelist; exit $EXIT_FAILURE'
 
@@ -998,6 +993,7 @@
 
       # Unlock the critical section if it was locked
       if test "$need_locks" != no; then
+       removelist=$lockfile
         $RM "$lockfile"
       fi
     }