Add a new `-weak' flag to tell libtool when not to propogate

dependency libraries from dlpreopened modules to libraries:

* config/ltmain.in: Support new -weak link mode option.  Adjust
help message.
* libltdl/Makefile.am (libltdlc_la_LDFLAGS): Use it.
* doc/libtool.texi (Linking with dlopened modules): Document it.
(Link mode): Mention -weak.
* NEWS: Updated.

ChangeLog

@@ -1,3 +1,15 @@
+2004-08-29  Gary V. Vaughan  <gary@gnu.org>
+
+	Add a new `-weak' flag to tell libtool when not to propogate
+	dependency libraries from dlpreopened modules to libraries:
+
+	* config/ltmain.in: Support new -weak link mode option.  Adjust
+	help message.
+	* libltdl/Makefile.am (libltdlc_la_LDFLAGS): Use it.
+	* doc/libtool.texi (Linking with dlopened modules): Document it.
+	(Link mode): Mention -weak.
+	* NEWS: Updated.
+
 2004-08-29  Gary V. Vaughan  <gary@gnu.org>
 
 	* config/ltmain.in (opt_help): Defer showing help messages until
@@ -65,6 +77,8 @@
 
 	* doc/libtool.texi (Cheap tricks): Escape the `@' for texinfo.
 
+2004-08-27  Gary V. Vaughan  <gary@gnu.org>
+
 	* doc/libtool.texi (Cheap tricks): Fix the instructions for making
 	a ltmain.in wrapping libtool in light of recent changes to the
 	version checking code.

NEWS

@@ -31,6 +31,8 @@ New in 1.5b: 2004-??-??; CVS version 1.5a, Libtool team:
 * If you configure libtool with --disable-shared (or if libtool does not
   support shared libraries on your platform) trying to build a library using
   `-shared' is a fatal error.
+* New link mode option `-weak' tells libtool when not to propogate dependency
+  libraries from dlpreopened modules.
 * libtoolize installs libtool.m4, (ltdl.m4 if used,) and various supporting
   m4 definitions to AC_CONFIG_MACRO_DIR.
 * Mode inferrence removed, shorthand for choosing modes added.

config/ltmain.in

@@ -517,6 +517,7 @@ The following components of LINK-COMMAND are treated specially:
   -static           do not do any dynamic linking of libtool libraries
   -version-info CURRENT[:REVISION[:AGE]]
 		    specify library version info [each variable defaults to 0]
+  -weak LIBNAME     declare that the target provides the LIBNAME interface
 
 All other options (arguments beginning with \`-') are ignored.
 
@@ -2343,6 +2344,7 @@ func_mode_link ()
     thread_safe=no
     vinfo=
     vinfo_number=no
+    weak_libs=
 
     func_infer_tag $base_compile
 
@@ -2457,21 +2459,24 @@ func_mode_link ()
 	  prev=
 	  continue
 	  ;;
+	framework)
+	  case $host in
+	    *-*-darwin*)
+	      case "$deplibs " in
+		*" $qarg.ltframework "*) ;;
+		*) deplibs="$deplibs $qarg.ltframework" # this is fixed later
+		   ;;
+	      esac
+	      ;;
+   	  esac
+	  prev=
+	  continue
+	  ;;
 	inst_prefix)
 	  inst_prefix_dir="$arg"
 	  prev=
 	  continue
 	  ;;
-	precious_regex)
-	  precious_files_regex="$arg"
-	  prev=
-	  continue
-	  ;;
-	release)
-	  release="-$arg"
-	  prev=
-	  continue
-	  ;;
 	objectlist)
 	  if test -f "$arg"; then
 	    save_arg=$arg
@@ -2576,6 +2581,16 @@ func_mode_link ()
 	  prev=
 	  continue
 	  ;;
+	precious_regex)
+	  precious_files_regex="$arg"
+	  prev=
+	  continue
+	  ;;
+	release)
+	  release="-$arg"
+	  prev=
+	  continue
+	  ;;
 	rpath | xrpath)
 	  # We need an absolute path.
 	  case $arg in
@@ -2598,6 +2613,24 @@ func_mode_link ()
 	  prev=
 	  continue
 	  ;;
+	shrext)
+  	  shrext_cmds="$arg"
+	  prev=
+	  continue
+	  ;;
+	weak)
+	  weak_libs="$weak_libs $arg"
+	  prev=
+	  continue
+	  ;;
+	xcclinker)
+	  linker_flags="$linker_flags $qarg"
+	  compiler_flags="$compiler_flags $qarg"
+	  prev=
+	  compile_command="$compile_command $qarg"
+	  finalize_command="$finalize_command $qarg"
+	  continue
+	  ;;
 	xcompiler)
 	  compiler_flags="$compiler_flags $qarg"
 	  prev=
@@ -2613,32 +2646,6 @@ func_mode_link ()
 	  finalize_command="$finalize_command $wl$qarg"
 	  continue
 	  ;;
-	xcclinker)
-	  linker_flags="$linker_flags $qarg"
-	  compiler_flags="$compiler_flags $qarg"
-	  prev=
-	  compile_command="$compile_command $qarg"
-	  finalize_command="$finalize_command $qarg"
-	  continue
-	  ;;
-	framework)
-	  case $host in
-	    *-*-darwin*)
-	      case "$deplibs " in
-		*" $qarg.ltframework "*) ;;
-		*) deplibs="$deplibs $qarg.ltframework" # this is fixed later
-		   ;;
-	      esac
-	      ;;
-   	  esac
-	  prev=
-	  continue
-	  ;;
-	shrext)
-  	  shrext_cmds="$arg"
-	  prev=
-	  continue
-	  ;;
 	*)
 	  eval "$prev=\"\$arg\""
 	  prev=
@@ -2692,6 +2699,11 @@ func_mode_link ()
 	continue
 	;;
 
+      -framework)
+	prev=framework
+	continue
+	;;
+
       -inst-prefix-dir)
 	prev=inst_prefix
 	continue
@@ -2804,11 +2816,6 @@ func_mode_link ()
 	continue
 	;;
 
-      -shrext)
-	prev=shrext
-	continue
-	;;
-
       -no-fast-install)
 	fast_install=no
 	continue
@@ -2881,6 +2888,11 @@ func_mode_link ()
 	continue
 	;;
 
+      -shrext)
+	prev=shrext
+	continue
+	;;
+
       -static)
 	# The effects of -static are defined in a previous loop.
 	# We used to do the same as -all-static on platforms that
@@ -2899,12 +2911,18 @@ func_mode_link ()
 	prev=vinfo
 	continue
 	;;
+
       -version-number)
 	prev=vinfo
 	vinfo_number=yes
 	continue
 	;;
 
+      -weak)
+        prev=weak
+	continue
+	;;
+
       -Wc,*)
 	args=`$echo "X$arg" | $Xsed -e "$sed_quote_subst" -e 's/^-Wc,//'`
 	arg=
@@ -2957,11 +2975,6 @@ func_mode_link ()
 	continue
 	;;
 
-      -framework)
-	prev=framework
-	continue
-	;;
-
       # Some other compiler flag.
       -* | +*)
 	# Unknown arguments in both finalize_command and compile_command need
@@ -3244,16 +3257,21 @@ func_mode_link ()
 	# Collect and forward deplibs of preopened libtool libs
 	for lib in $dlprefiles; do
 	  # Ignore non-libtool-libs
+	  dependency_libs=
 	  case $lib in
-	  */*.la | *\\*.la)
-	    . $lib
-	    deplibs="$deplibs $dependency_libs"
-	    ;;
-	  *.la)
-	    . ./$lib
-	    deplibs="$deplibs $dependency_libs"
-	    ;;
+	  *[\\/]*.la) . $lib   ;;
+	  *.la)	      . ./$lib ;;
 	  esac
+
+	  # Collect preopened libtool deplibs, except any this library
+	  # has declared as weak libs
+	  for deplib in $dependency_libs; do
+            deplib_base=`echo "$deplib" |$SED "$basename"`
+	    case " $weak_libs " in
+	    *" $deplib_base "*) ;;
+	    *) deplibs="$deplibs $deplib" ;;
+	    esac
+	  done
 	done
 	libs="$dlprefiles"
       fi
@@ -6547,6 +6565,9 @@ inherited_linker_flags='$inherited_linker_flags'
 # Libraries that this one depends upon.
 dependency_libs='$dependency_libs'
 
+# Names of additional weak libraries provided by this library
+weak_library_names='$weak_libs'
+
 # Version information for $libname.
 current=$current
 age=$age

doc/libtool.texi

@@ -166,6 +166,7 @@ Dlopened modules
 
 * Building modules::            Creating dlopenable objects and libraries.
 * Dlpreopening::                Dlopening that works on static platforms.
+* Linking with dlopened modules:: Using dlopenable modules in libraries.
 * Finding the dlname::          Choosing the right file to @code{dlopen}.
 * Dlopen issues::               Unresolved problems that need your attention.
 
@@ -1427,6 +1428,13 @@ existing projects where identical version numbers are already used across
 operating systems.  New projects should use the @samp{-version-info} flag
 instead.
 
+@item -weak @var{libname}
+if @var{output-file} is a libtool library, declare that it provides a
+weak @var{libname} interface.  This is a hint to libtool that there is
+no need to append @var{libname} to the list of dependency libraries of
+@var{output-file}, because linking against @var{output-file} already
+supplies the same interface (@pxref{Linking with dlopened modules}).
+
 @item -Wl,@var{flag}
 @itemx -Xlinker @var{flag}
 Pass a linker specific flag directly to the linker.
@@ -2884,6 +2892,7 @@ use libtool to generate dlopen-accessible modules.
 @menu
 * Building modules::            Creating dlopenable objects and libraries.
 * Dlpreopening::                Dlopening that works on static platforms.
+* Linking with dlopened modules:: Using dlopenable modules in libraries.
 * Finding the dlname::          Choosing the right file to @code{dlopen}.
 * Dlopen issues::               Unresolved problems that need your attention.
 @end menu
@@ -3071,6 +3080,112 @@ symbols preloaded into @samp{libhell}, you must prefix
 @end deftypefun
 
 
+@node Linking with dlopened modules
+@section Linking with dlopened modules
+@cindex linking, dlopen
+@cindex linking, dlpreopen
+
+When, say, an interpreter application uses dlopened modules to extend
+the list of methods it provides, an obvious abstraction for the
+maintainers of the interpreter is to have all methods (including the
+built in ones supplied with the interpreter) accessed through
+dlopen.  For one thing, the dlopening functionality will be tested
+even during routine invocations.  For another, only one subsystem has
+to be written for getting methods into the interpreter.
+
+The downside of this abstraction is, of course, that environments that
+provide only static linkage can't even load the intrinsic interpreter
+methods.  Not so!  We can statically link those methods by
+@strong{dlpreopening} them.
+
+Unfortunately, since platforms such as @sc{aix} and cygwin require
+that all library symbols must be resolved at compile time, the
+interpreter maintainers will need to provide a library to both its own
+dlpreopened modules, and third-party modules loaded by dlopen.  In
+itself, that is not so bad, except that the interpreter too must
+provide those same symbols otherwise it will be impossible to resolve
+all the symbols required by the modules as they are loaded.  Things
+are even worse if the code that loads the modules for the interpreter
+is itself in a library -- and that is usually the case for any
+non-trivial application.  Modern platforms take care of this by
+automatically loading all of a module's dependency libraries as the
+module is loaded (libltdl can do this even on platforms that can't do
+it by themselves).  In the end, this leads to problems with duplicated
+symbols and prevents modules from loading, and prevents the
+application from compiling when modules are preloaded.
+
+@example
+,-------------.    ,------------------.    ,-----------------.
+| Interpreter |---->     Module------------>   Third-party   |
+`-------------'    |     Loader       |    |Dlopened Modules |
+                   |        |         |    `-----------------'
+                   |,-------v--------.|             |
+                   ||  Dlpreopened   ||             |
+                   ||    Modules     ||             |
+                   |`----------------'|             |
+                   |        |         |             |
+                   |,-------v--------.|    ,--------v--------.
+                   ||Module Interface||    |Module Interface |
+                   ||    Library     ||    |     Library     |
+                   |`----------------'|    `-----------------'
+                   `------------------'
+@end example
+
+Libtool has the concept of @dfn{weak library interfaces} to circumvent
+this problem.  Recall that the code that dlopens method-provider
+modules for the interpreter application resides in a library: All of
+the modules and the dlopener library itself should be linked against
+the common library that resolves the module symbols at compile time.
+To guard against duplicate symbol definitions, and for dlpreopened
+modules to work at all in this scenario, the dlopener library must
+declare that it provides a weak library interface to the common
+symbols in the library it shares with the modules.  That way, when
+@command{libtool} links the @strong{Module Loader} library with some
+@strong{Dlpreopened Modules} that were in turn linked against the
+@strong{Module Interface Library}, it knows that the @strong{Module
+Loader} provides an already loaded @strong{Module Interface Library}
+to resolve symbols for the @strong{Dlpreopened Modules}, and doesn't
+ask the compiler driver to link an identical @strong{Module Interface
+Library} dependency library too.
+
+In conjunction with Automake, the @file{Makefile.am} for the
+@strong{Module Loader} might look like this:
+
+@example
+lib_LTLIBRARIES = libinterface.la libloader.la
+
+libinterface_la_SOURCES = interface.c interface.h
+libinterface_la_LDFLAGS = -version-info 3:2:1
+
+libloader_la_SOURCES    = loader.c
+libloader_la_LDFLAGS    = -weak libinterface.la \
+                          -version-info 3:2:1 \
+                          -dlpreopen ../modules/intrinsics.la
+libloader_la_LIBADD     = $(libinterface_la_OBJECTS)
+@end example
+
+And the @file{Makefile.am} for the @file{intrinsics.la} module in a
+sibling @file{modules} directory might look like this:
+
+@example
+AM_CPPFLAGS             = -I$(srcdir)/../libloader
+AM_LDFLAGS              = -no-undefined -module -avoid-version \
+                          -export-dynamic
+
+noinst_LTLIBRARIES      = intrinsics.la
+
+intrinsics_la_LIBADD    = ../libloader/libinterface.la
+
+../libloader/libinterface.la:
+        cd ../libloader; $(MAKE) $(AM_MAKEFLAGS) libinterface.la
+@end example
+
+@cindex -weak option
+For a more complex example, see the sources of @file{libltdl} in the
+Libtool distribution, which is built with the help of the @samp{-weak}
+option.
+
+
 @node Finding the dlname
 @section Finding the correct name to dlopen
 @cindex names of dynamic modules

libltdl/Makefile.am

@@ -62,7 +62,7 @@ libltdl_la_LIBADD	= libdlloader.la
 
 libltdlc_la_SOURCES	= $(libltdl_la_SOURCES)
 libltdlc_la_CPPFLAGS	= -DLTDLOPEN=libltdlc $(AM_CPPFLAGS)
-libltdlc_la_LDFLAGS	= $(LT_DLPREOPEN)
+libltdlc_la_LDFLAGS	= -weak libdlloader.la $(LT_DLPREOPEN)
 libltdlc_la_LIBADD	=  $(libdlloader_la_OBJECTS) $(libdlloader_la_LIBADD)
 
 ## These are installed as a subdirectory of pkgdatadir so that