Warn and return for duplicated plugin

If a plugin has been loaded already, we should warn and return, instead
of adding it on the plugin list.

	PR ld/20321
	* plugin.c (plugin_opt_plugin): Warn and return if plugin has
	been loaded already.
	* testsuite/ld-plugin/lto.exp: Run PR ld/20321 test.
	* testsuite/ld-plugin/pr20321.c: New file.

ld/ChangeLog

@@ -1,3 +1,11 @@
+2016-07-04  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR ld/20321
+	* plugin.c (plugin_opt_plugin): Warn and return if plugin has
+	been loaded already.
+	* testsuite/ld-plugin/lto.exp: Run PR ld/20321 test.
+	* testsuite/ld-plugin/pr20321.c: New file.
+
 2016-07-04  Nick Clifton  <nickc@redhat.com>
 
 	* scripttempl/ft32.sc (__PMSIZE_): If not defined, set to 256K.

ld/plugin.c

@@ -237,6 +237,7 @@ void
 plugin_opt_plugin (const char *plugin)
 {
   plugin_t *newplug;
+  plugin_t *curplug = plugins_list;
 
   newplug = xmalloc (sizeof *newplug);
   memset (newplug, 0, sizeof *newplug);
@@ -245,6 +246,18 @@ plugin_opt_plugin (const char *plugin)
   if (!newplug->dlhandle)
     einfo (_("%P%F: %s: error loading plugin: %s\n"), plugin, dlerror ());
 
+  /* Check if plugin has been loaded already.  */
+  while (curplug)
+    {
+      if (newplug->dlhandle == curplug->dlhandle)
+	{
+	  einfo (_("%P: %s: duplicated plugin\n"), plugin);
+	  free (newplug);
+	  return;
+	}
+      curplug = curplug->next;
+    }
+
   /* Chain on end, so when we run list it is in command-line order.  */
   *plugins_tail_chain_ptr = newplug;
   plugins_tail_chain_ptr = &newplug->next;

ld/testsuite/ld-plugin/lto.exp

@@ -198,6 +198,9 @@ set lto_link_tests [list \
   [list "Build libpr20267b.a" \
    "$plug_opt" "-flto $lto_no_fat" \
    {pr20267b.c} {} "libpr20267b.a"] \
+  [list "Build pr20321" \
+   "-Wl,-plugin,$plug_so" "-flto" \
+   {pr20321.c} {} "pr20321" "c" ".*: duplicated plugin"] \
 ]
 
 if { [at_least_gcc_version 4 7] } {

ld/testsuite/ld-plugin/pr20321.c

@@ -0,0 +1,4 @@
+int main(void)
+{
+  return 0;
+}