_krb5_load_plugins: Windows naming rules

Windows plugins must be loaded from the same directory as the
heimdal.dll in order to ensure that "DLL Hell" is not revisited.
Since plugins cannot be grouped in a directory by "module" name
the plugins must have a name of the form:

  plugin_<module>_<name>.dll

Construct the "plugin_<module>_" prefix string dynamically and
test for both the prefix and the ".dll" extension when building
the list of plugins to load.

Change-Id: I0ff7bf7287f8fb38061c226f0844d7d0f1fb59ec

lib/krb5/plugin.c

@@ -277,6 +277,14 @@ _krb5_load_plugins(krb5_context context, const char *name, const char **paths)
     const char **di;
     char *dirname = NULL;
     DIR *d;
+#ifdef _WIN32
+    const char * plugin_prefix;
+    size_t plugin_prefix_len;
+
+    if (asprintf(&plugin_prefix, "plugin_%s_", name) == -1)
+	return;
+    plugin_prefix_len = (plugin_prefix ? strlen(plugin_prefix) : 0);
+#endif
 
     HEIMDAL_MUTEX_lock(&plugin_mutex);
 
@@ -321,8 +329,20 @@ _krb5_load_plugins(krb5_context context, const char *name, const char **paths)
 		continue;
 
 #ifdef _WIN32
-            if (strncmp(n, "plugin_krb5_", sizeof("plugin_krb5_") - 1))
-                continue;
+	    /*
+	     * On Windows, plugins must be loaded from the same directory as
+	     * heimdal.dll (typically the assembly directory) and must have
+	     * the name form "plugin_<module>_<name>.dll".
+	     */
+	    {
+		char *ext;
+
+		if (strnicmp(n, plugin_prefix, plugin_prefix_len))
+		    continue;
+		ext = strrchr(n, '.');
+		if (ext == NULL || stricmp(ext, ".dll"))
+		     continue;
+	    }
 #endif
 
 	    ret = 0;
@@ -367,6 +387,10 @@ _krb5_load_plugins(krb5_context context, const char *name, const char **paths)
     free(dirname);
     HEIMDAL_MUTEX_unlock(&plugin_mutex);
     heim_release(module);
+#ifdef _WIN32
+    if (plugin_prefix)
+	free(plugin_prefix);
+#endif
 #endif /* HAVE_DLOPEN */
 }