Add support for legacy behaviour in udev_device_get_sysattr_value()

Wraps the changes made in systemd commits 5ae18ddc and 53726349
with a #if's on LIBUDEV_LEGACY_VERSION so that old behaviour
can be chosen at build-time if users find it necessary to support
legacy software dependent on the original behaviour

Original Authors:
	Kay Sievers

Signed-off-by:  Ian Stakenvicius <axs@gentoo.org>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>

src/libudev/Makefile.am

@@ -18,6 +18,7 @@ AM_CPPFLAGS = \
 	-include $(top_builddir)/config.h \
 	-DUDEV_CONF_FILE=\"$(udevconffile)\" \
 	-DUDEV_HWDB_BIN=\"$(udevhwdbbin)\" \
+	-DUDEV_VERSION=$(UDEV_VERSION) \
 	-I $(top_srcdir)/src/udev
 
 lib_LTLIBRARIES = \

src/libudev/libudev-device.c

@@ -1381,7 +1381,9 @@ _public_ const char *udev_device_get_sysattr_value(struct udev_device *udev_devi
         }
 
         if (S_ISLNK(statbuf.st_mode)) {
+#if LIBUDEV_LEGACY_VERSION < 199
                 struct udev_device *dev;
+#endif
 
                 /*
                  * Some core links return only the last element of the target path,
@@ -1398,6 +1400,11 @@ _public_ const char *udev_device_get_sysattr_value(struct udev_device *udev_devi
                         goto out;
                 }
 
+#if LIBUDEV_LEGACY_VERSION < 199
+#if LIBUDEV_LEGACY_VERSION >= 196
+            /* resolve only custom link to a device */
+            if (!streq(sysattr, "device")) {
+#endif
                 /* resolve link to a device and return its syspath */
                 util_strscpyl(path, sizeof(path), udev_device->syspath, "/", sysattr, NULL);
                 dev = udev_device_new_from_syspath(udev_device->udev, path);
@@ -1407,6 +1414,10 @@ _public_ const char *udev_device_get_sysattr_value(struct udev_device *udev_devi
                         val = udev_list_entry_get_value(list_entry);
                         udev_device_unref(dev);
                 }
+#if LIBUDEV_LEGACY_VERSION >= 196
+            }
+#endif
+#endif
 
                 goto out;
         }

src/libudev/libudev.h

@@ -20,6 +20,10 @@
 #ifndef _LIBUDEV_H_
 #define _LIBUDEV_H_
 
+#ifndef LIBUDEV_LEGACY_VERSION
+#define LIBUDEV_LEGACY_VERSION UDEV_VERSION
+#endif
+
 #include <stdarg.h>
 #include <sys/types.h>
 #include <sys/stat.h>