From 5f17edea5bd4f92efb35a2facaaacd0042f03359 Mon Sep 17 00:00:00 2001
From: Thomas Haller <thaller@redhat.com>
Date: Fri, 8 Oct 2021 15:17:43 +0200
Subject: [PATCH] std-aux: work around "-Wunused-but-set-variable" warning with
 clang in nm_auto()

We use the cleanup attribute heavily. It's useful for deferring
deallocation. For example, we have code like:

  gs_unref_object NMBluezManager *self_keep_alive = g_object_ref(self);

where we don't use the variable otherwise, except for owning (and
freeing) the reference. This already lead to a compiler warning about
unused variable, which we would workaround with

  _nm_unused gs_unref_object NMBluezManager *self_keep_alive = g_object_ref(self);

With clang 13.0.0~rc1-1.fc35, this got worse. Now for example also

    static inline void
    nm_strvarray_set_strv(GArray **array, const char *const *strv)
    {
        gs_unref_array GArray *array_old = NULL;

        array_old = g_steal_pointer(array);

        if (!strv || !strv[0])
            return;

        nm_strvarray_ensure(array);
        for (; strv[0]; strv++)
            nm_strvarray_add(*array, strv[0]);
    }

leads to a warning

    ./src/libnm-glib-aux/nm-shared-utils.h:3078:28: error: variable array_old set but not used [-Werror,-Wunused-but-set-variable]
        gs_unref_array GArray *array_old = NULL;
                               ^

This is really annoying. We don't want to plaster our code with _nm_unused,
because that might hide actual issues. But we also want to keep using this
pattern and need to avoid the warning.

A problem is also that GCC usually does not warn about truly unused
variables with cleanup attribute. Clang was very useful here to flag
such variables. But now clang warns about cases which are no bugs, which
is a problem. So this does loose some useful warnings. On the other hand,
a truly unused variable (with cleanup attribute) is ugly, but not an actual
problem.

Now, with clang 13, automatically mark nm_auto() variables as _nm_unused
as workaround.

(cherry picked from commit 1c85bc5ead2cd2fe7d38c4fb84ebf97ae70ab7c4)
---
 src/libnm-std-aux/nm-std-aux.h | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/src/libnm-std-aux/nm-std-aux.h b/src/libnm-std-aux/nm-std-aux.h
index 17b268816d..ca85e47e07 100644
--- a/src/libnm-std-aux/nm-std-aux.h
+++ b/src/libnm-std-aux/nm-std-aux.h
@@ -13,17 +13,26 @@
 
 /*****************************************************************************/
 
-#define _nm_packed        __attribute__((__packed__))
-#define _nm_unused        __attribute__((__unused__))
-#define _nm_used          __attribute__((__used__))
-#define _nm_pure          __attribute__((__pure__))
-#define _nm_const         __attribute__((__const__))
-#define _nm_printf(a, b)  __attribute__((__format__(__printf__, a, b)))
-#define _nm_align(s)      __attribute__((__aligned__(s)))
-#define _nm_section(s)    __attribute__((__section__(s)))
-#define _nm_alignof(type) __alignof(type)
-#define _nm_alignas(type) _nm_align(_nm_alignof(type))
-#define nm_auto(fcn)      __attribute__((__cleanup__(fcn)))
+#define _nm_packed             __attribute__((__packed__))
+#define _nm_unused             __attribute__((__unused__))
+#define _nm_used               __attribute__((__used__))
+#define _nm_pure               __attribute__((__pure__))
+#define _nm_const              __attribute__((__const__))
+#define _nm_printf(a, b)       __attribute__((__format__(__printf__, a, b)))
+#define _nm_align(s)           __attribute__((__aligned__(s)))
+#define _nm_section(s)         __attribute__((__section__(s)))
+#define _nm_alignof(type)      __alignof(type)
+#define _nm_alignas(type)      _nm_align(_nm_alignof(type))
+
+#if defined(__clang__) && __clang_major__ == 13
+/* Clang 13 can emit -Wunused-but-set-variable warning for cleanup variables
+ * that are only assigned (never used otherwise). Hack around */
+#define _nm_auto_extra _nm_unused
+#else
+#define _nm_auto_extra
+#endif
+
+#define nm_auto(fcn) _nm_auto_extra __attribute__((__cleanup__(fcn)))
 
 /* This is required to make LTO working.
  *