[XEN PATCH v1 2/3] ocaml/libs: Fill build failure due to unused variable in ocaml macro

[Javi Merino <javi.merino@xxxxxxxxx>]

On Fedora 40, the build fails with:

    gcc -m64 -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall 
-Wstrict-prototypes -Wno-unused-but-set-variable -Wno-unused-local-typedefs   
-Werror -O2 -fomit-frame-pointer 
-D__XEN_INTERFACE_VERSION__=__XEN_LATEST_INTERFACE_VERSION__ -MMD -MP -MF 
.domain_getinfo_stubs_v1.o.d -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE   -fPIC 
-I/usr/lib64/ocaml -I 
/build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../libs -I 
/build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../libs/xsd_glue
 
-I/build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../../../tools/include
 -D__XEN_TOOLS__ 
-I/build/tools/ocaml/libs\/xsd_glue/domain_getinfo_plugin_v1/../../../../../tools/include
  -c -o domain_getinfo_stubs_v1.o domain_getinfo_stubs_v1.c
    In file included from domain_getinfo_stubs_v1.c:10:
    domain_getinfo_stubs_v1.c: In function 'xsd_glue_failwith':
    /usr/lib64/ocaml/caml/memory.h:275:29: error: unused variable 'caml__frame' 
[-Werror=unused-variable]
      275 |   struct caml__roots_block *caml__frame = *caml_local_roots_ptr
          |                             ^~~~~~~~~~~
    domain_getinfo_stubs_v1.c:48:9: note: in expansion of macro 'CAMLparam0'
      48 |         CAMLparam0();
         |         ^~~~~~~~~~
    cc1: all warnings being treated as errors

The CAMLparam0 macro is defined in /usr/lib64/ocaml/caml/memory.h:255 as:

    #define CAMLparam0()                                                    \
      struct caml__roots_block** caml_local_roots_ptr =                     \
        (DO_CHECK_CAML_STATE ? Caml_check_caml_state() : (void)0,           \
         &CAML_LOCAL_ROOTS);                                                \
      struct caml__roots_block *caml__frame = *caml_local_roots_ptr

We can't modify the macro.  Mark the xsd_glue_failwith() function with
ignore "-Wunused-variable" to prevent gcc from failing the build due
to the unused variable.

Fixes: a6576011a4d2 ("ocaml/libs: Implement a dynamically-loaded plugin for 
Xenctrl.domain_getinfo")
Signed-off-by: Javi Merino <javi.merino@xxxxxxxxx>
---

While it is not ideal, this is the most effective option I have found
that avoids treating warnings as errors.  I can add a comment above
the pragma explaining why if we decide that this is the way forward.

 .../domain_getinfo_plugin_v1/domain_getinfo_stubs_v1.c         | 3 +++
 1 file changed, 3 insertions(+)

diff --git 
a/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/domain_getinfo_stubs_v1.c 
b/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/domain_getinfo_stubs_v1.c
index 7be386f4d4c2..df2b3c74c727 100644
--- 
a/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/domain_getinfo_stubs_v1.c
+++ 
b/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/domain_getinfo_stubs_v1.c
@@ -42,6 +42,8 @@ static struct custom_operations xsd_glue_xenctrl_ops = {
        .compare_ext = custom_compare_ext_default, /* Can't compare     */
 };
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-variable"
 static void Noreturn xsd_glue_failwith(
        xc_interface *xch, const char *func, unsigned int line)
 {
@@ -70,6 +72,7 @@ static void Noreturn xsd_glue_failwith(
 
        caml_raise_with_arg(*caml_named_value("xsg.error_v1"), msg);
 }
+#pragma GCC diagnostic pop
 #define xsd_glue_failwith(xch) xsd_glue_failwith(xch, __func__, __LINE__)
 
 CAMLprim value stub_xsd_glue_xc_interface_open(value unit)
-- 
2.46.1