Add "warn_unused_result" to attributes.h

A new ISC_ATTR_WARN_UNUSED_RESULT macro now defines
__attribute__((warn_unused_result)) and is used for
dns_rdata_fromstruct().

(cherry picked from commit 07c8c1d242)

configure.ac

@@ -491,6 +491,11 @@ AC_COMPILE_IFELSE(
 #
 AX_GCC_FUNC_ATTRIBUTE([returns_nonnull])
 
+#
+# check for GCC returns_nonnull attribute
+#
+AX_GCC_FUNC_ATTRIBUTE([warn_unused_result])
+
 #
 # how to link math functions?
 #

lib/dns/include/dns/rdata.h

@@ -468,6 +468,7 @@ dns_rdata_tofmttext(dns_rdata_t *rdata, const dns_name_t *origin,
  * it is undefined and falls back to the default value of 'width'
  */
 
+ISC_ATTR_WARN_UNUSED_RESULT
 isc_result_t
 dns_rdata_fromstruct(dns_rdata_t *rdata, dns_rdataclass_t rdclass,
 		     dns_rdatatype_t type, void *source, isc_buffer_t *target);

lib/isc/include/isc/attributes.h

@@ -35,6 +35,12 @@
 #define ISC_ATTR_RETURNS_NONNULL
 #endif
 
+#if HAVE_FUNC_ATTRIBUTE_WARN_UNUSED_RESULT
+#define ISC_ATTR_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+#else
+#define ISC_ATTR_WARN_UNUSED_RESULT
+#endif
+
 #ifdef HAVE_FUNC_ATTRIBUTE_MALLOC
 /*
  * Indicates that a function is malloc-like, i.e., that the

tests/dns/private_test.c

@@ -97,6 +97,7 @@ make_signing(signing_testcase_t *testcase, dns_rdata_t *private,
 static void
 make_nsec3(nsec3_testcase_t *testcase, dns_rdata_t *private,
 	   unsigned char *pbuf) {
+	isc_result_t result;
 	dns_rdata_nsec3param_t params;
 	dns_rdata_t nsec3param = DNS_RDATA_INIT;
 	unsigned char bufdata[BUFSIZ];
@@ -134,8 +135,9 @@ make_nsec3(nsec3_testcase_t *testcase, dns_rdata_t *private,
 	}
 
 	isc_buffer_init(&buf, bufdata, sizeof(bufdata));
-	dns_rdata_fromstruct(&nsec3param, dns_rdataclass_in,
-			     dns_rdatatype_nsec3param, &params, &buf);
+	result = dns_rdata_fromstruct(&nsec3param, dns_rdataclass_in,
+				      dns_rdatatype_nsec3param, &params, &buf);
+	assert_int_equal(result, ISC_R_SUCCESS);
 
 	dns_rdata_init(private);