Make use of GCC's __attribute__((unused)) (#6392)

* configure.ac: Add support for GCC's __attribute__((unused))

When compiling under GCC with -Wextra (along with -Wall) there are a lot
of compiler warnings such as

collectors/cgroups.plugin/cgroup-network.c:89:45: warning: unused parameter ‘scope’ [-Wunused-parameter]
   89 | struct iface *read_proc_net_dev(const char *scope, const char *prefix) {
      |                                 ~~~~~~~~~~~~^~~~~

Some arguments may be able to be got rid off, others won't.

GCC (and at least clang[0]) provide an __attribute__((unused))
annotation that can be used on function parameters (also on variables,
functions, labels, enums, structs etc) to inform the compiler of such
and will squash warnings of the above nature.

A check is added to configure.ac for the use of GCC (I believe $GCC will
be set to 'yes' for clang also) and if found it creates __always_unused
& __maybe_unused #define's set to __attribute__((unused)) otherwise it
just sets them empty.

If other compilers have a similar feature this check can be adjusted to
accommodate them.

The reason for the two defines is that some variables may always be
unused in a function, others may or may not depending on #ifdef's for
example. So we are able to document both cases.

Subsequent commits will start making use of these to squash such
compiler warnings.

[0]: https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/statsd.plugin: Mark a function argument as __maybe_unused

In collectors/statsd.plugin/statsd.c the app function argument to
STATSD_APP_CHART_DIM() might be unused if NETDATA_INTERNAL_CHECKS is not
defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/apps.plugin: Mark a function argument as __maybe_unused

In collectors/apps.plugin/apps_plugin.c the function
debug_print_process_tree() takes an argument 'msg' that might be unused
if NETDATA_INTERNAL_CHECKS is not defined, then debug_log() will be set
to a dummy function that takes no arguments.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* libnetdata/locks/locks: Mark function arguments as __maybe_unused

In libnetdata/locks/locks.c there a number of functions that take
arguments 'file', 'function' & 'line' that might be unused if
NETDATA_INTERNAL_CHECKS is not defined, then the debug() macro that it's
used in from libnetdata/log/log.h will be defined to a dummy function
where none of the arguments are used.

This fixes compiler warnings [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* libnetdata/socket/security: Mark a function argument as __maybe_unused

In libnetdata/socket/security.c the function security_info_callback()
takes an argument 'ret' that might be unused if NETDATA_INTERNAL_CHECKS
is not defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* collectors/cgroups.plugin: Mark a function argument as __maybe_unused

In collectors/cgroups.plugin/cgroup-network.c the function
read_proc_net_dev() takes an argument 'scope' that might be unused if
the NETDATA_INTERNAL_CHECKS is not defined.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall".

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* database/rrddim: Mark function arguments as __maybe_unused

In database/rrddim.c there a couple of functions that take a 'st'
argument that might be unused if NETDATA_INTERNAL_CHECKS is not defined,
then the debug() macro that it's used in from libnetdata/log/log.h will
be defined to a dummy function where none of the arguments are used.

This fixes compiler warnings [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

* database/rrdvar: Mark a function argument as __maybe_unused

In database/rrdvar.c the function rrdvar_create_and_index() take an
argument 'scope' that might be unused if NETDATA_INTERNAL_CHECKS is not
defined, then the debug() macro that it's used in from
libnetdata/log/log.h will be defined to a dummy function where none of
the arguments are used.

This fixes a compiler warning [-Wunused-parameter] when compiling with
-Wextra *and* -Wall.

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>

collectors/apps.plugin/apps_plugin.c

@@ -2227,7 +2227,7 @@ static inline int debug_print_process_and_parents(struct pid_stat *p, usec_t tim
     return indent + 1;
 }
 
-static inline void debug_print_process_tree(struct pid_stat *p, char *msg) {
+static inline void debug_print_process_tree(struct pid_stat *p, char *msg __maybe_unused) {
     debug_log("%s: process %s (%d, %s) with parents:", msg, p->comm, p->pid, p->updated?"running":"exited");
     debug_print_process_and_parents(p, p->stat_collected_usec);
 }

collectors/cgroups.plugin/cgroup-network.c

@@ -86,7 +86,7 @@ unsigned int read_iface_ifindex(const char *prefix, const char *iface) {
     return (unsigned int)ifindex;
 }
 
-struct iface *read_proc_net_dev(const char *scope, const char *prefix) {
+struct iface *read_proc_net_dev(const char *scope __maybe_unused, const char *prefix) {
     if(!prefix) prefix = "";
 
     procfile *ff = NULL;

collectors/statsd.plugin/statsd.c

@@ -1067,7 +1067,7 @@ static const char *valuetype2string(STATSD_APP_CHART_DIM_VALUE_TYPE type) {
 }
 
 static STATSD_APP_CHART_DIM *add_dimension_to_app_chart(
-        STATSD_APP *app
+        STATSD_APP *app __maybe_unused
         , STATSD_APP_CHART *chart
         , const char *metric_name
         , const char *dim_name

configure.ac

@@ -437,6 +437,14 @@ else
     AC_DEFINE_UNQUOTED([unlikely(x)], [(x)], [gcc branch optimization])
 fi
 
+if test "${GCC}" = "yes"; then
+    AC_DEFINE([__always_unused], [__attribute__((unused))], [gcc unused attribute])
+    AC_DEFINE([__maybe_unused], [__attribute__((unused))], [gcc unused attribute])
+else
+    AC_DEFINE([__always_unused], [], [dummy unused attribute])
+    AC_DEFINE([__maybe_unused], [], [dummy unused attribute])
+fi
+
 if test "${enable_pedantic}" = "yes"; then
     enable_strict="yes"
     CFLAGS="${CFLAGS} -pedantic -Wall -Wextra -Wno-long-long"

database/rrddim.c

@@ -466,7 +466,7 @@ inline void rrddim_is_obsolete(RRDSET *st, RRDDIM *rd) {
     rrdset_flag_set(st, RRDSET_FLAG_OBSOLETE_DIMENSIONS);
 }
 
-inline void rrddim_isnot_obsolete(RRDSET *st, RRDDIM *rd) {
+inline void rrddim_isnot_obsolete(RRDSET *st __maybe_unused, RRDDIM *rd) {
     debug(D_RRD_CALLS, "rrddim_isnot_obsolete() for chart %s, dimension %s", st->name, rd->name);
 
     rrddim_flag_clear(rd, RRDDIM_FLAG_OBSOLETE);
@@ -475,7 +475,7 @@ inline void rrddim_isnot_obsolete(RRDSET *st, RRDDIM *rd) {
 // ----------------------------------------------------------------------------
 // RRDDIM - collect values for a dimension
 
-inline collected_number rrddim_set_by_pointer(RRDSET *st, RRDDIM *rd, collected_number value) {
+inline collected_number rrddim_set_by_pointer(RRDSET *st __maybe_unused, RRDDIM *rd, collected_number value) {
     debug(D_RRD_CALLS, "rrddim_set_by_pointer() for chart %s, dimension %s, value " COLLECTED_NUMBER_FORMAT, st->name, rd->name, value);
 
     now_realtime_timeval(&rd->last_collected_time);

database/rrdvar.c

@@ -68,7 +68,8 @@ inline void rrdvar_free(RRDHOST *host, avl_tree_lock *tree, RRDVAR *rv) {
     freez(rv);
 }
 
-inline RRDVAR *rrdvar_create_and_index(const char *scope, avl_tree_lock *tree, const char *name, RRDVAR_TYPE type, RRDVAR_OPTIONS options, void *value) {
+inline RRDVAR *rrdvar_create_and_index(const char *scope __maybe_unused, avl_tree_lock *tree, const char *name,
+                                       RRDVAR_TYPE type, RRDVAR_OPTIONS options, void *value) {
     char *variable = strdupz(name);
     rrdvar_fix_name(variable);
     uint32_t hash = simple_hash(variable);

libnetdata/locks/locks.c

@@ -82,7 +82,8 @@ int __netdata_mutex_unlock(netdata_mutex_t *mutex) {
     return ret;
 }
 
-int netdata_mutex_init_debug( const char *file, const char *function, const unsigned long line, netdata_mutex_t *mutex) {
+int netdata_mutex_init_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                             const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
     usec_t start = 0;
     (void)start;
 
@@ -98,7 +99,8 @@ int netdata_mutex_init_debug( const char *file, const char *function, const unsi
     return ret;
 }
 
-int netdata_mutex_lock_debug( const char *file, const char *function, const unsigned long line, netdata_mutex_t *mutex) {
+int netdata_mutex_lock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                             const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
     usec_t start = 0;
     (void)start;
 
@@ -114,7 +116,8 @@ int netdata_mutex_lock_debug( const char *file, const char *function, const unsi
     return ret;
 }
 
-int netdata_mutex_trylock_debug( const char *file, const char *function, const unsigned long line, netdata_mutex_t *mutex) {
+int netdata_mutex_trylock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
     usec_t start = 0;
     (void)start;
 
@@ -130,7 +133,8 @@ int netdata_mutex_trylock_debug( const char *file, const char *function, const u
     return ret;
 }
 
-int netdata_mutex_unlock_debug( const char *file, const char *function, const unsigned long line, netdata_mutex_t *mutex) {
+int netdata_mutex_unlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                               const unsigned long line __maybe_unused, netdata_mutex_t *mutex) {
     usec_t start = 0;
     (void)start;
 
@@ -219,7 +223,8 @@ int __netdata_rwlock_trywrlock(netdata_rwlock_t *rwlock) {
 }
 
 
-int netdata_rwlock_destroy_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_destroy_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                 const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -235,7 +240,8 @@ int netdata_rwlock_destroy_debug( const char *file, const char *function, const
     return ret;
 }
 
-int netdata_rwlock_init_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_init_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                              const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -251,7 +257,8 @@ int netdata_rwlock_init_debug( const char *file, const char *function, const uns
     return ret;
 }
 
-int netdata_rwlock_rdlock_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_rdlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -267,7 +274,8 @@ int netdata_rwlock_rdlock_debug( const char *file, const char *function, const u
     return ret;
 }
 
-int netdata_rwlock_wrlock_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_wrlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -283,7 +291,8 @@ int netdata_rwlock_wrlock_debug( const char *file, const char *function, const u
     return ret;
 }
 
-int netdata_rwlock_unlock_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_unlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -299,7 +308,8 @@ int netdata_rwlock_unlock_debug( const char *file, const char *function, const u
     return ret;
 }
 
-int netdata_rwlock_tryrdlock_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_tryrdlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                   const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 
@@ -315,7 +325,8 @@ int netdata_rwlock_tryrdlock_debug( const char *file, const char *function, cons
     return ret;
 }
 
-int netdata_rwlock_trywrlock_debug( const char *file, const char *function, const unsigned long line, netdata_rwlock_t *rwlock) {
+int netdata_rwlock_trywrlock_debug(const char *file __maybe_unused, const char *function __maybe_unused,
+                                   const unsigned long line __maybe_unused, netdata_rwlock_t *rwlock) {
     usec_t start = 0;
     (void)start;
 

libnetdata/socket/security.c

@@ -20,7 +20,7 @@ int netdata_validate_server =  NETDATA_SSL_VALID_CERTIFICATE;
  * @param where the variable with the flags set.
  * @param ret the return of the caller
  */
-static void security_info_callback(const SSL *ssl, int where, int ret) {
+static void security_info_callback(const SSL *ssl, int where, int ret __maybe_unused) {
     (void)ssl;
     if (where & SSL_CB_ALERT) {
         debug(D_WEB_CLIENT,"SSL INFO CALLBACK %s %s", SSL_alert_type_string(ret), SSL_alert_desc_string_long(ret));