tracepoints: gnu-zero-variadic-macro-arguments warnings #26916

These occur for all tracepoints when using Clang 15 and a system installed systemtap:

# clang --version
Debian clang version 15.0.7

./autogen.sh
./configure CC=clang-15 CXX=clang++-15
make
...
net_processing.cpp:4892:5: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
    TRACE6(net, inbound_message,
    ^
./util/trace.h:18:50: note: expanded from macro 'TRACE6'
#define TRACE6(context, event, a, b, c, d, e, f) DTRACE_PROBE6(context, event, a, b, c, d, e, f)
                                                 ^
/usr/include/aarch64-linux-gnu/sys/sdt.h:494:3: note: expanded from macro 'DTRACE_PROBE6'
  STAP_PROBE6(provider,probe,parm1,parm2,parm3,parm4,parm5,parm6)
  ^
/usr/include/aarch64-linux-gnu/sys/sdt.h:380:3: note: expanded from macro 'STAP_PROBE6'
  _SDT_PROBE(provider, name, 6, (arg1, arg2, arg3, arg4, arg5, arg6))
  ^
/usr/include/aarch64-linux-gnu/sys/sdt.h:78:75: note: expanded from macro '_SDT_PROBE'
    __asm__ __volatile__ (_SDT_ASM_BODY(provider, name, _SDT_ASM_ARGS, (n)) \
                                                                          ^
/usr/include/aarch64-linux-gnu/sys/sdt.h:283:9: note: macro '_SDT_ASM_BODY' defined here
#define _SDT_ASM_BODY(provider, name, pack_args, args, ...)                   \
        ^
1 warning generated.

FWIW, on Ubuntu Lunar with systemtap-sdt-dev-4.8 being installed, warnings are gone when configuring with --enable-c++20.

FWIW, on Ubuntu Lunar with systemtap-sdt-dev-4.8 being installed, warnings are gone when configuring with --enable-c++20.

fwiw I don't think systemtap 4.8 is the reason there are no warnings for you. The changes form 4.7 to 4.8 in includes/sys/sdt.h file are minimal and shouldn't affect this.

FWIW, on Ubuntu Lunar with systemtap-sdt-dev-4.8 being installed, warnings are gone when configuring with --enable-c++20.

fwiw I don't think systemtap 4.8 is the reason there are no warnings for you. The changes form 4.7 to 4.8 in includes/sys/sdt.h file are minimal and shouldn't affect this.

Correct, the reason is --enable-c++20. See https://en.cppreference.com/w/cpp/preprocessor/replace.

Updated op to note that this is now system-installed systemtap only.

This is still present with #26593 as it also uses an underlying variadic macro with 0 arguments via _SDT_ASM_BODY -> _SDT_PROBE -> _SDT_PROBE_N -> STAP_PROBEV -> TRACEPOINT.

I tried to directly call the STAP_PROBE1 macro for the single argument case (on top of #26593), which would mean that all our remaining TRACEPOINT macros were invoked with >= 1 argument:

// A USDT tracepoint with one argument.
#define TRACEPOINT1(context, event) \
    if (TRACEPOINT_ACTIVE(context, event)) \
        STAP_PROBE1(context, event, arg)

...but this apparently does not stop the warning, even though we are now never using an empty variadic argument:

wallet/spend.cpp:954:5: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]
    TRACEPOINT(coin_selection, selected_coins, wallet.GetName().c_str(), GetAlgorithmName(result.GetAlgo()).c_str(), result.GetTarget(), result.GetWaste(), result.GetSelectedValue());
    ^
./util/trace.h:48:9: note: expanded from macro 'TRACEPOINT'
        STAP_PROBEV(context, event, __VA_ARGS__)
        ^
/usr/include/x86_64-linux-gnu/sys/sdt.h:422:3: note: expanded from macro 'STAP_PROBEV'
  _SDT_PROBE_N(provider, name, _SDT_NARG(0, ##__VA_ARGS__), ##__VA_ARGS__)
  ^
/usr/include/x86_64-linux-gnu/sys/sdt.h:420:3: note: expanded from macro '_SDT_PROBE_N'
  _SDT_PROBE(provider, name, N, (__VA_ARGS__))
  ^
/usr/include/x86_64-linux-gnu/sys/sdt.h:78:75: note: expanded from macro '_SDT_PROBE'
    __asm__ __volatile__ (_SDT_ASM_BODY(provider, name, _SDT_ASM_ARGS, (n)) \
                                                                          ^
/usr/include/x86_64-linux-gnu/sys/sdt.h:289:9: note: macro '_SDT_ASM_BODY' defined here
#define _SDT_ASM_BODY(provider, name, pack_args, args, ...)                   \
        ^

It seems we have a few options therefore:

1. Do nothing and wait until we move to c++20 (no timeline)
2. Exclude the warning with global compile flags: CXXFLAGS += -Wno-gnu-zero-variadic-macro-arguments
3. Add pragmas inside the macros to disable the warnings (diff based on #26593):

diff --git a/src/util/trace.h b/src/util/trace.h
index 816d8c295..e3fc0d8f4 100644
--- a/src/util/trace.h
+++ b/src/util/trace.h
@@ -32,15 +32,29 @@
 #define TRACEPOINT_ACTIVE(context, event) context##_##event##_semaphore > 0
 
 // A USDT tracepoint with no arguments.
+// Remove pragmas when we switch to c++20
 #define TRACEPOINT0(context, event) \
-    if (TRACEPOINT_ACTIVE(context, event)) \
-        STAP_PROBE(context, event)
+    do { \
+        _Pragma("clang diagnostic push") \
+        _Pragma("clang diagnostic ignored \"-Wgnu-zero-variadic-macro-arguments\"") \
+        if (TRACEPOINT_ACTIVE(context, event)) { \
+            STAP_PROBE(context, event); \
+        } \
+        _Pragma("clang diagnostic pop") \
+    } while(0)
 
 // A USDT tracepoint with one to twelve arguments. It's checked that the
 // tracepoint is active before preparing its arguments.
+// Remove pragmas when we switch to c++20
 #define TRACEPOINT(context, event, ...) \
-    if (TRACEPOINT_ACTIVE(context, event)) \
-        STAP_PROBEV(context, event, __VA_ARGS__)
+    do { \
+        _Pragma("clang diagnostic push") \
+        _Pragma("clang diagnostic ignored \"-Wgnu-zero-variadic-macro-arguments\"") \
+        if (TRACEPOINT_ACTIVE(context, event)) { \
+            STAP_PROBEV(context, event, __VA_ARGS__); \
+        } \
+        _Pragma("clang diagnostic pop") \
+    } while(0)
 
 #else