[PATCH v2] libgo: Portable access to thread ID in struct sigevent

[soeren@soeren-tempel.net]

From: Sören Tempel <soeren@soeren-tempel.net>

Tested on x86_64 Arch Linux (glibc) and Alpine Linux (musl libc).

Previously, libgo relied on the _sigev_un implementation-specific
field in struct sigevent, which is only available on glibc. This
patch uses the sigev_notify_thread_id macro instead which is mandated
by timer_create(2). In theory, this should work with any libc
implementation for Linux. Unfortunately, there is an open glibc bug
as glibc does not define this macro. For this reason, a glibc-specific
workaround is required. Other libcs (such as musl) define the macro
and don't require the workaround.

This makes go_signal compatible with musl libc.

See: https://sourceware.org/bugzilla/show_bug.cgi?id=27417

Signed-off-by: Sören Tempel <soeren@soeren-tempel.net>
---
Changes since v1: Add workaround for glibc.

 libgo/go/runtime/os_linux.go |  4 +++-
 libgo/runtime/go-signal.c    | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/libgo/go/runtime/os_linux.go b/libgo/go/runtime/os_linux.go
index 96fb1788..6653d85e 100644
--- a/libgo/go/runtime/os_linux.go
+++ b/libgo/go/runtime/os_linux.go
@@ -22,6 +22,8 @@ type mOS struct {
 	profileTimerValid uint32
 }
 
+func setProcID(uintptr, int32)
+
 func getProcID() uint64 {
 	return uint64(gettid())
 }
@@ -365,7 +367,7 @@ func setThreadCPUProfiler(hz int32) {
 	var sevp _sigevent
 	sevp.sigev_notify = _SIGEV_THREAD_ID
 	sevp.sigev_signo = _SIGPROF
-	*((*int32)(unsafe.Pointer(&sevp._sigev_un))) = int32(mp.procid)
+	setProcID(uintptr(unsafe.Pointer(&sevp)), int32(mp.procid))
 	ret := timer_create(_CLOCK_THREAD_CPUTIME_ID, &sevp, &timerid)
 	if ret != 0 {
 		// If we cannot create a timer for this M, leave profileTimerValid false
diff --git a/libgo/runtime/go-signal.c b/libgo/runtime/go-signal.c
index 528d9b6d..c56350cc 100644
--- a/libgo/runtime/go-signal.c
+++ b/libgo/runtime/go-signal.c
@@ -16,6 +16,11 @@
   #define SA_RESTART 0
 #endif
 
+// Workaround for https://sourceware.org/bugzilla/show_bug.cgi?id=27417
+#if __linux__ && !defined(sigev_notify_thread_id)
+  #define sigev_notify_thread_id _sigev_un._tid
+#endif
+
 #ifdef USING_SPLIT_STACK
 
 extern void __splitstack_getcontext(void *context[10]);
@@ -183,6 +188,16 @@ setSigactionHandler(struct sigaction* sa, uintptr handler)
 	sa->sa_sigaction = (void*)(handler);
 }
 
+void setProcID(uintptr_t, int32_t)
+	__asm__ (GOSYM_PREFIX "runtime.setProcID");
+
+void
+setProcID(uintptr_t ptr, int32_t v)
+{
+	struct sigevent *s = (void *)ptr;
+	s->sigev_notify_thread_id = v;
+}
+
 // C code to fetch values from the siginfo_t and ucontext_t pointers
 // passed to a signal handler.