Support SIGEV_THREAD_ID

[James Y Knight <jyknight@google.com>]

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There seems to be some debate in glibc over whether this API should be
supported, due to the long-standing debate about "pthread_t" vs
"kernel tid" APIs. (And this API uses kernel tids, of course.)

One proposal from previous glibc discussion was to add a
SIGEV_PTHREAD_ID, which takes a pthread_t, instead of a kernel tid.
Nobody has done this yet, and I'd note that if it is done, that is not
at all incompatible with also continuing to support SIGEV_THREAD_ID.
(Just like both sched_setaffinity and pthread_setaffinity_np exist
without issue).

Despite that discussion, SIGEV_THREAD_ID functionality does in fact
work with glibc -- it provides the SIGEV_THREAD_ID define in its
headers, and it defines the same 'struct sigevent' as the kernel does,
including a _tid member. The only thing it's missing is the field name
"sigev_notify_thread_id" -- so users are simply doing "#define
sigev_notify_thread_id _sigev_un._tid" as a workaround (ugh).

However, it does _not_ work today with musl, as musl's timer_create
function translates the user-facing struct to a separate kernel-facing
structure.

Given long-standing usage of this feature, and given that potential
future directions are additive, not conflicting, ISTM reasonable to
just add support for this to musl.

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From 0a0aef759f216444f558f427466b47f38d678052 Mon Sep 17 00:00:00 2001
From: James Y Knight <jyknight@google.com>
Date: Sun, 30 Jun 2019 21:55:20 -0400
Subject: [PATCH] Add support for SIGEV_THREAD_ID and sigev_notify_thread_id.

This is like SIGEV_SIGNAL, but targeted to a particular thread's
tid, rather than the process.
---
 include/signal.h        | 16 +++++++++++++---
 src/time/timer_create.c |  8 ++++++--
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/signal.h b/include/signal.h
index 5c48cb83..735e0ac7 100644
--- a/include/signal.h
+++ b/include/signal.h
@@ -180,14 +180,24 @@ struct sigevent {
 	union sigval sigev_value;
 	int sigev_signo;
 	int sigev_notify;
-	void (*sigev_notify_function)(union sigval);
-	pthread_attr_t *sigev_notify_attributes;
-	char __pad[56-3*sizeof(long)];
+	union {
+		char __pad[64 - 2*sizeof(int) - sizeof(union sigval)];
+		pid_t sigev_notify_thread_id;
+		struct {
+			void (*sigev_notify_function)(union sigval);
+			pthread_attr_t *sigev_notify_attributes;
+		} __sev_thread;
+	} __sev_fields;
 };
 
+#define sigev_notify_thread_id __sev_fields.sigev_notify_thread_id
+#define sigev_notify_function __sev_fields.__sev_thread.sigev_notify_function
+#define sigev_notify_attributes __sev_fields.__sev_thread.sigev_notify_attributes
+
 #define SIGEV_SIGNAL 0
 #define SIGEV_NONE 1
 #define SIGEV_THREAD 2
+#define SIGEV_THREAD_ID 4 /* Linux extension */
 
 int __libc_current_sigrtmin(void);
 int __libc_current_sigrtmax(void);
diff --git a/src/time/timer_create.c b/src/time/timer_create.c
index c5e40a19..5a3f4d3a 100644
--- a/src/time/timer_create.c
+++ b/src/time/timer_create.c
@@ -83,11 +83,15 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 	switch (evp ? evp->sigev_notify : SIGEV_SIGNAL) {
 	case SIGEV_NONE:
 	case SIGEV_SIGNAL:
+	case SIGEV_THREAD_ID:
 		if (evp) {
 			ksev.sigev_value = evp->sigev_value;
 			ksev.sigev_signo = evp->sigev_signo;
 			ksev.sigev_notify = evp->sigev_notify;
-			ksev.sigev_tid = 0;
+			if (evp->sigev_notify == SIGEV_THREAD_ID)
+				ksev.sigev_tid = evp->sigev_notify_thread_id;
+			else
+				ksev.sigev_tid = 0;
 			ksevp = &ksev;
 		}
 		if (syscall(SYS_timer_create, clk, ksevp, &timerid) < 0)
@@ -115,7 +119,7 @@ int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict
 
 		ksev.sigev_value.sival_ptr = 0;
 		ksev.sigev_signo = SIGTIMER;
-		ksev.sigev_notify = 4; /* SIGEV_THREAD_ID */
+		ksev.sigev_notify = SIGEV_THREAD_ID;
 		ksev.sigev_tid = td->tid;
 		if (syscall(SYS_timer_create, clk, &ksev, &timerid) < 0)
 			timerid = -1;
-- 
2.22.0.709.g102302147b-goog