From d3a628a8ec1d4b1a2ec080328a41c21e48f04d06 Mon Sep 17 00:00:00 2001
From: Dirk Engling <erdgeist@erdgeist.org>
Date: Thu, 29 Oct 2026 01:16:53 +0100
Subject: add wake-up tester that helped figuring out how to tickle hanging udp
 readers

---
 tests/udp_wake_probe.c | 42 ++++++++++++++++++++++++++++++++----------
 1 file changed, 32 insertions(+), 10 deletions(-)

(limited to 'tests/udp_wake_probe.c')

diff --git a/tests/udp_wake_probe.c b/tests/udp_wake_probe.c
index 8a0f7a8..e948c34 100644
--- a/tests/udp_wake_probe.c
+++ b/tests/udp_wake_probe.c
@@ -1,13 +1,17 @@
-/* Three ways to get a thread out of a blocking recvfrom, tested rather than
-   assumed: close the socket, shut it down, or give it a receive timeout.
+/* Four ways to get a thread out of a blocking recvfrom, tested rather than
+   assumed: close the socket, shut it down, give it a receive timeout, or
+   send it a signal.
 
-   This decides how udp workers can be stopped. They sit in recvfrom and are
-   never joined today, and the sockets of a SO_REUSEPORT group all share one
-   address, so a worker cannot be singled out by sending it a datagram.
+   This decides how udp workers are stopped. The sockets of a SO_REUSEPORT
+   group all share one address, so a worker cannot be singled out by sending
+   it a datagram, and the two mechanisms differ in what they cost while the
+   tracker is running: a receive timeout wakes every worker on every tick
+   forever, a signal and a shutdown cost nothing until the one moment they
+   are used.
 
      cc -O2 -pthread -o /tmp/wake tests/udp_wake_probe.c && /tmp/wake
 
-   On Linux 6.1: close does not wake it, shutdown does, SO_RCVTIMEO does. */
+   On Linux 6.1: close does not wake it, the other three do. */
 #include <errno.h>
 #include <netinet/in.h>
 #include <pthread.h>
@@ -15,9 +19,12 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/socket.h>
+#include <signal.h>
 #include <sys/time.h>
 #include <unistd.h>
 
+#define WAKE_SIGNAL SIGUSR1
+
 static _Atomic int returned;
 static _Atomic int saved_errno;
 static int         sock;
@@ -31,6 +38,11 @@ static void       *reader(void *args) {
   return NULL;
 }
 
+/* A handler that does nothing is all it takes: the point is the return from
+   recvfrom with EINTR, not anything the handler does. SA_RESTART must stay
+   off or the kernel resumes the call instead of reporting it. */
+static void noop(int s) { (void)s; }
+
 static int bind_udp(int timeout_ms) {
   struct sockaddr_in address;
   int                s = socket(AF_INET, SOCK_DGRAM, 0);
@@ -50,7 +62,7 @@ static int bind_udp(int timeout_ms) {
   return s;
 }
 
-static void try_it(const char *name, int timeout_ms, int do_close, int do_shutdown) {
+static void try_it(const char *name, int timeout_ms, int do_close, int do_shutdown, int do_signal) {
   pthread_t thread;
   int       waited;
 
@@ -62,6 +74,8 @@ static void try_it(const char *name, int timeout_ms, int do_close, int do_shutdo
 
   if (do_shutdown)
     shutdown(sock, SHUT_RD);
+  if (do_signal)
+    pthread_kill(thread, WAKE_SIGNAL);
   if (do_close)
     close(sock);
 
@@ -92,8 +106,16 @@ static void try_it(const char *name, int timeout_ms, int do_close, int do_shutdo
 }
 
 int main(void) {
-  try_it("close() the socket", 0, 1, 0);
-  try_it("shutdown(SHUT_RD)", 0, 0, 1);
-  try_it("SO_RCVTIMEO 500 ms", 500, 0, 0);
+  struct sigaction sa;
+
+  memset(&sa, 0, sizeof(sa));
+  sa.sa_handler = noop;
+  sa.sa_flags   = 0;
+  sigaction(WAKE_SIGNAL, &sa, NULL);
+
+  try_it("close() the socket", 0, 1, 0, 0);
+  try_it("shutdown(SHUT_RD)", 0, 0, 1, 0);
+  try_it("SO_RCVTIMEO 500 ms", 500, 0, 0, 0);
+  try_it("pthread_kill(SIGUSR1)", 0, 0, 0, 1);
   return 0;
 }
-- 
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