Introduce interprocess signaling on Windows

This change gets rsync working without any warning or errors. On Windows
we now create a bunch of C:\var\sig\x\y.pid shared memory files, so sigs
can be delivered between processes. WinMain() creates this file when the
process starts. If the program links signaling system calls then we make
a thread at startup too, which allows asynchronous delivery each quantum
and cancelation points can spot these signals potentially faster on wait

See #1240

test/libc/calls/sigsuspend_test.c

@@ -76,10 +76,6 @@ TEST(sigsuspend, testSignalQueuingSelf) {
 }
 
 TEST(sigsuspend, testSignalQueuingIpc) {
-  if (IsWindows()) {
-    // xxx: probably need a signal server to do this kind of signalling
-    return;
-  }
   int pid, ws;
   sigset_t neu, old, bits;
   struct sigaction oldusr1, oldusr2;

test/libc/proc/fork_test.c

@@ -103,8 +103,6 @@ static void OnSigusr2(int sig) {
 }
 
 TEST(fork, childToChild) {
-  if (IsWindows())
-    return;  // :'(
   sigset_t mask, oldmask;
   int ws, parent, child1, child2;
   gotsigusr1 = false;

test/libc/proc/handkill_test.c

@@ -125,8 +125,6 @@ TEST(handkill, thread2thread_async) {
 }
 
 TEST(handkill, process_async) {
-  if (IsWindows())
-    return;
   SPAWN(fork);
   shm->ready = true;
   while (!shm->got_signal)
@@ -142,8 +140,6 @@ TEST(handkill, process_async) {
 }
 
 TEST(handkill, process_pause) {
-  if (IsWindows())
-    return;
   SPAWN(fork);
   shm->ready = true;
   pause();

test/libc/proc/posix_spawn_test.c

@@ -262,8 +262,6 @@ void EmptySigHandler(int sig) {
 }
 
 TEST(posix_spawn, etxtbsy) {
-  if (IsWindows())
-    return;  // can't deliver signals between processes
   if (IsXnu())
     return;  // they don't appear impacted by this race condition
   if (IsNetbsd())

test/libc/proc/system_test.c

@@ -166,13 +166,12 @@ TEST(system, notequals) {
 }
 
 TEST(system, usleep) {
-  ASSERT_EQ(0, GETEXITSTATUS(system("usleep & kill $!")));
+  ASSERT_EQ(0, GETEXITSTATUS(system("usleep & kill $!; wait")));
 }
 
 TEST(system, kill) {
   int ws = system("kill -TERM $$; usleep");
-  if (!IsWindows())
-    ASSERT_EQ(SIGTERM, WTERMSIG(ws));
+  ASSERT_EQ(SIGTERM, WTERMSIG(ws));
 }
 
 TEST(system, exitStatusPreservedAfterSemiColon) {

test/libc/stdio/popen_test.c

@@ -120,8 +120,6 @@ void OnSig(int sig) {
 }
 
 TEST(popen, complicated) {
-  if (IsWindows())
-    return;  // windows treats sigusr1 as terminate
   char cmd[64];
   signal(SIGUSR1, OnSig);
   sprintf(cmd, "read a ; test \"x$a\" = xhello && kill -USR1 %d", getpid());

test/posix/interprocess_signaling_test.c

@@ -0,0 +1,67 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <signal.h>
+#include <stdatomic.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+atomic_int *got;
+
+void onsig(int sig) {
+  *got = sig;
+}
+
+int main(int argc, char *argv[]) {
+
+  // create process shared memory
+  got = mmap(0, 4, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+  if (got == MAP_FAILED)
+    return 5;
+
+  // listen for signal
+  if (signal(SIGUSR1, onsig))
+    return 6;
+
+  // block signals
+  sigset_t full;
+  if (sigfillset(&full))
+    return 7;
+  if (sigprocmask(SIG_BLOCK, &full, 0))
+    return 8;
+
+  // create child process
+  int pid;
+  if (!(pid = fork())) {
+    sigset_t empty;
+    sigemptyset(&empty);
+    sigsuspend(&empty);
+    *got |= 128;
+    _exit(0);
+  }
+
+  // send signal
+  if (kill(pid, SIGUSR1))
+    return 9;
+
+  // wait for child to die
+  int ws;
+  if (wait(&ws) != pid)
+    return 10;
+  if (ws)
+    return 11;
+  if (*got != (128 | SIGUSR1))
+    return 12;
+}