Avoid calling poll_oneoff with zero subscriptions.

With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.

libc-bottom-half/cloudlibc/src/libc/poll/poll.c

@@ -79,7 +79,13 @@ int poll(struct pollfd *fds, size_t nfds, int timeout) {
       __wasi_poll_oneoff(subscriptions, events, nevents, &nevents);
 #endif
   if (error != 0) {
-    errno = error;
+    // WASI's poll requires at least one subscription, or else it returns
+    // `EINVAL`. Since a `poll` with nothing to wait for is valid in POSIX,
+    // return `ENOTSUP` to indicate that we don't support that case.
+    if (nevents == 0)
+      errno = ENOTSUP;
+    else
+      errno = error;
     return -1;
   }
 

libc-bottom-half/cloudlibc/src/libc/sys/select/pselect.c

@@ -108,7 +108,13 @@ int pselect(int nfds, fd_set *restrict readfds, fd_set *restrict writefds,
       __wasi_poll_oneoff(subscriptions, events, nevents, &nevents);
 #endif
   if (error != 0) {
-    errno = error;
+    // WASI's poll requires at least one subscription, or else it returns
+    // `EINVAL`. Since a `pselect` with nothing to wait for is valid in POSIX,
+    // return `ENOTSUP` to indicate that we don't support that case.
+    if (nevents == 0)
+      errno = ENOTSUP;
+    else
+      errno = error;
     return -1;
   }
 

libc-top-half/musl/include/unistd.h

@@ -128,7 +128,9 @@ char *getcwd(char *, size_t);
 unsigned alarm(unsigned);
 #endif
 unsigned sleep(unsigned);
+#ifdef __wasilibc_unmodified_upstream /* WASI has no pause */
 int pause(void);
+#endif
 
 #ifdef __wasilibc_unmodified_upstream /* WASI has no fork/exec */
 pid_t fork(void);