From 11ec052f76954720524b22d07eb775aa41feffcd Mon Sep 17 00:00:00 2001
From: Dan Gohman <sunfish@mozilla.com>
Date: Thu, 16 Jan 2020 14:52:04 -0800
Subject: [PATCH] Avoid calling `poll_oneoff` with zero subscriptions.

With https://github.com/WebAssembly/WASI/pull/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    |  8 +++++++-
 .../cloudlibc/src/libc/sys/select/pselect.c        |  8 +++++++-
 libc-bottom-half/sources/pause.c                   | 14 --------------
 libc-top-half/musl/include/unistd.h                |  2 ++
 4 files changed, 16 insertions(+), 16 deletions(-)
 delete mode 100644 libc-bottom-half/sources/pause.c

diff --git a/libc-bottom-half/cloudlibc/src/libc/poll/poll.c b/libc-bottom-half/cloudlibc/src/libc/poll/poll.c
index be76ecd7c..8d7fcaa2b 100644
--- a/libc-bottom-half/cloudlibc/src/libc/poll/poll.c
+++ b/libc-bottom-half/cloudlibc/src/libc/poll/poll.c
@@ -64,7 +64,13 @@ 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;
   }
 
diff --git a/libc-bottom-half/cloudlibc/src/libc/sys/select/pselect.c b/libc-bottom-half/cloudlibc/src/libc/sys/select/pselect.c
index bd1d2fdc0..9ab292aa8 100644
--- a/libc-bottom-half/cloudlibc/src/libc/sys/select/pselect.c
+++ b/libc-bottom-half/cloudlibc/src/libc/sys/select/pselect.c
@@ -90,7 +90,13 @@ 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;
   }
 
diff --git a/libc-bottom-half/sources/pause.c b/libc-bottom-half/sources/pause.c
deleted file mode 100644
index 0e3e71250..000000000
--- a/libc-bottom-half/sources/pause.c
+++ /dev/null
@@ -1,14 +0,0 @@
-#include <stddef.h>
-#include <errno.h>
-#include <wasi/api.h>
-#include <unistd.h>
-
-int pause(void) {
-    size_t n;
-    __wasi_errno_t error = __wasi_poll_oneoff(0, 0, 0, &n);
-    if (error != 0) {
-        errno = error;
-        return -1;
-    }
-    __builtin_trap();
-}
diff --git a/libc-top-half/musl/include/unistd.h b/libc-top-half/musl/include/unistd.h
index c6fe070bb..e55f638e9 100644
--- a/libc-top-half/musl/include/unistd.h
+++ b/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);