Update poll/select/pause to return -EINTR when they cannot block

ChangeLog.md

@@ -33,6 +33,10 @@ See docs/process.md for more on how version tagging works.
   out to `wasm-bindgen` in the users's path and integrate the wasm-bindgen JS
   with the normal Emscripten JS. Some wasm-bindgen features may not yet be fully
   supported. (#23493)
+- The select() and poll() syscalls now fail with EINTR when no FDs are active
+  and they are asked to block in a build that does not support blocking (i.e.
+  now JSPI or ASYNCIFY).  Previously they would return 0 but without any result
+  FSs set (i.e. as if the timeout at expired). (#27049)
 
 6.0.0 - 06/04/26
 ----------------
@@ -84,7 +88,7 @@ See docs/process.md for more on how version tagging works.
 5.0.7 - 04/30/26
 ----------------
 - mimalloc was updated to 3.3.1. (#26696)
-- The `WASM_JS_TYPES` setting was removed, as the corresponsing propsal was
+- The `WASM_JS_TYPES` setting was removed, as the corresponding proposal was
   pushed back to phase 1. (#26739)
 - The `-sDETERMINISTIC` setting was removed.  This setting just injected
   `src/deterministic.js` as a `--pre-js`.  For now, this file remains part of

src/lib/libsyscall.js

@@ -634,9 +634,15 @@ var SyscallsLibrary = {
 #endif
 
     var count = doPoll(fds, nfds, 0, undefined);
+    if (!count && timeout) {
+      // We cannot actually block here since we are not in an async context,
+      // so return -EINTR, as if we were inturrupted by a signal.
 #if ASSERTIONS
-    if (!count && timeout != 0) warnOnce('non-zero poll() timeout not supported: ' + timeout)
+      warnOnce('non-zero poll() timeout not supported: ' + timeout)
+
 #endif
+      return -{{{ cDefs.EINTR }}};
+    }
     return count;
   },
   // The shared readiness derivation: one pass over the pollfds, writing

system/lib/libc/musl/src/select/select.c

@@ -22,7 +22,7 @@ int select(int n, fd_set *restrict rfds, fd_set *restrict wfds, fd_set *restrict
 
 	if (s<0 || us<0) return __syscall_ret(-EINVAL);
 #ifdef __EMSCRIPTEN__
-	return emscripten_select(n, rfds, wfds, efds, tv);
+	return __syscall_ret(emscripten_select(n, rfds, wfds, efds, tv));
 #else
 	if (us/1000000 > max_time - s) {
 		s = max_time;
@@ -85,7 +85,7 @@ static int emscripten_select(int nfds, fd_set *readfds, fd_set *writefds, fd_set
 	int rtn = __syscall_poll((intptr_t)fds, n, timeout);
 	if (rtn < 0) {
 		free(fds);
-		return -1;
+		return rtn;
 	}
 
 	// Part 2: Translate the result of poll into the results of select();

system/lib/wasmfs/syscalls.cpp

@@ -1391,7 +1391,7 @@ int __syscall_poll(intptr_t fds_, int nfds, int timeout) {
 
   // Process the list of FDs and compute their revents masks. Count the number
   // of nonzero such masks, which is our return value.
-  int nonzero = 0;
+  int count = 0;
   for (nfds_t i = 0; i < nfds; i++) {
     auto* pollfd = &fds[i];
     auto fd = pollfd->fd;
@@ -1426,14 +1426,17 @@ int __syscall_poll(intptr_t fds_, int nfds, int timeout) {
     // TODO: set the state based on the state of the other end of the pipe, for
     //       pipes (POLLERR | POLLHUP)
     if (mask) {
-      nonzero++;
+      count++;
     }
     pollfd->revents = mask;
   }
   // TODO: This should block based on the timeout. The old FS did not do so due
   //       to web limitations, which we should perhaps revisit (especially with
   //       pthreads and asyncify).
-  return nonzero;
+  if (timeout && !count) {
+    return -EINTR;
+  }
+  return count;
 }
 
 // libc routes zero-timeout poll() calls here (see musl's poll.c). WasmFS's

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
-  "a.out.js": 268274,
-  "a.out.nodebug.wasm": 587640,
-  "total": 855914,
+  "a.out.js": 268293,
+  "a.out.nodebug.wasm": 587659,
+  "total": 855952,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/core/test_pipe_select.c

@@ -5,6 +5,10 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <string.h>
+#include <errno.h>
+#ifdef __EMSCRIPTEN__
+#include <emscripten/threading.h>
+#endif
 
 int pipe_a[2];
 
@@ -35,6 +39,40 @@ int main() {
   assert(get_available(pipe_a[0]) == strlen(t));
   assert(get_available(pipe_a[1]) == strlen(t));
 
+
+
+  // Test select with timeout when no FDs are ready
+  {
+    int pipe_b[2];
+    assert(pipe(pipe_b) == 0);
+
+    fd_set fds;
+    FD_ZERO(&fds);
+    FD_SET(pipe_b[0], &fds);
+    struct timeval tv = {0, 10000}; // 10ms
+
+#ifdef __EMSCRIPTEN__
+    if (emscripten_is_main_runtime_thread()) {
+      printf("Main thread: expecting EINTR\n");
+      int res = select(pipe_b[0] + 1, &fds, NULL, NULL, &tv);
+      assert(res == -1);
+      assert(errno == EINTR);
+    } else {
+      printf("Worker thread: expecting timeout\n");
+      int res = select(pipe_b[0] + 1, &fds, NULL, NULL, &tv);
+      assert(res == 0);
+    }
+#else
+    // Native: should timeout
+    printf("Native: expecting timeout\n");
+    int res = select(pipe_b[0] + 1, &fds, NULL, NULL, &tv);
+    assert(res == 0);
+#endif
+
+    close(pipe_b[0]);
+    close(pipe_b[1]);
+  }
+
   close(pipe_a[0]);
   close(pipe_a[1]);
   return 0;

test/sockets/sdl2_net_server.c

@@ -62,7 +62,7 @@ void main_loop() {
 
   /* Check the sd if there is a pending connection.
    * If there is one, accept that, and open a new socket for communicating */
-  SDLNet_CheckSockets(state.socketSet, 20);
+  SDLNet_CheckSockets(state.socketSet, 0);
   int serverSocketActivity = SDLNet_SocketReady(state.sd);
 
   if (serverSocketActivity)

test/sockets/test_sockets_partial_client.c

@@ -45,9 +45,14 @@ void iter() {
   FD_SET(sockfd, &fdr);
   res = select(64, &fdr, NULL, NULL, NULL);
   if (res == -1) {
-    perror("select failed");
-    finish(EXIT_FAILURE);
-  } else if (!FD_ISSET(sockfd, &fdr)) {
+    if (errno != EINTR) {
+      perror("select failed");
+      finish(EXIT_FAILURE);
+    }
+    return;
+  }
+
+  if (!FD_ISSET(sockfd, &fdr)) {
     return;
   }
 

test/sockets/test_sockets_partial_server.c

@@ -82,6 +82,9 @@ void iter() {
   if (clientfd) FD_SET(clientfd, &fdw);
   res = select(64, &fdr, &fdw, NULL, NULL);
   if (res == -1) {
+    if (errno == EINTR) {
+      return;
+    }
     perror("select failed");
     exit(EXIT_SUCCESS);
   }

test/sockets/test_sockets_select_server_closes_connection_client_rw.c

@@ -44,6 +44,7 @@ void main_loop() {
   int selectRes;
   ssize_t transferAmount;
   fd_set sett;
+  struct timeval zero_timeout = {0, 0};
 
   switch (state) {
     case 0:
@@ -53,7 +54,7 @@ void main_loop() {
       // select should tell us 0 handles are ready
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, &sett, NULL, NULL, NULL);
+      selectRes = select(64, &sett, NULL, NULL, &zero_timeout);
       if (selectRes != 0) {
         printf("case 0: read select != 0 (%d)\n", selectRes);
         finish(EXIT_FAILURE);
@@ -63,7 +64,7 @@ void main_loop() {
       // the connection either is setting up or is established and writing is possible
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, NULL, &sett, NULL, NULL);
+      selectRes = select(64, NULL, &sett, NULL, &zero_timeout);
       if (selectRes == -1) {
         printf("case 0: write select == -1\n");
         finish(EXIT_FAILURE);
@@ -86,7 +87,7 @@ void main_loop() {
       // has sent the data and then closed the connection
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, &sett, NULL, NULL, NULL);
+      selectRes = select(64, &sett, NULL, NULL, &zero_timeout);
       if (selectRes == -1) {
         printf("case 1: read selectRes == -1\n");
         finish(EXIT_FAILURE);
@@ -114,7 +115,7 @@ void main_loop() {
       // succeed, but the socket should not set in the set.
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, NULL, &sett, NULL, NULL);
+      selectRes = select(64, NULL, &sett, NULL, &zero_timeout);
       if (selectRes != 0 || FD_ISSET(sockfd, &sett)) {
         printf("case 2: write selectRes != 0 || FD_ISSET(sockfd, &sett)\n");
         finish(EXIT_FAILURE);
@@ -124,7 +125,7 @@ void main_loop() {
       // has to succeed because there is still data in the inQueue
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, &sett, NULL, NULL, NULL);
+      selectRes = select(64, &sett, NULL, NULL, &zero_timeout);
       if (selectRes != 1) {
         printf("case 2: read selectRes != 1\n");
         finish(EXIT_FAILURE);
@@ -152,7 +153,7 @@ void main_loop() {
       // should succeed
       FD_ZERO(&sett);
       FD_SET(sockfd, &sett);
-      selectRes = select(64, &sett, NULL, NULL, NULL);
+      selectRes = select(64, &sett, NULL, NULL, &zero_timeout);
       if (selectRes != 1) {
         printf("case 3: read selectRes != 1\n");
         finish(EXIT_FAILURE);

test/unistd/misc.out

@@ -10,7 +10,7 @@ link: -1, errno: 34
 lockf(good): 0, errno: 0
 lockf(bad): -1, errno: 8
 nice: -1, errno: 63
-pause: 0, errno: 0
+pause: -1, errno: 27
 pipe(good): 0, errno: 0
 pipe(bad): -1, errno: 21
 pipe2(good): 0, errno: 0