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

ChangeLog.md

@@ -63,11 +63,15 @@ See docs/process.md for more on how version tagging works.
 - The `PThread.runningWorkers` field was removed from the `PThread` object.
   If you have JS code that was depending on this you can transition to using the
   `PThread.pthreads` object. (#26998)
+- 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)
 
 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

@@ -658,9 +658,14 @@ var SyscallsLibrary = {
     }
 #endif
 
+    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;
   },
   __syscall_getcwd__deps: ['$lengthBytesUTF8', '$stringToUTF8'],

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();

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/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);
   }