fcntl cannot clear `O_NONBLOCK` once it has been set

[hly2019]

Version of emscripten/emsdk:

5.0.6

Basically, the pattern:

int fl = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, fl | O_NONBLOCK);
/* ... */
fcntl(fd, F_SETFL, fl & ~O_NONBLOCK); // switch back to blocking

works on Linux but not under Emscripten. The subsequent F_GETFL still reports O_NONBLOCK set.

Failing command line in full:

$ emcc -O0 -Wall fsetfl.c -o a.out.js
$ node a.out.js
after-set   O_NONBLOCK=1 (expect 1)
after-clear O_NONBLOCK=1 (expect 0)

The same program under native clang prints after-clear O_NONBLOCK=0 as expected.

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(void) {
    int fd = open("/tmp/x", O_RDWR | O_CREAT | O_TRUNC, 0644);
    int fl0 = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, fl0 | O_NONBLOCK);
    int fl1 = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, fl1 & ~O_NONBLOCK);
    int fl2 = fcntl(fd, F_GETFL, 0);
    printf("after-set   O_NONBLOCK=%d (expect 1)\n", (fl1 & O_NONBLOCK) != 0);
    printf("after-clear O_NONBLOCK=%d (expect 0)\n", (fl2 & O_NONBLOCK) != 0);
    close(fd);
    return 0;
}