WASMFS fdatasync(bad_fd) returns positive value instead of `-1`

[hly2019]

Version of emscripten/emsdk:

5.0.6

POSIX: fdatasync() on an invalid fd must return -1 with errno set. Under WASMFS where -sWASMFS=1, fdatasync() returns a positive value and leaves errno = 0.

Failing command line in full:

$ emcc -O0 -Wall -sWASMFS=1 fdatasync.c -o a.out.js
$ node a.out.js
fdatasync(9999): ret=8 errno=0

The same program on native Linux prints fdatasync(9999): ret=-1 errno=9. Default emcc builds also print fdatasync(9999): ret=-1 errno=8.

Example code snippet:

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

int main(void) {
    errno = 0;
    int r = fdatasync(9999);
    printf("fdatasync(9999): ret=%d errno=%d\n", r, errno);
    return (r == -1) ? 0 : 1;
}