ftruncate on a writable fd fails when the file's mode lacks write bits

[Ryan2026R]

Version of emscripten/emsdk:

5.0.6

The write access is determined at open() time in POSIX: once a fd is opened O_WRONLY/O_RDWR, write() and ftruncate() must succeed regardless of the file's current mode bits. Under MEMFS, it seems that ftruncate() re-checks the mode via FS.nodePermissions(node, 'w') and fails with EACCES even though write() on the same fd succeeded.

Failing command line in full:

$ emcc -O0 -Wall ftrunc.c -o a.out.js
$ node a.out.js
write: ret=5 (expected 5)
ftruncate: ret=-1 errno=2

The same program on native Linux prints ftruncate: ret=0 errno=0.

Example code snippet:

#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdio.h>
#include <errno.h>

int main(void) {
    mkdir("/tmp/xsv_test", 0755);
    int fd = open("/tmp/xsv_test/test_perm.txt",
                  O_WRONLY | O_CREAT | O_TRUNC, 0444); // read-only
    ssize_t w = write(fd, "hello", 5); // succeeds
    printf("write: ret=%zd (expected 5)\n", w);
    errno = 0;
    int ret = ftruncate(fd, 0); // fails on Emscripten MEMFS
    printf("ftruncate: ret=%d errno=%d\n", ret, errno);
    close(fd);
    return (ret == 0) ? 0 : 1;
}

Under -sNODERAWFS=1 the Node-backed ftruncate works correctly.