fix assumptions about how fd_prestat_get may be called

[Techatrix]

The current implementation of fd_prestat_get does not appears to handle the following cases properly:

• fd_prestat_get is called on the same file descriptor again
• fd_prestat_get is called on stdio file descriptors (fd < 3)
• fd_prestat_get is called in non sequential order of file descriptors

For testing, I used the ./wasm-wasi/example project and modified the hello.c and extension.ts:

#include <stdio.h>
#include <string.h>

// Relies on https://github.com/WebAssembly/wasi-libc

int main() {
  int i = 2;
  while (i--) {
    __wasi_fd_t fd = 6;
    while (fd--) {
      __wasi_prestat_t prestat;
      __wasi_errno_t ret = __wasi_fd_prestat_get(fd, &prestat);
      printf("fd_prestat_get(%d): %s (%d)\n", fd, strerror(ret), ret);
    }
    printf("\n");
  }
}

I modified the call to createProcess in extension.ts to add an additional preopen directory:

const process = await wasm.createProcess('hello', module, {
	stdio: pty.stdio,
	trace: true,
	mountPoints: [{ kind: "memoryFileSystem", fileSystem: await wasm.createMemoryFileSystem(), mountPoint: "/foo" }]
});

Before:

fd_prestat_get(5): Success (0)
fd_prestat_get(4): Success (0)
fd_prestat_get(3): Bad file descriptor (8)
fd_prestat_get(2): Invalid argument (28)
fd_prestat_get(1): Invalid argument (28)
fd_prestat_get(0): Invalid argument (28)

fd_prestat_get(5): Invalid argument (28)
fd_prestat_get(4): Invalid argument (28)
fd_prestat_get(3): Invalid argument (28)
fd_prestat_get(2): Invalid argument (28)
fd_prestat_get(1): Invalid argument (28)
fd_prestat_get(0): Invalid argument (28)

After:

fd_prestat_get(5): Bad file descriptor (8)
fd_prestat_get(4): Success (0)
fd_prestat_get(3): Success (0)
fd_prestat_get(2): Bad file descriptor (8)
fd_prestat_get(1): Bad file descriptor (8)
fd_prestat_get(0): Bad file descriptor (8)

fd_prestat_get(5): Bad file descriptor (8)
fd_prestat_get(4): Success (0)
fd_prestat_get(3): Success (0)
fd_prestat_get(2): Bad file descriptor (8)
fd_prestat_get(1): Bad file descriptor (8)
fd_prestat_get(0): Bad file descriptor (8)

The new output matches exactly what I got from wasmtime with wasmtime run --dir=.::/foo --dir=.::/bar hello.wasm.

[andy0130tw]

I discovered that I have to deal with the same problem in my test program. However this broke the tests, as the tests all request fd=4 as the only preopen (the first preopen should be 3). Here is my patch to the test: agda-web@9681eb1.