Fix fileobj I/O un-deterministic behavior

[mthrok]

Ever since the file-like object support was added in #1158, the test
was occasionally failing in CI. This PR fixes this.

What issues are resolved by this PR?

The failing tests exhibit flakiness in the following three areas;

1. info_test
   For mp3 and vorbis formats, the number of reported frames were most times zero (this is due to the container format), but occasionally, the test reported a correct number. It should be always reporting the same number.
2. load_test
   The shape of the Tensor loaded with load function occasionally had different number of frames than what actually the file has.
3. save_test
   The saved data occasionally lacked some frames at the end.

On which environment were the issues happening?

unix system, all Python environment

What was the cause and what is the fix?

When opening file (for both read and write), libsox initializes the dedicated data structure called sox_format_t, and set various attributes. One of which is seekable and this value is supposed to be false for file-like objects. In the investigation #1229, it turned out that this attribute is occasionally true which contributed to the un-deterministic behavior described above. The following code from libsox determines if the opened FILE* is seekable, by simply checking if the associated file descriptor is a regular file. [src]

static sox_bool is_seekable(sox_format_t const * ft)
{
  struct stat st;

  assert(ft);
  if (!ft->fp)
    return sox_false;
  fstat(fileno((FILE*)ft->fp), &st);
  return ((st.st_mode & S_IFMT) == S_IFREG);
}

The problem is that fileno returns -1 for the case of file-like object. This is because for file-like object, we use sox_open_mem_read or sox_open_mem_stream_write, which calls fmemopen and open_memstream respectively, and the resulting *FILE object does not have the associated file descriptor. In this case, fstat received -1 as the file descriptor value and (I guess) the stat structure is untouched. Thus the attribute of st_mode has an un-initialized value, which result in un-deterministic behavior.

This PR adds patch to check the the value of file descriptor and mark seekable=false if it does not have a valid file descriptor.

How do we know this patch fixes the issue?

I took the detailed note on #1229. The following script will keep running the tests.

https://github.com/mthrok/audio/blob/10ba189d8bc5f0beb5f7200fcb59778acc490033/debug_fileobj.sh#L1

Before applying the patch, it typically took ~15 mins for one of the tests to fail. After this patch is applied, they do not fail. I kept running the test over night and none of the tests failed.

TODOs

• Cherry-pick to release/0.8
• Upstream the patch to sox project
  • ⚠️ the upstream has a fix https://sourceforge.net/p/sox/code/ci/bb38934e11035c8fab141f70dabda3afdd17da36/ , but this fix seems to make is_seekable return true for the in-memory file object case (need to verify this), which is the opposite of the behavior we want in torchaudio implementation.
• Fix in fbcode