catch and report out-of-memory error

[bob-carpenter]

This is from mippeqf (Markus) on the Stan forums:

When the storage directory set under model$sample(output_dir = …) is full, some chains stop unexpectedly with the error message basic_ios::clear: iostream error. If detectable, it would be great if the error message could pass this reason along.

[WardBrian]

One solution may be to install an extra top-level exception handler in main.cpp which is more specific:

// new:
  } catch (std::system_error &e) {
    // system_error is thrown by std::ofstream but the
    // message is not always helpful, so we also print errno
    std::cerr << e.what() << ": " << std::strerror(errno) << std::endl;
    return cmdstan::return_codes::NOT_OK;
// already there:
  } catch (const std::exception &e) {
    std::cerr << e.what() << std::endl;
    return cmdstan::return_codes::NOT_OK;
  }

Unfortunately, it seems like strerror(errno) is the only way to actually get something useful out of the errors that the iostream throws, but I see conflicting reports about how portable this is

[bob-carpenter]

I think you can make it more specific with bad_alloc. Doc is:

std::bad_alloc is the type of the object thrown as exceptions by the allocation functions to report failure to allocate storage.

[WardBrian]

That is for out of memory (RAM), not disk space

[bob-carpenter]

After some searching, it seems like @WardBrian's approach with std::strerror(errno) is the only thing that's viable for reporting the exact error. We could trap or inspect if there's been an error multiple ways:

• check for .fail() on the file output stream, https://en.cppreference.com/w/cpp/io/basic_ios/fail
• check for .bad() on the stream, https://en.cppreference.com/w/cpp/io/basic_ios/bad
• check forstd::ferror(f) on the file f if we open it with fopen: see https://en.cppreference.com/w/cpp/io/c/ferror