[libcxx] Avoid double frees of file descriptors in the fallback ifstream/ofstream codepath

[ldionne]

Do I understand this is triggered by the test suite when we build the dylib with _LIBCPP_FILESYSTEM_USE_FSTREAM, and so this doesn't need a test added?

If so, LGTM, otherwise please add a test.

[mstorsjo]

In D90601#2369985, @ldionne wrote:

Do I understand this is triggered by the test suite when we build the dylib with _LIBCPP_FILESYSTEM_USE_FSTREAM, and so this doesn't need a test added?

Indeed, it's triggered by some of the fs.op.copy* tests, but whether the C runtime errors out on it (and how loudly) varies I guess. Afaik most of the actual deployments of libc++ std::fs so far has been using the copyfile/sendfile implementations.

libcxx/src/filesystem/operations.cpp

#elif defined(_LIBCPP_FILESYSTEM_USE_FSTREAM)
   bool copy_file_impl(FileDescriptor& read_fd, FileDescriptor& write_fd, error_code& ec) {
     ifstream in;
     in.__open(read_fd.fd, ios::binary);
     if (!in.is_open()) {
       // This assumes that __open didn't reset the error code.
       ec = capture_errno();
       return false;
     }
+    read_fd.fd = -1;
     ofstream out;
     out.__open(write_fd.fd, ios::binary);
     if (!out.is_open()) {
       ec = capture_errno();
       return false;
     }
+    write_fd.fd = -1;
 
     if (in.good() && out.good()) {
       using InIt = istreambuf_iterator<char>;
       using OutIt = ostreambuf_iterator<char>;
       InIt bin(in);
       InIt ein;
       OutIt bout(out);
       copy(bin, ein, bout);