[libcxx] [test] [support] Use socket()+bind() to create unix sockets portably

[mgorny]

Summary

Replace the mknod() call with socket() + bind() for creating unix
sockets. The mknod() method is not portable and does not work
on NetBSD while binding the socket should work on all systems supporting
unix sockets.

// NB: I think this should solve the issues w/ FreeBSD/Darwin too but I don't have those systems to test

[mgorny]

@emaste, maybe you could take a look at this too. I think it would solve the problem for FreeBSD as well.

[dim]

LGTM.

[mgorny]

@dim, thanks for the review. Should I also try removing the following restriction?

#if !defined(__APPLE__) && !defined(__FreeBSD__) // No support for domain sockets
        {env.create_socket("socket"), file_type::socket},
#endif

[dim]

In D55576#1332370, @mgorny wrote:

@dim, thanks for the review. Should I also try removing the following restriction?

#if !defined(__APPLE__) && !defined(__FreeBSD__) // No support for domain sockets
        {env.create_socket("socket"), file_type::socket},
#endif

Well, test/support/filesystem_test_helper.hpp also has this comment:

  // OS X and FreeBSD doesn't support socket files so we shouldn't even
  // allow tests to call this unguarded.
#if !defined(__FreeBSD__) && !defined(__APPLE__)
    std::string create_socket(std::string file) {
        file = sanitize_path(std::move(file));
        fs_helper_run(fs_make_cmd("create_socket", file));
        return file;
    }
#endif

but it looks like you now fixed it with this change to filesystem_dynamic_test_helper.py. So I guess after this, it should be safe to remove the FreeBSD specific exception. I don't know about macOS, though.

test/support/filesystem_dynamic_test_helper.py

 import sys
 import os
+import socket
 import stat
 
 # Ensure that this is being run on a specific platform
 assert sys.platform.startswith('linux') or sys.platform.startswith('darwin') \
     or sys.platform.startswith('cygwin') or sys.platform.startswith('freebsd') \
     or sys.platform.startswith('netbsd')
 
 def env_path():
def create_hardlink(source, link):
     os.link(sanitize(source), sanitize(link))
 
 
 def create_fifo(source):
     os.mkfifo(sanitize(source))
 
 
 def create_socket(source):
-    mode = 0o600 | stat.S_IFSOCK
-    os.mknod(sanitize(source), mode)
+    sock = socket.socket(socket.AF_UNIX)
+    sanitized_source = sanitize(source)
+    # AF_UNIX sockets may have very limited path length, so split it
+    # into chdir call (with technically unlimited length) followed
+    # by bind() relative to the directory
+    os.chdir(os.path.dirname(sanitized_source))
+    sock.bind(os.path.basename(sanitized_source))
 
 
 if __name__ == '__main__':
     command = " ".join(sys.argv[1:])
     eval(command)
     sys.exit(0)