umockdev-record: Check attributes with select() before read()

This might help against attributes which immediately block a read() even
in non-blocking mode.

See issue #81

src/umockdev-record.vala

@@ -216,6 +216,11 @@ print_device_attributes(string devpath, string subdir)
                 continue;
             }
 
+            // trying to read some attributes hangs forever even with O_NONBLOCK, so check if there's anything
+            Posix.fd_set read_fds;
+            Posix.FD_ZERO(out read_fds);
+            Posix.FD_SET(fd, ref read_fds);
+            if (Posix.select(fd + 1, &read_fds, null, null, {0, 0}) > 0) {
                 ssize_t contents_len = Posix.read(fd, contents, contents.length - 1);
                 if (contents_len >= 0) {
                     contents[contents_len] = 0;
@@ -223,6 +228,7 @@ print_device_attributes(string devpath, string subdir)
                 } else {
                     debug("Cannot read %s, ignoring: %s", attr_path, strerror(errno));
                 }
+            }
 
             Posix.close(fd);
         } else if (FileUtils.test(attr_path, FileTest.IS_DIR)) {