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function wp_ajax_add_meta() {
	check_ajax_referer( 'add-meta', '_ajax_nonce-add-meta' );
	$c    = 0;
	$pid  = (int) $_POST['post_id'];
	$post = get_post( $pid );

	if ( isset( $_POST['metakeyselect'] ) || isset( $_POST['metakeyinput'] ) ) {
		if ( ! current_user_can( 'edit_post', $pid ) ) {
			wp_die( -1 );
		}

		if ( isset( $_POST['metakeyselect'] ) && '#NONE#' === $_POST['metakeyselect'] && empty( $_POST['metakeyinput'] ) ) {
			wp_die( 1 );
		}

		// If the post is an autodraft, save the post as a draft and then attempt to save the meta.
		if ( 'auto-draft' === $post->post_status ) {
			$post_data                = array();
			$post_data['action']      = 'draft'; // Warning fix.
			$post_data['post_ID']     = $pid;
			$post_data['post_type']   = $post->post_type;
			$post_data['post_status'] = 'draft';
			$now                      = time();

			$post_data['post_title'] = sprintf(
				/* translators: 1: Post creation date, 2: Post creation time. */
				__( 'Draft created on %1$s at %2$s' ),
				gmdate( __( 'F j, Y' ), $now ),
				gmdate( __( 'g:i a' ), $now )
			);

			$pid = edit_post( $post_data );

			if ( $pid ) {
				if ( is_wp_error( $pid ) ) {
					$x = new WP_Ajax_Response(
						array(
							'what' => 'meta',
							'data' => $pid,
						)
					);
					$x->send();
				}

				$mid = add_meta( $pid );
				if ( ! $mid ) {
					wp_die( __( 'Please provide a custom field value.' ) );
				}
			} else {
				wp_die( 0 );
			}
		} else {
			$mid = add_meta( $pid );
			if ( ! $mid ) {
				wp_die( __( 'Please provide a custom field value.' ) );
			}
		}

		$meta = get_metadata_by_mid( 'post', $mid );
		$pid  = (int) $meta->post_id;
		$meta = get_object_vars( $meta );

		$x = new WP_Ajax_Response(
			array(
				'what'         => 'meta',
				'id'           => $mid,
				'data'         => _list_meta_row( $meta, $c ),
				'position'     => 1,
				'supplemental' => array( 'postid' => $pid ),
			)
		);
	} else { // Update?
		$mid   = (int) key( $_POST['meta'] );
		$key   = wp_unslash( $_POST['meta'][ $mid ]['key'] );
		$value = wp_unslash( $_POST['meta'][ $mid ]['value'] );

		if ( '' === trim( $key ) ) {
			wp_die( __( 'Please provide a custom field name.' ) );
		}

		$meta = get_metadata_by_mid( 'post', $mid );

		if ( ! $meta ) {
			wp_die( 0 ); // If meta doesn't exist.
		}

		if (
			is_protected_meta( $meta->meta_key, 'post' ) || is_protected_meta( $key, 'post' ) ||
			! current_user_can( 'edit_post_meta', $meta->post_id, $meta->meta_key ) ||
			! current_user_can( 'edit_post_meta', $meta->post_id, $key )
		) {
			wp_die( -1 );
		}

		if ( $meta->meta_value !== $value || $meta->meta_key !== $key ) {
			$u = update_metadata_by_mid( 'post', $mid, $value, $key );
			if ( ! $u ) {
				wp_die( 0 ); // We know meta exists; we also know it's unchanged (or DB error, in which case there are bigger problems).
			}
		}

		$x = new WP_Ajax_Response(
			array(
				'what'         => 'meta',
				'id'           => $mid,
				'old_id'       => $mid,
				'data'         => _list_meta_row(
					array(
						'meta_key'   => $key,
						'meta_value' => $value,
						'meta_id'    => $mid,
					),
					$c
				),
				'position'     => 0,
				'supplemental' => array( 'postid' => $meta->post_id ),
			)
		);
	}
	$x->send();
}