template_redirect to accompany with a shortcode

Question

In one of my plugin's shortcode I'm handing user login / registration on the fly with a post submission. So the whole thing is happening within the shortcode like:

<?php
function this_shortcode() {
    ob_start();
    //if isset(login)... process login, get $user_id
    //else if isset(registration)... process registration, get $user_id
    //else if already registered, get $current_user->ID as $user_id

    /**
     * if !empty($user_id){
     *  $p_id = wp_insert_post();
     *  if !empty($p_id) {
     *      echo success;
     *      **I NEED ONLY WHEN THE REDIRECTION SHOULD TAKE PLACE**
     *  } else {
     *      echo error;
     *  }
     * } else {
     * }
     */

    <form>
       <!-- registration/login + post insertion form here -->
    </form>

    return ob_get_clean();
}
add_shortcode( 'my-shortcode', 'this_shortcode' );

I've already learnt that, wp_redirect() is not the right way to redirect from shortcode. So, what I'm trying is:

function project_registration_login_redirection(){
    global $post;
    if( has_shortcode( $post->post_content, 'my-shortcode' ) ) {
        if ( isset( $_POST['my_submit'] ) ) {
            wp_redirect( home_url('/that-page') );
            exit();
        }
    }
}
add_action( 'template_redirect', 'project_registration_login_redirection' );

But it's clear that, this one redirects without any check, even though the form validation is showing error there, and the wp_insert_post() is returning empty.

How can I process the form somewhere, but do the redirection only when the form returns a valid feedback?

I tried:

global $post, $p_id;
if ( isset( $_POST['my_submit'] ) && !empty( $p_id ) ) {

but it's not working.

Answer 1

The answer below doesn't directly answer your question but provides a possible alternative solution.

Drop has_shortcode( $post->post_content, 'my-shortcode' ) statement. There is no need to check this if you are validating nonce. So the validation should look something like this.

function project_registration_login_redirection(){

    if ( !isset( $_POST['my_submit'] ) ) {
        return;
    }

    // form validation here
    global $errors; // a global var so you can show errors on your form if any

    // I usually have a seperate validation class but you may do it different way.
    // this method is useful to feed mock array for testing purpose
    $validator = new My_Form_validator();
    $errors = $validator->validate($_POST);

    if(!empty($errors){
        return;
    }

    // no error found

    $data = $validator->get_data();
    // process form data here

    wp_redirect(home_url('that-page'));
    exit;

}
add_action( 'template_redirect', 'project_registration_login_redirection' );

hope it helps.