client variable created in for loop is replacing previous one in previous thread

main.odin

package main

import "core:fmt"
import "core:net"
import "core:io"
import "core:thread"
import "core:time"
import "core:strings"

Client :: struct {
  socket: net.TCP_Socket,
  endpoint: net.Endpoint,
  endpoint_as_string: string,
}

new_client :: proc(socket: net.TCP_Socket, endpoint: net.Endpoint) -> Client {
  return Client{socket, endpoint, net.endpoint_to_string(endpoint)};
}

main :: proc() {
  endpoint, _ := net.parse_endpoint("127.0.0.1:8080");
  socket, err := net.listen_tcp(endpoint);
  defer net.close(socket);

  if err != nil {
    fmt.println("Error listening on socket: ", err);
    return;
  }

  fmt.println("Listening at ", net.endpoint_to_string(endpoint));

  for {
    connection, endpoint,  err := net.accept_tcp(socket);
    if err != nil {
      fmt.println("Error accepting connection: ", err);
      return;
    }

    client := new_client(connection, endpoint);
    fmt.println("Accepted connection from ", client.endpoint_as_string);

    t := thread.create(proc(t: ^thread.Thread) {
      client := cast (^Client) t.data;
      handle_client(client);
    });
    if t == nil {
      fmt.eprintln("Error creating thread");
      net.close(client.socket);
      continue;
    }


    t.user_index = len(threads);
    t.data = &client;
    append(&threads, t);
    thread.start(t);

    fmt.println("Started thread ", t.user_index);
    fmt.println("Number of threads: ", len(threads));
    fmt.println(&client);
  }
}

threads : [dynamic]^thread.Thread;

@(init)
create_thread_pool :: proc() {
    threads := make([dynamic]^thread.Thread, 0);
    thread_cleaner();
}

destroy_thread_pool :: proc() {
    delete(threads);
}

thread_cleaner :: proc() {
  t := thread.create(proc(t : ^thread.Thread) {
    for {
      time.sleep(1 * time.Second);

      if len(threads) == 0 {
        continue;
      }

			for i := 0; i < len(threads); {
				if t := threads[i]; thread.is_done(t) {
					fmt.printf("Thread %d is done\n", t.user_index);
					thread.destroy(t);

					ordered_remove(&threads, i);
				}
				else {
          i += 1;
        }
			}
		}
  });
  thread.start(t);
}


handle_client :: proc(client: ^Client) {
  buffer_size :: 10240;
  read_buffer : [buffer_size]byte = [buffer_size]byte{};
  bytes_read : int = 0;

  err : net.Network_Error = nil;

  for {
    bytes_read, err = net.recv_tcp(client.socket, read_buffer[:]);

    if err != nil {
      fmt.println("Error reading from socket: ", err);
      break;
    }

    if bytes_read == 0 {
      break;
    }

    net.send_tcp(client.socket, []u8{'o', 'k', '\n'});
    fmt.println("Received message from ", client.endpoint_as_string, ":", string(read_buffer[:]));
  }

  fmt.println("Closing connection with ", client.endpoint_as_string);
  net.close(client.socket);
}

solution.odin

package main

import "core:fmt"
import "core:net"
import "core:io"
import "core:thread"
import "core:time"
import "core:strings"

Client :: struct {
  socket: net.TCP_Socket,
  endpoint: net.Endpoint,
  endpoint_as_string: string,
}

new_client :: proc(socket: net.TCP_Socket, endpoint: net.Endpoint) -> ^Client {
  c := new(Client);
  c.socket = socket;
  c.endpoint = endpoint;
  c.endpoint_as_string = net.endpoint_to_string(endpoint);
  return c;
}

main :: proc() {
  endpoint, _ := net.parse_endpoint("127.0.0.1:8080");
  socket, err := net.listen_tcp(endpoint);
  defer net.close(socket);

  if err != nil {
    fmt.println("Error listening on socket: ", err);
    return;
  }

  fmt.println("Listening at ", net.endpoint_to_string(endpoint));

  for {
    connection, endpoint,  err := net.accept_tcp(socket);
    if err != nil {
      fmt.println("Error accepting connection: ", err);
      return;
    }

    client := new_client(connection, endpoint);
    fmt.println("Accepted connection from ", client.endpoint_as_string);

    t := thread.create(proc(t: ^thread.Thread) {
      client := cast (^Client) t.data;
      handle_client(client);
    });
    if t == nil {
      fmt.eprintln("Error creating thread");
      net.close(client.socket);
      continue;
    }


    t.user_index = len(threads);
    t.data = client;
    append(&threads, t);
    thread.start(t);

    fmt.println("Started thread ", t.user_index);
    fmt.println("Number of threads: ", len(threads));
    fmt.println(client);
  }
}

threads : [dynamic]^thread.Thread;

@(init)
create_thread_pool :: proc() {
    threads := make([dynamic]^thread.Thread, 0);
    thread_cleaner();
}

destroy_thread_pool :: proc() {
    delete(threads);
}

thread_cleaner :: proc() {
  t := thread.create(proc(t : ^thread.Thread) {
    for {
      time.sleep(1 * time.Second);

      if len(threads) == 0 {
        continue;
      }

			for i := 0; i < len(threads); {
				if t := threads[i]; thread.is_done(t) {
					fmt.printf("Thread %d is done\n", t.user_index);
					thread.destroy(t);

					ordered_remove(&threads, i);
				}
				else {
          i += 1;
        }
			}
		}
  });
  thread.start(t);
}


handle_client :: proc(client: ^Client) {
  buffer_size :: 10240;
  read_buffer : [buffer_size]byte = [buffer_size]byte{};
  bytes_read : int = 0;

  err : net.Network_Error = nil;

  for {
    bytes_read, err = net.recv_tcp(client.socket, read_buffer[:]);

    if err != nil {
      fmt.println("Error reading from socket: ", err);
      break;
    }

    if bytes_read == 0 {
      break;
    }

    net.send_tcp(client.socket, []u8{'o', 'k', '\n'});
    fmt.println("Received message from ", client.endpoint_as_string, ":", string(read_buffer[:]));
  }

  fmt.println("Closing connection with ", client.endpoint_as_string);
  net.close(client.socket);
}

Explanation of solution:

so Client{} vs new(Client) is like a:
creating and returning value 
vs 
creating value and putting to heap and returning address to it?

in main.odin we were returning instance of struct as value and putting value to same stack variable client, so in next iteration we were rewriting same stack variable with new value.

in solution.odin we create instance of struct and put it to heap and return pointer (^Client) which stack variable client is holding and passing it to thread. so next iteration simply replaces stack variable with new pointer which means previous instance is living in different memory address than new.