fix(balancer) remove old host when deleting target

[GoneGo1ng]

When deleting target, the old host should be removed. Instead of forcibly recreating a balancer.

Fix #6983

[CLAassistant]

All committers have signed the CLA.

[GoneGo1ng]

@fffonion please review this change.

[gszr]

As #6983 turned out to be a documentation issue, and is now closed, this seems unrelated. @GoneGo1ng, please add further info. Thanks!

[GoneGo1ng]

@gszr Sorry, I replied too late.

This is my upstream details.

curl -X GET http://172.16.192.153:8001/upstreams/test | python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   901  100   901    0     0   9963      0 --:--:-- --:--:-- --:--:--  9901
{
    "algorithm": "round-robin",
    "client_certificate": null,
    "created_at": 1624244553,
    "hash_fallback": "none",
    "hash_fallback_header": null,
    "hash_on": "none",
    "hash_on_cookie": null,
    "hash_on_cookie_path": "/",
    "hash_on_header": null,
    "healthchecks": {
        "active": {
            "concurrency": 10,
            "healthy": {
                "http_statuses": [
                    200,
                    302
                ],
                "interval": 5,
                "successes": 1
            },
            "http_path": "/hello",
            "https_sni": null,
            "https_verify_certificate": false,
            "timeout": 1,
            "type": "http",
            "unhealthy": {
                "http_failures": 1,
                "http_statuses": [
                    429,
                    404,
                    500,
                    501,
                    502,
                    503,
                    504,
                    505
                ],
                "interval": 5,
                "tcp_failures": 1,
                "timeouts": 1
            }
        },
        "passive": {
            "healthy": {
                "http_statuses": [
                    200,
                    201,
                    202,
                    203,
                    204,
                    205,
                    206,
                    207,
                    208,
                    226,
                    300,
                    301,
                    302,
                    303,
                    304,
                    305,
                    306,
                    307,
                    308
                ],
                "successes": 0
            },
            "type": "http",
            "unhealthy": {
                "http_failures": 0,
                "http_statuses": [
                    429,
                    500,
                    503
                ],
                "tcp_failures": 0,
                "timeouts": 0
            }
        },
        "threshold": 0
    },
    "host_header": null,
    "id": "ce249d0b-073f-4eef-a9f1-ccfbefee623a",
    "name": "test",
    "slots": 1000,
    "tags": null
}

Then, I updated "http_path".

curl -H "Content-Type:application/json" -X PATCH http://172.16.192.153:8001/upstreams/test -d \
> '{"healthchecks": {"active": {"http_path": "/ping"}},"name": "test"}' | python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   967  100   900  100    67  11048    822 --:--:-- --:--:-- --:--:-- 11111
{
    "algorithm": "round-robin",
    "client_certificate": null,
    "created_at": 1624244553,
    "hash_fallback": "none",
    "hash_fallback_header": null,
    "hash_on": "none",
    "hash_on_cookie": null,
    "hash_on_cookie_path": "/",
    "hash_on_header": null,
    "healthchecks": {
        "active": {
            "concurrency": 10,
            "healthy": {
                "http_statuses": [
                    200,
                    302
                ],
                "interval": 5,
                "successes": 1
            },
            "http_path": "/ping",
            "https_sni": null,
            "https_verify_certificate": false,
            "timeout": 1,
            "type": "http",
            "unhealthy": {
                "http_failures": 1,
                "http_statuses": [
                    429,
                    404,
                    500,
                    501,
                    502,
                    503,
                    504,
                    505
                ],
                "interval": 5,
                "tcp_failures": 1,
                "timeouts": 1
            }
        },
        "passive": {
            "healthy": {
                "http_statuses": [
                    200,
                    201,
                    202,
                    203,
                    204,
                    205,
                    206,
                    207,
                    208,
                    226,
                    300,
                    301,
                    302,
                    303,
                    304,
                    305,
                    306,
                    307,
                    308
                ],
                "successes": 0
            },
            "type": "http",
            "unhealthy": {
                "http_failures": 0,
                "http_statuses": [
                    429,
                    500,
                    503
                ],
                "tcp_failures": 0,
                "timeouts": 0
            }
        },
        "threshold": 0
    },
    "host_header": null,
    "id": "ce249d0b-073f-4eef-a9f1-ccfbefee623a",
    "name": "test",
    "slots": 1000,
    "tags": null
}

Something strange happened. When I update http_path, the old http_path is still being requested. So I created this PR to solve this issue.

[GIN] 2021/06/21 - 15:27:51 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:27:51 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:27:56 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:27:56 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:00 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:01 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:05 | 200 |            0s |  172.16.192.153 |GET      "/ping"
[GIN] 2021/06/21 - 15:28:05 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:05 | 200 |            0s |  172.16.192.153 |GET      "/ping"
[GIN] 2021/06/21 - 15:28:10 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:10 | 200 |            0s |  172.16.192.153 |GET      "/ping"
[GIN] 2021/06/21 - 15:28:15 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:15 | 200 |            0s |  172.16.192.153 |GET      "/ping"
[GIN] 2021/06/21 - 15:28:20 | 404 |            0s |  172.16.192.153 |GET      "/hello"
[GIN] 2021/06/21 - 15:28:20 | 200 |            0s |  172.16.192.153 |GET      "/ping"