Fix HTTPRequestComponent::get_string return value

[martgras]

What does this implement/fix?

the static variable initialization happens only once therefore http_request.get_string() returns stale values.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes esphome/issues#2903

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

esphome:
  name: testm5       
esp32:
  board: pico32
  framework:
    type: arduino
logger:
  level: VERBOSE
wifi:
  networks:  
      ssid: !secret wifi_sid
      password: !secret wifi_password
http_request:
  useragent: esphome/device
  id: httpget
  timeout: 20s
sensor:
  - platform: uptime
    name: Uptime Sensor
    id: uptime_sensor
    update_interval: 20s
    on_value:
      then:
        - http_request.get:
            url: http://worldtimeapi.org/api/timezone/Europe/Berlin
            on_response:
              then:
                - logger.log:
                    format: "Got %s"
                    args: ['id(httpget).get_string()']

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[roitagar]

Regarding trying to avoid copying - will it work if you make the local static variable of type String rather than std::string?
It'll allow use of a Move Constructor..

[oxan]

On ESP8266 getClient() returns a String& to a member variable, and I don't think it likes if we move from that object.

[roitagar]

I think the compiler handles it automatically, so ESP8266 will use a copy-constructor and ESP32 will use a move-constructor.

EDIT: Obviously I mean copy-assignment vs. move-assignment and not constructor - as the static object is already constructed at that point..

[oxan]

Hmm, I think you're right and that'd work. Happy to see (another) PR change it into that :)

[roitagar]

@oxan

Happy to see (another) PR change it into that :)

#2988 😃