System.disableUpdates() operates asynchronously

[m-mcgowan]

This PR will be cherry-picked into another PR based on release/1.2.1 (for 1.2.1-rc.3).

Problem

For a full description see the clubhouse story. In a nutshell, System.disableUpdates() should not block the application thread to send the event to the cloud.

Solution

The internal function system_send_event gains an additional flag to indicate the event should be processed asynchronously. This is a temporary solution until we introduce futures to all synchronous system APIs so that they may be used synchronously or asynchronously as chosen by the caller.

Steps to Test

Testing was done by using the app below both with and without the asynchornous event flag handling. The connection to the cloud was interrupted by turning off the mobile hotspot and the LED observed to see if the application thread continued running. Without asynchronous events, the LED stopped blinking immediately. With asynchronous events, it continued blinking for a few seconds (blocking on queuing the asynchronous request to the system event queue - we should consider allowing the queue to allocate more resources or make some functions optional when the queue cannot accept more items.)

Example App

#include "application.h"

SYSTEM_THREAD(ENABLED);
void setup()
{
    Serial.begin();
    pinMode(D7, OUTPUT);
}

#define TIME(fn, maximum) \
    { int start = millis(); \
    fn; \
    int duration = millis()-start; \
    if (duration > maximum) { \
        Serial.printlnf("function took too long: %d ms", duration); \
    }}

//ApplicationWatchdog wd(5000, System.reset);

void loop()
{
    digitalWrite(D7, false);
    TIME(System.enableUpdates(), 2);
    delay(100);
    digitalWrite(D7, true);
    TIME(System.disableUpdates(), 2);
    delay(100);
}

References

• [CH33758]

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

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• [bugfix] System.disableUpdates() operates asynchronously #1801

[sergeuz]

This is fine as a temporary solution. Do you plan to implement support for "cancellable events" in the scope of this PR as well, or should we merge it as is for now?

[sergeuz]

For visibility: this will only work if name and data are statically allocated (which is true for the updates/enabled and updates/forced events)