Fix flush method root cause - handle async callbacks in _doSend with proper error handling

[Copilot]

The Sender.flush() method was not properly handling callbacks or returning promises because the root cause was in the _doSend() function, which couldn't handle asynchronous callbacks from preparePayload().

Root Cause

The issue was that _doSend() assumed preparePayload() would always execute its callback synchronously, but when compression is enabled, the callback executes asynchronously after stream processing completes.

// Before: Broken for async callbacks
_sendPostMgr.preparePayload((processedPayload: IPayloadData) => {
    result = sendPostFunc(processedPayload, onComplete, !isAsync);
}, _zipPayload, payloadData, !isAsync);
return result;  // ❌ Returns undefined when callback is async

Solution

Fixed _doSend() to handle both synchronous and asynchronous callback execution using direct resolve capture with proper error handling:

// After: Handles both sync and async callbacks with error handling
let result: void | IPromise<boolean>;
let callbackExecuted = false;
let resolveFn: any;
let rejectFn: any;

_sendPostMgr.preparePayload((processedPayload: IPayloadData) => {
    result = sendPostFunc(processedPayload, onComplete, !isAsync);
    callbackExecuted = true;
    if (resolveFn) {
        doAwait(result as any, resolveFn, rejectFn);
    }
}, _zipPayload, payloadData, !isAsync);

if (callbackExecuted) {
    return result;  // Synchronous case
}

// Asynchronous case - return promise that captures both resolve and reject
return createPromise<boolean>((resolve, reject) => {
    resolveFn = resolve;
    rejectFn = reject;
});

Also simplified the flush method's return logic using doAwaitResponse to handle both promise and non-promise results consistently.

Breaking Changes

This release contains a potential breaking change to the flush method signature in the IChannelControls interface. The parameter name has been changed from async to isAsync to avoid potential conflicts with the async keyword.

Interface change:

// Before:
flush?(async: boolean, callBack?: (flushComplete?: boolean) => void, sendReason?: SendRequestReason): boolean | void | IPromise<boolean>;

// After: 
flush?(isAsync: boolean, callBack?: (flushComplete?: boolean) => void, sendReason?: SendRequestReason): boolean | void | IPromise<boolean>;

If you have custom channels or plugins that implement the IChannelControls interface directly, you will need to update the parameter name from async to isAsync in your implementation.

Benefits

• Efficient async support: await applicationInsights.flush() now works correctly with compression enabled
• Proper error handling: Promise rejections are now correctly propagated through the async callback chain
• No polling overhead: Uses direct callback invocation instead of scheduleTimeout polling
• Handles both sync/async: Works whether preparePayload executes callback immediately or asynchronously
• Cleaner code: Simplified return logic and eliminated complex nested exception handling
• Backward compatible: Existing usage patterns continue to work

Fixes #2628.

---

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[MSNev]

SenderTests > flush method handles asynchronous callback execution without callback
2025-07-29T20:13:18.4900180Z >> Message: flush should return promise-like object when async=true and no callback
2025-07-29T20:13:18.4901108Z >> Actual: false
2025-07-29T20:13:18.4901489Z >> Expected: true
2025-07-29T20:13:18.4902738Z >> at SenderTests.test (http://localhost:9001/channels/applicationinsights-channel-js/Tests/Unit/dist/aichannel.tests.js:6121:34)
2025-07-29T20:13:18.4904972Z >> at Object.testMethod (http://localhost:9001/channels/applicationinsights-channel-js/node_modules/@microsoft/ai-test-framework/dist/es5/ai-test-framework.js:53630:48)
2025-07-29T20:13:18.4906682Z >> at runTest (http://localhost:9001/common/Tests/External/qunit-2.9.3.js:2725:35)
2025-07-29T20:13:18.4908159Z >> at Test.run (http://localhost:9001/common/Tests/External/qunit-2.9.3.js:2708:9)
2025-07-29T20:13:18.4909176Z >> at http://localhost:9001/common/Tests/External/qunit-2.9.3.js:2972:16
2025-07-29T20:13:18.4921097Z >> at processTaskQueue (http://localhost:9001/common/Tests/External/qunit-2.9.3.js:2293:26)

[Copilot]

SenderTests > flush method handles asynchronous callback execution without callback
2025-07-29T20:13:18.4900180Z >> Message: flush should return promise-like object when async=true and no callback
2...

Fixed the failing test. The issue was that when isAsync=true and no callback is provided, the flush method was returning a boolean value instead of a promise as required by the interface. Updated the logic to return createPromise<boolean>((resolve) => resolve(!rsp.rejected)) in this case. Commit: c7775f5

[Copilot]

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