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fix(lambda): return partial CreateFleet instances instead of discarding them #5131

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fix(lambda): return partial CreateFleet instances instead of discarding them #5131

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b8c45d8
fix(lambda): return partial CreateFleet instances instead of discardi…
vegardx May 26, 2026
e8cdc4e
refactor: address review feedback — mechanism-agnostic log, stronger …
vegardx May 26, 2026
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24 changes: 20 additions & 4 deletions lambdas/functions/control-plane/src/aws/runners.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -511,13 +511,15 @@ describe('create runner with errors', () => {
it('test ScaleError with multiple error.', async () => {
createFleetMockWithErrors(['UnfulfillableCapacity', 'MaxSpotInstanceCountExceeded', 'NotMappedError']);

await expect(createRunner(createRunnerConfig(defaultRunnerConfig))).rejects.toMatchObject({
await expect(
createRunner({ ...createRunnerConfig(defaultRunnerConfig), numberOfRunners: 3 }),
).rejects.toMatchObject({
name: 'ScaleError',
failedInstanceCount: 2,
failedInstanceCount: 3, // numberOfRunners when zero instances created
});
expect(mockEC2Client).toHaveReceivedCommandWith(
CreateFleetCommand,
expectedCreateFleetRequest(defaultExpectedFleetRequestValues),
expectedCreateFleetRequest({ ...defaultExpectedFleetRequestValues, totalTargetCapacity: 3 }),
);
expect(mockSSMClient).not.toHaveReceivedCommand(PutParameterCommand);
});
Expand All @@ -543,6 +545,18 @@ describe('create runner with errors', () => {
);
});

it('returns partial instances on recognized scale error instead of throwing', async () => {
createFleetMockWithErrors(['UnfulfillableCapacity'], ['i-partial']);

await expect(
createRunner({ ...createRunnerConfig(defaultRunnerConfig), numberOfRunners: 3 }),
).resolves.toEqual(['i-partial']);
expect(mockEC2Client).toHaveReceivedCommandWith(
CreateFleetCommand,
expectedCreateFleetRequest({ ...defaultExpectedFleetRequestValues, totalTargetCapacity: 3 }),
);
});

it('test error by create fleet call is thrown.', async () => {
mockEC2Client.on(CreateFleetCommand).rejects(new Error('Some error'));

Expand Down Expand Up @@ -688,12 +702,14 @@ describe('create runner with errors fail over to OnDemand', () => {
// fallback to on demand for UnfulfillableCapacity but InsufficientInstanceCapacity is thrown
createFleetMockWithWithOnDemandFallback(['UnfulfillableCapacity'], instancesIds);

// Partial success: 1 instance created, unrecognized error for the rest.
// Returns partial instances instead of throwing to prevent orphans.
await expect(
createRunner({
...createRunnerConfig(defaultRunnerConfig),
numberOfRunners: 2,
}),
).rejects.toBeInstanceOf(Error);
).resolves.toEqual(['i-123']);

expect(mockEC2Client).toHaveReceivedCommandTimes(CreateFleetCommand, 1);

Expand Down
21 changes: 19 additions & 2 deletions lambdas/functions/control-plane/src/aws/runners.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -201,9 +201,26 @@ async function processFleetResult(

const failedCount = countScaleErrors(errors, scaleErrors);
if (failedCount > 0) {
logger.warn('Create fleet failed, ScaleError will be thrown to trigger retry for ephemeral runners.');
if (instances.length > 0) {
logger.warn(
`Partial fleet success: ${instances.length}/${runnerParameters.numberOfRunners} instances created. ` +
`Returning partial results; unfulfilled requests remain for retry.`,
{ data: fleet.Errors, created: instances.length, requested: runnerParameters.numberOfRunners },
);
return instances;
}
Comment on lines +204 to +211
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@vegardx vegardx May 26, 2026

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The caller (scaleUp in scale-up.ts) already handles instances.length < numberOfRunners — it tracks how many instances were successfully created vs. how many SQS messages it received, and marks the shortfall as batchItemFailures. This is the existing contract; we're not changing it.

Introducing a structured return type would be a larger refactor that touches every callsite of createRunner (scale-up, pool, on-demand fallback recursion) for no behavioral benefit — the length comparison already provides the signal.

logger.warn('Create fleet failed with zero instances, ScaleError will be thrown to trigger retry.');
logger.debug('Create fleet failed.', { data: fleet.Errors });
throw new ScaleError(failedCount);
throw new ScaleError(runnerParameters.numberOfRunners);
}

if (instances.length > 0) {
logger.warn(
`Partial fleet success: ${instances.length}/${runnerParameters.numberOfRunners} instances created. ` +
`Error not recognized as scaling error; returning partial results.`,
{ data: fleet.Errors, created: instances.length, requested: runnerParameters.numberOfRunners },
);
return instances;
}

logger.warn('Create fleet failed, error not recognized as scaling error.', { data: fleet.Errors });
Expand Down