fix: throw reject when SingleThreadExecutor drainTo in progress and queue is empty

[nodece]

Fix #4465

Motivaction

In SingleThreadExecutor, the runner drains all tasks from the queue into localTasks. Although the queue is empty in memory at this point, the tasks are still pending execution in localTasks—so logically, the queue is still "full."

Calling execute() during this phase should not enqueue a new runnable into the queue, as doing so would exceed the intended capacity. This can lead to increased memory usage and potential OutOfMemory (OOM) issues.

Changes

To address this, we introduce a variable to track the total number of pending runnables. This counter is used to control whether a new task should be added:

• The counter is incremented when a runnable is added to the queue.

• The counter is decremented when a runnable is actually executed.

This ensures accurate tracking of pending tasks and prevents overfilling the logical task queue.

[nodece]

@lhotari Could you have a chance to review?

[StevenLuMT]

Boundary value confirmation is required

[StevenLuMT]

LGTM

[xiezhx9]

squash into one commit might be better

[nodece]

squash into one commit might be better

The code owner can use squash merge to resolve this.

[nodece]

@lhotari @merlimat Could you have a chance to review?

[jiazhai]

+1. LTGM. Good Catch.