[WIP] Fix race condition causing panic in WorkerPool

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Problem

There's a race condition in telegram/media.go that causes a panic:

panic: send on closed channel
goroutine 329 [running]:
github.com/amarnathcjd/gogram/telegram.(*WorkerPool).AddWorker(0xc000140510, 0xc0005c72f0)
        /home/ubuntu/jb/vendor/github.com/amarnathcjd/gogram/telegram/media.go:61 +0xfa

Root Cause

The issue occurs in the WorkerPool implementation:

1. In initializeWorkers() (line 1212), workers are created asynchronously in a goroutine
2. The defer w.Close() statement in the calling function (e.g., UploadFile or DownloadMedia) can execute before the goroutine finishes
3. WorkerPool.Close() closes the free channel
4. When the goroutine tries to call AddWorker(), it attempts to send on the closed channel, causing a panic

Solution

The fix involves two changes:

1. Make AddWorker safe: Add a check to detect if the channel is closed before attempting to send. Use a select with default case to avoid blocking and panicking.

2. Add a closed flag: Track whether the worker pool has been closed to prevent adding workers after closure.

Changes Required

In telegram/media.go:

1. Add a closed flag to the WorkerPool struct
2. Update AddWorker() to check if the pool is closed and use a non-blocking send
3. Update Close() to set the closed flag before closing the channel
4. Update the select statement in AddWorker() to handle the closed channel gracefully

This will prevent the panic while maintaining the existing functionality of the worker pool.

This pull request was created as a result of the following prompt from Copilot chat.

Problem

There's a race condition in telegram/media.go that causes a panic:

panic: send on closed channel
goroutine 329 [running]:
github.com/amarnathcjd/gogram/telegram.(*WorkerPool).AddWorker(0xc000140510, 0xc0005c72f0)
        /home/ubuntu/jb/vendor/github.com/amarnathcjd/gogram/telegram/media.go:61 +0xfa

Root Cause

The issue occurs in the WorkerPool implementation:

1. In initializeWorkers() (line 1212), workers are created asynchronously in a goroutine
2. The defer w.Close() statement in the calling function (e.g., UploadFile or DownloadMedia) can execute before the goroutine finishes
3. WorkerPool.Close() closes the free channel
4. When the goroutine tries to call AddWorker(), it attempts to send on the closed channel, causing a panic

Solution

The fix involves two changes:

1. Make AddWorker safe: Add a check to detect if the channel is closed before attempting to send. Use a select with default case to avoid blocking and panicking.

2. Add a closed flag: Track whether the worker pool has been closed to prevent adding workers after closure.

Changes Required

In telegram/media.go:

1. Add a closed flag to the WorkerPool struct
2. Update AddWorker() to check if the pool is closed and use a non-blocking send
3. Update Close() to set the closed flag before closing the channel
4. Update the select statement in AddWorker() to handle the closed channel gracefully

This will prevent the panic while maintaining the existing functionality of the worker pool.

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[IndrajeethY]

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[xAmarnath]

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