Can Semaphores Emulate Buffered Locking in Go?

Buffered Lock Patterns

In Go, the concept of a buffered channel enables non-blocking operations until the channel's buffer is filled. Inspired by this mechanism, the inquiry arises: is there a generalized pattern akin to buffered channels for "buffered locking," where a resource can be locked for a finite set of clients?

Answer: Semaphores

The primitive that fulfills this requirement is a semaphore. Constructed with a buffered channel, a semaphore imposes a limit on the number of concurrent clients that can access a resource.

Consider the following implementation using a buffered channel:

var semaphore = make(chan struct{}, 4) // allow four concurrent users

func f() {
    // Grab the lock. Blocks as long as 4 other invocations of f are still running.
    semaphore <- struct{}{}

    // Release the lock once we're done.
    defer func() { <-semaphore }()

    // Do work...
}

In this scenario, the f function acquires the lock by sending a value into the semaphore channel. If the channel is full, representing the maximum allowed concurrency, f blocks until another client releases the lock by receiving a value from the channel. The defer statement ensures that the lock is released when the function returns.

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