Add support for GetOrTrySetFunc

[blakearnold]

I'm trying to set values using GetOrSetFunc, but sometimes there is a transient error when retrieving the value. This requires extra logic for handling an error.

Example:

	var err error
	result, _ := c.cachedBalance.GetOrSetFunc(
		pubkey, func() (balance *rpc.GetBalanceResult) {
			balance, err = c.rpcClient.GetBalance(ctx, accountID)
			return balance
		},
	)
	if err != nil {
                // delete from balance so it will be refetched next time
		c.cachedBalance.Delete(accountID)
		return nil, err
	}
	if result == nil || result.Value() == nil {
		return nil, fmt.Errorf("balance not found")
	}
	return result.Value(), nil

it would be nice to add a GetOrTrySetFunc so I can structure the code as so:

	var err error
	result, _ := c.cachedBalance.GetOrTrySetFunc(
		pubkey, func() (balance *rpc.GetBalanceResult, cancel bool) {
			balance, err = c.rpcClient.GetBalance(ctx, pubkey, rpc.CommitmentConfirmed)
			if err != nil {
				return nil, true
			}
			return balance, false
		},
	)
	if err != nil {
		return nil, err
	}
	return result.Value(), nil

[davseby]

Interesting. I'm not a big fan of doing any IO operations inside the SetFunc as that locks the whole cache. Perhaps, something like this would be sufficient for you?

	if item := c.cachedBalance.Get(pubkey); item != nil {
		return item.Value(), nil
	}

	// Could be replaced with deterministic workers or 
	// named-based lock for better performance.
	c.mu.Lock()
	defer c.mu.Unlock()

	// Ensure the item is still not in the 
	// cache after acquiring the lock.
	if item := c.cachedBalance.Get(pubkey); item != nil {
		return item.Value(), nil
	}

	balance, err = c.rpcClient.GetBalance(ctx, pubkey, rpc.CommitmentConfirmed)
	if err != nil {
		return nil, err
	}

	c.cachedBalance.Set(pubkey, balance, cache.DefaultExpiration)

	return balance, nil

We'll have this in mind, still need to think about it for a minute. Perhaps, there is a way of fetching items in a non-blocking fashion using cache loader that also can return or resolve errors. Not sure though.

[blakearnold]

True, this is not ideal without locking per key.

I've been using Map in https://github.com/puzpuzpuz/xsync, which has LoadOrTryCompute, which locks per key. I was trying to move over to ttl cache to be able to set TTLs, but seams that I'll need to add additional code to get it supported.

[davseby]

True, this is not ideal without locking per key.

I've been using Map in https://github.com/puzpuzpuz/xsync, which has LoadOrTryCompute, which locks per key. I was trying to move over to ttl cache to be able to set TTLs, but seams that I'll need to add additional code to get it supported.

Interesting concept, I'll look into it.

[lukasbindreiter]

True, this is not ideal without locking per key.

Seems a bit related to: #122