I would consider an alternative logic: define the MinIdleConnsHeadroom as the percentage of conns using the last specified period.

Let me explain myself: consider a 1 minute period, if a connection has been used during that last 1 minute period, then we don't consider it "idle" at all: it is idle in terms of the network, but we know that some load spike during the last minute has used it, so we shouldn't really close it. Let's call these connections used during the last 1 minute "recently used".

Let's say we have 100 active connections, then with MinIdleConnsHeadroom=50 we should leave 50 really-idle connections on top of those 100 recently used, to account for the possible spikes.

If the use decreases, let's say to 50 active conns, then we'll close connections until we have 75 (50 recently used + 25 idle). This will not happen in a single close, as the number of active connections will naturally decrease over time.

I see two advantages in this:

• The number of idle connections we keep depends on the number of used ones, rather than an absolute minimum.
• The logic is (IMO) simpler to implement: take the slice of conns, count the recently used ones, and then skip the percentage of idle we want to keep, and close the rest.

This is just an alternative idea to consider, I'm not saying there's something wrong with your logic 👍

I would consider an alternative logic: define the MinIdleConnsHeadroom as the percentage of conns using the last specified period.

I like your idea because you don't have to think about the absolute minimum value to set. I'm going to change the implementation.

Left some opinionated comments, they're not blockers, but consider thinking about them.

Thanks @colega for your review. I should have addressed all comments, plus added few more test cases and made Close() idempotent. Can you take another look, please?