Accepted proposal: grow an existing map with maps.Grow

Liz Fong-Jones’s proposal to add maps.Grow was accepted on October 29. It gives you a way to make room in a map that already exists, much like make(M, n) does for a new one. A Go map grows on its own as you add entries. Each time it runs out of room, the runtime allocates bigger storage, rehashes the entries into it, and leaves the old storage for the garbage collector. One grow is cheap, but a loop that adds thousands of entries can trigger many of them, and each one burns CPU and creates more garbage. If you know the final size when you create the map, you pass it to make and skip all that extra work. But that only works if you’re the one creating the map. If someone else created it and you’re only adding entries, you’re stuck with however much room it has. net/http/httputil.ReverseProxy hits this on every request. It copies the upstream response headers into the client’s response: func copyHeader(dst, src http.Header) { for k, vv := range src { for _, v := range vv { dst.Add(k, v)…

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