Reduce bytes to strings allocs in decoder

[marselester]

Instead of allocating when converting small byte slices to strings in the decoder, we can write to a 4kb buffer and use unsafe.String which refers to that buffer. This approach was successfully tested in https://github.com/marselester/systemd which is used by https://github.com/parca-dev/parca-agent.

Note, BenchmarkUnixFDs showed 8 allocs reduction. With larger dbus messages, the savings should be significant.

$ go test -bench='^BenchmarkUnixFDs$' -benchmem
# new
BenchmarkUnixFDs-2   	    8052	    241806 ns/op	   11181 B/op	     357 allocs/op
# old
BenchmarkUnixFDs-2   	    8899	    265314 ns/op	   11187 B/op	     365 allocs/op

The caveat is that we should bump Go to 1.20, I am not sure if that's something you're comfortable with.

[guelfey]

Thanks. Generally I'm fine with this, though I wouldn't want to bump the minimum version to Go 1.20 just for this. But using build tags it should be possible to wrap this functionality such that it's only used when using Go 1.20 - just put the actual stringConverter in a file with //+build go1.20 and a stub implementation that just uses a normal string conversion in a file with //+build !go1.20.

[marselester]

That's a good idea to leverage build tags. Maybe I am doing something wrong, but for some reason go1.20 tag clashes with go 1.12 version defined in the go.mod.

$ go test .
# github.com/godbus/dbus/v5 [github.com/godbus/dbus/v5.test]
./btos_new.go:10:9: unsafe.String requires go1.20 or later (-lang was set to go1.12; check go.mod)
FAIL	github.com/godbus/dbus/v5 [build failed]
FAIL

[marselester]

We can use the old way of converting strings in all the Go versions instead of the build tags.

func toString(b []byte) string {
	var s string
	h := (*reflect.StringHeader)(unsafe.Pointer(&s))
	h.Data = uintptr(unsafe.Pointer(&b[0]))
	h.Len = len(b)

	return s
}

[guelfey]

Thanks!