altsrc for MultiStringFlag

[nkuba]

Is it possible to read with altsrc the MultiStringFlag value to a destination?

My destination is []string, so I wrapped StringSliceFlag with MultiStringFlag.

NetworkPeersFlag = &cli.MultiStringFlag{
	Target: &cli.StringSliceFlag{
		Name:  "network.peers",
	},
	Destination: &clientConfig.Peers,
}

There is no dedicated function in altsrc for MultiStringFlag and setting it with altsrc.NewStringSliceFlag(NetworkPeersFlag.Target) doesn't work.

[joeycumines]

Mmm ok, so I implemented the cli.SliceFlag wrapper (note: MultiStringFlag = SliceFlag[*StringSliceFlag, []string, string]), but I have not used the altsrc package myself, so I don't necessarily have all the context here.

tl;dr it looks like an issue with the altsrc package, and it doesn't appear to work correctly with cli.StringSliceFlag.Destination, either

The way cli.SliceFlag works is by utilising flag.Value.Set as a hook. It appears that altsrc.StringSliceFlag works by replacing the original flag.Value, in the flag set, which presumably only works for the cli.Context.Get (etc) mechanism.

cli/altsrc/flag.go

Line 94 in c8147a4

underlyingFlag.Value = &sliceValue

This behavior seems silly, but I'm guessing it was added due to the "append" behavior of the (base) flag.Value.Set implementations, used for all the slice flags.

Anyhow, on closer inspection, it looks like it probably doesn't work properly for the Destination field of the base cli.StringSliceFlag, either. Unfortunately, the way the different sources for flag values hang together is rather convoluted, and the Destination semantics for the slice flag implementations in particular have only recently worked "correctly", in terms of core functionality (and not considering altsrc).