Proposal: cmd/go/internal/lockedfile: add Append function

[Mottl]

This is a proposal of adding Append function to cmd/go/internal/lockedfile.
Append will act as Write, except that it will not truncate a file if it already exists.

[bcmills]

Note that cmd/go/internal/lockedfile is internal, and thus cannot be imported from outside of its part of the source tree.

You probably want to pair this with a proposal to move lockedfile to publicly-visible location. 🙂

[Mottl]

Implementation: https://github.com/Mottl/go/blob/a270937c38ebad5598988f195d9104e0bf7f3d62/src/cmd/go/internal/lockedfile/lockedfile.go#L124-L137

[rsc]

A minor change to an internal-only interface does not really need a proposal.
The proposal here is really to export lockedfile as a public API.
I don't think we have enough experience yet to decide about that.
Maybe in six months, for the Go 1.14 cycle.