Subcommands should have first-class support for Context.

[danjacques]

Currently, this package is completely Context-unaware. Context is, however, the sort of thing the top-level main method may want to establish or control from outside the scope of the subcommand (e.g., for signal-based cancellation, factory installation, etc.).

Some solutions, such as https://godoc.org/github.com/luci/luci-go/common/cli, operate by bolting this support onto subcommands. This results in some pretty wonky code, weird inversions, and the creation (or discarding) of multiple Context.

A better solution would be to have subcommands.Run accept a Context and forward it to its CommandRun.

[maruel]

ok