other_approach.go

package gps

import (
	"context"
	"sync/atomic"
)

func (sm *SourceMgr) ListPackages(ctx context.Context, id ProjectIdentifier, v Version) (PackageTree, error) {
	subctx, cancel := context.WithCancel(ctx)
	quitchan := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			// cancel initiated by the caller
		case <-sm.cm.quitchan:
			// quitchan closed centrally; trigger the cancel func for the subctx
			cancel()
		case <-quitchan:
			// local quitchan was closed
		}
	}

	// do stuff
	// ...

	// close the quitchan to kill the goroutine
	close(quitchan)
}