util/eventbus: add a Monitor type to manage subscriber goroutines (#17127)

A common pattern in event bus usage is to run a goroutine to service a
collection of subscribers on a single bus client. To have an orderly shutdown,
however, we need a way to wait for such a goroutine to be finished.

This commit adds a Monitor type that makes this pattern easier to wire up:
rather than having to track all the subscribers and an extra channel, the
component need only track the client and the monitor.  For example:

   cli := bus.Client("example")
   m := cli.Monitor(func(c *eventbus.Client) {
     s1 := eventbus.Subscribe[T](cli)
     s2 := eventbus.Subscribe[U](cli)
     for {
       select {
       case <-c.Done():
         return
       case t := <-s1.Events():
          processT(t)
       case u := <-s2.Events():
          processU(u)
       }
     }
   })

To shut down the client and wait for the goroutine, the caller can write:

   m.Close()

which closes cli and waits for the goroutine to finish. Or, separately:

   cli.Close()
   // do other stuff
   m.Wait()

While the goroutine management is not explicitly tied to subscriptions, it is a
common enough pattern that this seems like a useful simplification in use.

Updates #15160

Change-Id: I657afda1cfaf03465a9dce1336e9fd518a968bca
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

0 files changed, 0 insertions, 0 deletions