PlannedReparent: create a new context for calling UndoDemoteMaster

[deepthi]

If the calling context for PRS expires during the time that PRS is running and we have already demoted the old master, we attempt to undo the demotion.
There's clearly a bug in https://github.com/vitessio/vitess/blob/master/go/vt/wrangler/reparent.go#L609:

		if err != nil || (ctx.Err() != nil && ctx.Err() == context.DeadlineExceeded) {
			// If we fail to promote the new master, try to roll back to the
			// original master before aborting.
			undoCtx, undoCancel := context.WithTimeout(ctx, *topo.RemoteOperationTimeout)
			defer undoCancel()

We are using the original context to call UndoDemoteMaster (or rather as the base for a new context) and we don't know if that context is valid any longer.
We can fix this by using the background context to call UndoDemoteMaster.

cc @zmagg

Signed-off-by: deepthi deepthi@planetscale.com

[rafael]

However, this does not guarantee that it will get called because, remember, the original context used for calling PRS has already expired so execution of PRS might terminate at any point.

@deepthi could you clarify this? This is not obvious to me. I think if you pass a fresh context to wr.tmc.UndoDemoteMaster and this is called in a synchronous context, I don't see how execution of PRS might terminate before finishing the undo.

[deepthi]

However, this does not guarantee that it will get called because, remember, the original context used for calling PRS has already expired so execution of PRS might terminate at any point.

@deepthi could you clarify this? This is not obvious to me. I think if you pass a fresh context to wr.tmc.UndoDemoteMaster and this is called in a synchronous context, I don't see how execution of PRS might terminate before finishing the undo.

When run using vtctlclient, the call to PRS is itself an RPC to vtctld and its context is the one passed all the way through. Things are different if you use vtctl instead.
I think you are right.. so all we need is a 1-line fix.

[enisoc]

LGTM