Cancellation channels are often derived from a Context, which
returns a directional `<-chan struct{}` from Done(). In order to use
this with parts of of the consul API, one is required to create a new
channel and dispatch a separate goroutine to watch for context
cancellation and close the new channel.
Changing the signature for the methods that take cancellation channels
will allow easier integration with existing uses of Context. Since the
cancellation pattern only reads from these channels, there should be no
backwards incompatibility with existing codebases, and most of the
methods already accept only the correct type.
Adds the ability to simply check whether a TCP socket accepts
connections to determine if it is healthy. This is a light-weight -
though less comprehensive than scripting - method of checking network
service health.
The check parameter `tcp` should be set to the `address:port`
combination for the service to be tested. Supports both IPv6 and IPv4,
in the case of a hostname that resolves to both, connections will be
attempted via both protocol versions, with the first successful
connection returning a successful check result.
Example check:
```json
{
"check": {
"id": "ssh",
"name": "SSH (TCP)",
"tcp": "example.com:22",
"interval": "10s"
}
}
```
This status must be one of the valid check statuses: 'passing', 'warning', 'critical', 'unknown'.
If the status field is not present or the empty string, the default of 'critical' is used.
James Bardin