= cockpit.js: Raw Channels

At a low level Cockpit communicates with the system via messages passed
through various channels. These are usually exposed via higher level
APIs, such as the `+cockpit.spawn()+` function. It is rare to use raw
channels directly.

[[cockpit-channels-channel]]
== cockpit.channel()

....
channel = cockpit.channel(options)
....

This function creates a new channel for communication with the system.
It returns a new channel object. The `+options+` argument is a plain
object. At least the `+"payload"+` option is required, and based on the
payload type, other options may be required.

`+"binary"+`::
  Set to `+true+` to transfer binary payloads. Both messages sent via
  link:#cockpit-channels-send[`+channel.send()+`] and those received via
  link:#cockpit-channels-message[`+channel.onmessage+`] should be arrays
  of bytes, either `+Uint8Array+` or `+Array+` depending on browser
  support.
`+"payload"+`::
  The payload type for the channel. Only specific payload types are
  supported.
`+"superuser"+`::
  Set to `+"require"+` to open this channel as root. If the currently
  logged in user is not permitted to become root (eg: via `+pkexec+`)
  then the `+channel+` will immediately be
  link:#cockpit-channels-close-ev[closed] with a `+"access-denied"+`
  problem code.
  +
  Set to `+"try"+` to try to open the channel as root, but if that
  fails, then fall back to an unprivileged channel.

The channel object returned has the following fields and methods and
events. You should call the `+channel.close()+` method when done with
the channel.

A valid channel will always be returned and the is ready to
`+channel.send()+`. The channel may
link:#cockpit-channels-close-ev[close shortly afterword] due to a
failure.

[[cockpit-channels-binary]]
== channel.binary

Will be `+true+` for an binary channel. Will be set to `+false+` if the
channel is textual.

[[cockpit-channels-options]]
== channel.options

The options used to open this channel. This should not be changed.

[[cockpit-channels-valid]]
== channel.valid

Will be `+true+` for an open channel. Will be set to `+false+` if the
channel closes.

[[cockpit-channels-send]]
== channel.send()

....
channel.send(data)
....

Send a message over the channel. The contents of the message depends on
the payload type of the channel. If a binary channel, then `+data+` is
expected to be an `+Array+` of bytes or a `+Uint8Array+`. If not binary,
then the `+data+` will be converted to a string if not already a string.

[[cockpit-channels-control]]
== channel.control()

....
channel.control(options)
....

Notify the channel to tune certain parameters on the fly. The
`+options+` is a plain javascript object, and the contents depend on the
`+"payload"+` of the channel.

One common operation is to set `+"command"+` to `+"done"+` in the
options field. To indicate that no further messages will be sent through
the channel.

[[cockpit-channels-wait]]
== channel.wait()

....
promise = channel.wait([callback])
....

Returns a `+promise+` that is ready when the channel is ready, or fails
if the client closes. If a `+callback+` is specified, it is attached to
the promise. The promise will be rejected or resolved with the contents
`+options+` passed to the
link:#cockpit-channels-onready[channel.onready] and
link:#cockpit-channels-close-ev[channel.onclose] events respectively.

In general it's not necessary to wait for the channel before starting to
use the channel.

[[cockpit-channels-close]]
== channel.close()

....
channel.close([options])
....

Close the channel.

If `+options+` is present it can be a plain javascript object containing
additional channel close options to send to the peer. If closing for
because of a problem, set the `+"problem"+` field to a
link:#cockpit-problems[problem code]. If `+options+` is not an object it
will be treated as a `+"problem"+`.

The link:#cockpit-channels-close-ev[close event] will fire. A channel
can also be closed by a peer or if the underlying transport closes.

[[cockpit-channels-message]]
== channel.onmessage

....
channel.addEventListener("message", function(event, data) { ... })
....

An event triggered when the channel receives a message. The message is
passed as a string to the handler in the `+data+`. In the case of binary
channels `+data+` is an `+Uint8Array+` or an `+Array+` of bytes if the
former is not supported by the browser. The contents of the message
depends on the payload type of the channel.

[[cockpit-channels-oncontrol]]
== channel.oncontrol

....
channel.addEventListener("control", function(event, options) { ... })
....

An event triggered when the channel receives an control message in the
middle of the flow. One particular use is when the `+command+` is set to
`+"done"+` then no further messages will be received in the channel. The
exact form of these messages depend on the `+"payload"+` of the channel.

[[cockpit-channels-onready]]
== channel.onready

....
channel.addEventListener("ready", function(event, options) { ... })
....

An event triggered when the other end of the channel is ready to start
processing messages. This indicates the channel is completely open. It
is possible to start sending messages on the channel before this point.

[[cockpit-channels-close-ev]]
== channel.onclose

....
channel.addEventListener("close", function(event, options) { ... })
....

An event triggered when the channel closes. This can happen either
because link:#cockpit-channels-close[channel.close()] function was
called, or if the peer closed the channel, or the underlying transport
closes.

The `+options+` will contain various close information, including a
`+"problem"+` field which will be set if the channel was closed because
of a problem.

[[cockpit-transport-origin]]
== cockpit.transport.origin

....
cockpit.transport.origin
....

The HTTP origin that is being used by the underlying channel transport.
This is read-only, you should not assign a value. If the browser
supports `+window.location.origin+` then this will be identical to that
value.

[[cockpit-transport-host]]
== cockpit.transport.host

....
cockpit.transport.host
....

The host that this transport is going to talk to by default. This is
read-only, you should not assign a value. If the value is null that
means that the transport has not been setup yet.

[[cockpit-transport-csrf-token]]
== cockpit.transport.csrf_token

....
cockpit.transport.csrf_token
....

A cross site request forgery token for use with external channels. This
becomes valid once the connection is properly established.

[[cockpit-transport-options]]
== cockpit.transport.options

....
cockpit.transport.options
....

Initialization options received over the underlying channel transport.
These will be empty until connection is properly established.

[[cockpit-transport-wait]]
== cockpit.transport.wait()

....
cockpit.transport.wait(callback)
....

Call the `+callback+` function once the underlying channel transport is
initialized. This will start the initialization if not already in
progress or completed. If the channel transport is already initialized,
then `+callback+` will be called immediately.

In general it's not necessary to wait for the transport before starting
to open channels.

[[cockpit-transport-close]]
== cockpit.transport.close()

....
cockpit.transport.close([problem])
....

Close the underlying channel transport. All channels open channels will
close. The `+problem+` argument should be a problem code string. If not
specified it will default to `+"disconnected"+`.

[[cockpit-transport-filter]]
== cockpit.transport.filter()

....
cockpit.transport.filter((message, channelid, control) =>  { ... })
....

Add a filter to the underlying channel transport. All incoming messages
will be passed to each of the filter callbacks that are registered.

This function is rarely used.

Filter callbacks are called in the order they are registered. If a
filter callback returns `+false+` then the message will not be
dispatched further, whether to other filters, or to channels, etc.

The `+message+` is the string or array with the raw message including,
the framing. The `+channelid+` is the channel identifier or an empty
string for control messages. If `+control+` is set then this is a
control message,d and the `+control+` argument contains the parsed JSON
object of the control message.

[[cockpit-transport-inject]]
== cockpit.transport.inject()

....
cockpit.transport.inject(message, [out])
....

Inject a message into the underlying channel transport. The `+message+`
should be a `+string+` or an array of bytes, and should be valid
according to the Cockpit message protocol. If the `+out+` argument is
equal to `+false+` then the message will be injected as an incoming
message as if it was received on the underlying channel transport.

This function is rarely used. In general you should only `+inject()+`
messages you got from a `+filter()+`.

[[cockpit-base64-encode]]
== cockpit.base64_encode()

....
string = cockpit.base64_encode(data)
....

Encode binary data into a string using the Base64 encoding. The `+data+`
argument can either be a `+string+`, an `+Array+`, an `+ArrayBuffer+` or
a `+Uint8Array+`. The return value is a string.

[[cockpit-base64-decode]]
== cockpit.base64_decode()

....
data = cockpit.base64_decode(string, [constructor])
....

Decode binary data from a Base64 encoded string. The `+string+` argument
should be a javascript string. The returned `+data+`> will be an array
of bytes.

You can pass `+Uint8Array+`, `+Array+` or `+String+` as an alternate
`+constructor+` if you want the decoded data in an alternate form. The
default is to return an `+Array+`. Note that if you use a `+String+` for
the decoded data, then you must guarantee that the data does not contain
bytes that would be invalid for a string.
